Study heat transfer by conduction and convection

23.7.0 Conduction of heat
23.7.01 Conduction of heat
Experiments
23.7.1 Boil water in a balloon
23.7.2 Boil water in a paper cup
23.7.3 Boil water in a test-tube
23.7.4 Boil water in a paper cup
23.7.5 Boil water in an aluminium pot and stainless steel pot
23.7.6 Conduction of heat by a coin on paper
23.7.7 Conduction of heat by different metals
23.7.8 Conduction of heat by metal bars
27.7.9 Conduction of heat by metals
27.7.9 Davy lamp
23.7.10 Conduction of heat by wood
23.7.11 Conduction in a metal bar
23.7.12 Cook an egg on a piece of paper
23.7.13 Copper coil candle snuffer
23.7.14 Copper coil snuffer conducts heat
23.7.15 Hammer hardwood peg
23.7.16 Heat conductivity ring
23.7.17 Heat insulation
23.7.18 Heat paper without burning
23.7.19 Melt ice blocks
23.7.20 Reduce heat loss with insulation
23.7.21 Relative conductivity
23.7.22 Touch wood and iron in the sun

23.8.0 Convection
23.8.01 Convection
23.8.1 Convection smoke box
23.8.2 Convection cells
23.8.3 Convection currents and ventilation
23.8.4 Convection currents between jars of water
23.8.5 Convection currents from an ink bottle
23.8.6 Convection currents in a container
23.8.7 Convection currents in a test-tube
23.8.8 Convection currents in water
23.8.9 Convection smoke box with two chimneys
23.8.10 Convection tube
23.8.11 Feel convection currents in a test-tube
23.8.12 Trace convection currents

23.9.01 Radiation
23.9.1 Absorption of radiation
23.9.2 Black and white surfaces affect radiation
23.9.3 Black body radiation
23.9.4 Colour temperature
23.9.5 Different surfaces affect heat radiation and absorption
23.9.6 Feel radiation
23.9.7 Focus radiant heat waves
23.9.8 Heat radiation decreases with distance
23.9.9 Heat transferred by radiation
23.9.10 Holes in carbon blocks
23.9.11 Leslie's cube
23.9.12 Light a match with reflectors
23.9.13 Plate in a furnace
23.9.14 Radiant heat passes through glass
23.9.15 Radiant heat using parabolic reflectors and a thermopile
23.9.16 Radiation from shiny surface and black surface
23.9.17 Reflection of radiant heat waves
23.9.18 Surface colour and the heat absorbed
23.9.19 Surface radiation from an engine
23.9.20 Teapot 23.9.21 Thermopile
23.9.22 Thermoscope to compare absorption of radiation
23.9.23 Transfer heat by radiation

23.7.01 Conduction of heat, thermal conductivity
The process of transformation of energy from one object to another is caused by heat motion of molecules and atoms and is called heat transfer.
Heat transfers from an object or a part of it in higher temperature to an object or a part of it in lower temperature.
When the temperatures of the two objects are equal, they are in a state of heat equilibrium.
Heat transfer can occur in solids, liquids and gases.

23.7.1 Water is a poor conductor of heat, boil water in a balloon
1. Boil water in a balloon.
Make a tripod big enough to suspend a balloon full of water over a burning candle.
Put aluminium foil on the table.
Put the tripod on the foil for safety.
Attach the balloon full of water so that the bottom of the balloon will just touch the candle flame.
A balloon filled with water will not pop like a similar balloon filled with air.
2. Fill a children's balloon with water, suspend a thermometer in the balloon and suspend the balloon over a burner.
Observer the increased temperature of the water without damage to the balloon.
3. Put 120 mL of water in a rubber balloon and tie the inlet.
Suspend the balloon over a lighted candle, but do not let the rubber touch the hot wick.
The balloon will explode after a few minutes.
Inflate another balloon with air and suspend it over a lighted candle.
This balloon explodes almost immediately.
Water at has a higher specific heat capacity, about 4.2 J.g-1 K-1, than air, slightly above 1.0 J.g-1 K-1, so it can absorb more heat than air for any degree rise in temperature.

23.7.2 Water is a poor conductor of heat, boil water in a paper cup
1. Use your bare fingers to hold the bottom of a test-tube containing water.
Tilt the test-tube over a flame so that you can heat the water in the upper part of the test-tube.
You can hold the bottom of the test-tube until the water in the upper part boils, because water is a poor conductor of heat.
2. Boil water in a paper cup.
Pour water in a paper cup and hold it over a flame.
The paper will not catch fire, because the water keeps the temperature of the paper cup lower than its ignition point.
3. Put small pieces of ice in the bottom of a test-tube containing water.
Heat the water near the top of the test-tube with a spirit burner.
The water will start to boil, yet the ice will not melt.
The warmed water is already at the top, so no convection takes places, and the conduction by water is very small.
Little heat transfers to the ice.
4. Put a small fish in a test-tube full of water.
Tilt the test-tube and heat the top 1 cm of water.
The water will boil and not harm the fish.
However, this experiment upsets some students.

23.7.3 Water is a poor conductor of heat, boil water in a test-tube
See diagram 23.1.8: Heat water in test-tubes.
1. Use your bare fingers to hold the bottom of a test-tube containing water.
Tilt the test-tube over a flame so that you can heat the water in the upper part of the test-tube.
You can hold the bottom of the test-tube until the water in the upper part boils, because water is a poor conductor of heat.
2. Boil water in the top of a test-tube while ice is held at the bottom.
Put small pieces of ice in the bottom of a test-tube containing water.
Heat the water near the top of the test-tube using a spirit burner.
The water will start to boil, yet the ice will not melt.
The warmed water is already at the top, so no convection takes places, and the conduction by water is very small.
Little heat transfers to the ice.
3. Put small pieces of ice in the bottom of a test-tube containing water.
Heat the water near the top of the test-tube with a spirit burner.
The water will start to boil, yet the ice will not melt.
The warmed water is already at the top, so no convection takes places, and the conduction by water is very small.
Little heat transfers to the ice.
4. Put a small fish in a test-tube full of water.
Tilt the test-tube and heat the top 1 cm of water.
The water will boil and not harm the fish.
However, this experiment upsets some students.

23.7.4 Water is a poor conductor of heat
Water is a poor conductor of heat, boil water in a paper cup, heat water in a paper bag
1. Boil water in a paper cup.
Pour some water in a paper cup and hold it over a flame.
The water boils without burning the paper cup.
As a control, heat a paper cup not containing water or use a propane torch to burn away the top part of the cup above the water level.
2. Boil water in a paper pan.
Draw two concentric squares on copy paper with sides 13 cm and 18 cm.
Cut out the outer square and fold along the edges of the 13 cm square to make right angle corners.
Staple the flap at each corner.
Heat a sheet of metal gauze over a stove or Bunsen burner on a tripod until the metal gauze is red hot.
Fill the paper pan one third full of water.
Lift each end of the paper pan and place it on the hot metal gauze.
Observe steam coming from the surface of the water before it boils.
Some teacher add a raw egg to the water in the pan.
Water absorbs the heat without the temperature being high enough to scorch the paper, because of its high heat capacity.
Also, when the water turns to steam latent heat of vaporization is absorbed by the water.
3. Pour some water in a paper bag.
Agitate the paper bag to coat the inside with water, then pour out the excess water.
Put a small soft chocolate, e.g. "Chocolate Kiss", in the paper bag.
Heat the paper bag over a 100 W light globe.
The chocolate melts, but the paper bag does not get hot.
The chocolate was melted by the radiant heat from the light globe.
4. Boil water in a paper cup.
Use several disposable paper cups or make a square paper cup as in the diagram.
Heat a paper cup with a spirit burner and the paper cup burns out instantly.
Put two metallic rods over an iron heating stand.
Put a paper cup containing water on the two metallic rods and heat with the spirit burner.
The water boils without the cup catching on fire.
You can try the experiment with a plastic cup, but different plastics have different melting temperatures.

23.7.5 Boil water in aluminium pot and stainless steel pot
Use similar sizes of aluminium pot and a stainless steel pot.
Add the same volume of water.
Heat the two pots simultaneously.
Note the time to boiling in each pot.
As stainless steel is a good conductor of heat, the temperature in the part of bottom and wall of pot not touching the flame is almost the same as the part that is directly heated.
This allows convection of heated water to occur in many small regions, so you can see steam bubbles in the whole surface of water, evenly distributed and similar in size.
In the aluminium pot, you see only a raised boiling liquid column in the centre of an area on the surface of water just above to the bottom of pot heated by flame.
In the aluminium pot, the small bottom of pot heated absorbs the heat of vaporization mainly and convection currents starting from this small area extend to all the liquid in the pot.
Before the violent boiling appears, the original vaporization happens in a circle that the surface touches with the wall of the stainless steel, so you can see many small steam bubbles.
This is because in such place the temperature is higher and the pressure inside the liquid is less.

23.7.6 Conduction of heat by a coin on paper
See diagram 23.1.7: Coin on paper.
Hold a piece of paper above a candle flame: it will char if brought near.
Put a metal coin or a key on the paper and hold it over the candle flame.
The metal conducts the heat away from the paper and leaves a pattern where the metal touches the paper.

23.7.7 Conduction of heat by different metals
See diagram 23.1.4: Conduction of heat by different metals.
See diagram 23.1.2: Heat different solids.
Feel heat energy 1. Use an iron rod, copper rod and glass rod that are the same length and diameter.
Hold one end of each rod with the other end over a Bunsen burner flame.
2. Repeat the experiment with rods of different diameters.
The bar that feels hot first shows the fastest rate of heat conduction.
3. Hold a wire coat hanger horizontally over a flame with your fingers, a small distance from directly above the flame.
Soon the wire becomes too hot to hold.
Move your fingers back, but keep the coat hanger in the same position.
Feel heat moving along the wire.

23.7.8 Conduction of heat by metal bars, melt paraffin wax
See diagram 23.7.2: Paraffin wax blobs on wire.
1. Use a bar of copper, brass or aluminium at least 30 cm long.
Place blobs of melted paraffin wax at 3 cm intervals.
While the paraffin blobs are still soft, push the pointed ends of nails or tacks into them.
Heat one end of the box with a flame.
Note the evidence that heat moves along the bar by conduction.
2. Use lengths of metal bars with the same lengths and diameters.
The metals should have big differences in heat conduction coefficient, e.g. lead, iron, aluminium and copper.
Remove the bottom of a metal can and cut out three legs.
Punch several holes in the wall of the can and insert the metal bars so that they are all in contact at the centre of the can.
Attach pins to the ends of the metal bars with paraffin wax.
Place a spirit burner below the apparatus to heat the bars evenly.
Observe the dropping of the pins.
3. Show the rates of heat conduction in different metals.
Clamp four different metals in the disc of the conductometer.
Place a small chip of paraffin on each free end of the metals.
Heat the conductivity disc and observe the paraffin chips melting and falling off in the order of decreasing conductivity of the metals.
4. The apparatus consists of four metal strips, aluminium, brass, copper and steel that meet together at a central point.
Place a small piece of paraffin wax in a depression in the outer end of each metal strip.
Direct a Bunsen burner flame at the junction of the metal strips and note the order of melting, which shows the relative thermal conductivity of the metals.
5. Prepare metal wires with the same diameter, e.g.copper, iron and aluminium.
Cut the wires the same length and twist them together, but keep one end open.
Put the open end of the wires into melted wax liquid, and take out to let the wax harden and form a wax drop at the end of the wires.
Heat the other end of the wires over a Bunsen burner.
Rotate the wires as you heat to heat each wire evenly.
Note which wax drop at the end of a wire melts first.
6. Use identical lengths of different metal bars, e.g. copper, brass, aluminium.
Try to use rods of the same diameter.
Put blobs of melted candle wax at intervals along the bars.
Push small nails or metal pieces into the wax while the wax blobs are still soft.
Heat one end of the bars.
The blobs of wax melt and the nails fall down as heat moves along the bars.
The metals do not conduct heat equally.
7. Waxed balls drop off different metal rods connected to a heat source as the heat is conducted along the metal rods.
Dip metal rods in wax then watch as the wax melts off.
8. Four rods, steel, brass, aluminium, copper, with liquid crystal thermometers are placed in hot water to show different conduction rates.

27.7.9 Conduction of heat by metals, Davy lamp
See diagram 23.119: Davy lamp
1. Davy lamp
Hold a sieve or a piece of metal gauze, e.g. 1 mm iron gauze or metal fly-wire screen, over the flame of a small candle. (Not fibreglass or plastic fly-wire screen!)
As you lower the wire gauze on the flame, the flame becomes smaller because the wire conducts the heat away from the flame so the temperature of the flame is lowered.
Also, as you lower the wire gauze on the flame, the flame does not go through the wire netting because heat is conducted away from the flame by the wires.
Sir Humphry Davy in 1816 used this observation to invent the miners' safety lamp that has metal gauze around the flame in the lamp to conduct away the heat so that the flame is not hot enough to ignite explosive gas in the coal mine.
2. Put a spirit burner under a tripod stand and cover the stand with 1 mm iron gauze.
Turn on the gas and ignite it above the metal gauze.
The gas burns only above the wire gauze screen, because the wire gauze conducts away the heat and prevents the gas below the gauze from reaching ignition temperature.
3. Every substance has own ignition temperature, i.e. the temperature to which you must heat it before it will burn in air.
Hold wire netting or a wire sieve above a lighted candle.
Move the wire netting downwards and observe any change of the candlelight.
The candle flame becomes dim because wire netting transfers the heat energy from the candle.
The candle flame not only becomes small but also is hindered crossing through the wire netting.
4. Be careful not to turn on the gas for too long time!
Place a Bunsen burner under a tripod covered with wire netting.
Turn on the gas then try to light the gas above and below the wire netting with a lighted match.
Only the gas above the wire netting can be lit because conduction of heat energy makes the gas under the wire netting unable to reach ignition temperature.
In a model Davy lamp, a candle enclosed in a cylinder of wire gauze does not light a jet of gas played on it from a rubber tube.
Use a block of wood or Plasticine (modelling clay) as a base.
Be Careful!
Do not leave the gas jet turned on for extended periods.
Disperse the released gas by ventilating the room.
Remember to turn off the gas!
5. Place a candle on a board and light the candle.
Place wire netting in the shape of a column above the candle.
Prepare a rubber tube to lead to a combustible gas.
Place the nozzle of the gas on top of the wire netting then turn on the gas so that the gas flows on the top of the wire netting.
The gas does not burn because the high conductivity of metal makes the temperature outside the wire netting not reach the ignition temperature of the gas.
6. A Bunsen burner will burn on top and bottom of two copper screens a few cm apart.
A Bunsen burner flame will not strike through to the other side of fine copper wire gauze.
7. Heat platinum wire in a flask until it glows dull red then evacuate the flask and the wire will glow more brightly at the same voltage.
8. Use your fingers hold a wire coat hanger horizontally over a flame a small distance from directly above the flame.
Soon the wirebecomes too hot to hold.
Move your fingers back but keep the coat hanger in the same position.
Feel heat moving along the wire.
9. Use identical lengths of different metal bars or rods with the same diameter, e.g. copper, brass, aluminium, iron.
Put blobs of melted candle wax at the same intervals along the bars.
Push small nails or metal pieces into the wax while the wax blobs are still soft.
Heat one end of each metal bar.
The blobs of wax melt and the nails fall down as heat moves along the bar.
The metals do not conduct heat equally.
10. Hold a piece of paper above a candle flame.
The paper chars.

23.7.10 Conduction of heat by wood, anisotropic conduction
Conductivity is greater along the grain in wood, so heat the centre of a thin board covered with a layer of paraffin and watch the melting pattern.

23.7.11 Conduction of heat in a metal bar
See diagram 23.7.2: Paraffin blobs on wire.
Use a bar of copper, brass or aluminium at least 30 cm long.
Place blobs of melted paraffin wax at 3 cm intervals.
While the paraffin blobs are still soft, push the pointed ends of nails or tacks into them.
Heat one end of the box with a flame.
Note the evidence that heat moves along the bar by conduction.

23.7.12 Cook an egg on a piece of paper
Cooking food keeps the temperature of the surface of the cooking vessel to the temperature of boiling water, 100 oC.
Use a small camping gas stove, an A4 sized piece of clean white paper, a little cooking oil, an old metal coat hanger, a few large paper clips, a metal spatula and an egg.
Make a square paper frying pan from a wire coat hanger with a paper dish fixed with paper clips.
Put drops of cooking oil on the paper to prevent the egg sticking to it.
Break an egg into the paper frying pan then hold it above a burner so that the paper above the flame is covered by egg.
The egg white and yolk contain water that turn into steam at 100 oC that remains at that temperature.
The paper may char around the edges of the egg.

23.7.13 Copper coil candle snuffer

23.7.13 Copper coil candle snuffer
See diagram 23.7.6: Copper coil candle snuffer.
Ignition temperature of a gas is the temperature at which total heat lost from conduction, convection and radiation is less than the heat produced by the combustion of the gas.
To show that metal is a good heat conductor of heat energy and that a certain temperature is a necessary conditions for burning, make a screw coil by rolling with thick copper wire or brass netting.
1. Light a small candle.
Hold the coil high above the candle flame and slowly move it down towards the flame.
Observe the change in candle light.
The candle light will reduce gradually then go out not, because of absence of oxygen, but because the wire transfers away the heat energy around the candle quickly the temperature around the candle is lower than the ignition temperature.
If the candle flame is too big, it may produce enough heat energy to compensate for the heat energy transferred by the wires so that the flame will not go out.
Note that the flame will not go out if you heat wire netting to a higher temperature so that its ability to transfer heat energy is lower.
2. Place a coil of heavy copper or aluminium wire over the flame of a candle.
The flame goes out.
You can snuff out a candle flame by depriving it of oxygen, but here the oxygen can easily get to the flame.
The fire goes out, because the coil of heavy wire conducts the heat away from the flame so fast that the temperature is lowered below the ignition temperature.
This shows that copper and aluminium are good conductors of heat.
If the flame is too large, it will produce heat energy too rapidly to be carried away by the coil.

23.7.14 Copper coil snuffer conducts heat
See diagram 23.7.6: Copper coil candle snuffer.
Place a coil of heavy copper or aluminium wire over the flame of a small size candle.
Why does the flame go out?
You can snuff out a candle flame by depriving it of oxygen, but here the oxygen can easily get to the flame.
The fire goes out, because the wire conducts the heat away from the flame so fast that the temperature is lowered below the kindling point.
This shows that copper and aluminium are good conductors of heat.
If the flame is too large, it will produce heat energy too rapidly to be carried away by the coil.
If the coil is already hot before the experiment, the temperature of the flame may not be lowered enough to put it out.

23.7.15 Hammer hardwood peg
The face of the hammer feels hot, but soon cools although the top of the hardwood peg still feels warm.
Some of the work done by the hitter is turned into heat energy, parts going to the wood and part to the iron.
The wood is a bad conductor of heat so that most of the heat is localized in the top of the peg, so the temperature of the peg rises.

23.7.16 Heat conductivity ring
Metal Strips: brass, aluminium, iron, copper
The heat conductivity ring is a metal ring with other identical metal strips attached over.
The strips have dimples punched in them to hold wax.
After heating the ring, the time taken to melt the wax is a measurement of comparative thermal conductivity of the metals.

23.7.17 Heat insulation, properties of common materials
See diagram 23.1.5 Four big and four small beakers
* Set up four big beakers and four small ones, as shown in diagram 23.1.5, above.
Pour the same amount of hot water into each small beaker, then put each small beaker containing hot water into each big one.
* Select three kinds of heat insulators such as pieces of polyester plastics, pieces of papers and pieces of wood.
Fill the space between a big beaker and small beaker with these materials.
Compare the degree of this heat insulation by measuring the drop in temperature of the water in small beakers at the same time.
* The fourth large beaker contains only air, and it is a control, against which you can compare the other beakers.
Controlling other variables to make a reliable comparison between them is necessary.
* The water must be the same temperature in each beaker, the quantities of the materials filled in each beaker must be identical, the original temperature of the large beaker should be the same.
* Put a thermometer in each beaker and cover with a piece of paper.
Record the temperature in each small beaker at one minute intervals.
Keep doing this at least 10 minutes.
You can judge which is the best heat insulating material according to these 10 data in each group.
Plot a graph of temperature against time.
Draw all three grapH on the one sheet of graph paper to see the conclusion clearly.

23.7.18 Heat paper without burning, coin on paper, hanging thread
Coin on paper conducts heat, paper that cannot be lighted
See diagram 23.1.7 Coin on paper, Hanging thread.
1. Place a coin on piece of paper and hold it high above a burning candle.
Lower the paper and coin towards the candle flame.
The paper in contact with the coin will not be burnt, because the metal in the coin conducts away the heat.
The paper not in contact with the coin will be burnt and leave a shape of the coin formed by the trace of burning.
Stretch the paper level to contact keep good contact between paper and coin.
Repeat the experiment with the same paper with no coin on it.
All the paper will be burnt.
2. Wrap soft thread around a long screw.
Leave a small length of thread hanging down.
Set light to the end of the thread hanging down.
The flame goes out at the place of contact with the screw, because metal in the screw conducts away heat so the thread cannot reach the temperature needed for burning (ignition temperature).
Repeat the experiment with a piece of wood roughly in the shape of the screw.
The thread burns completely, because wood cannot conduct heat away from the place of burning.

23.7.19 Melt ice blocks
Place blocks of ice as follows:
* in the sun and sheltered from the wind,
* in the sun and in the wind,
* sheltered from the sun and wind,
* in an ice chest or insulated portable cooler, e.g. "Esky".
List the blocks of ice in order of complete melting.
The ice will melt most rapidly in (2), then (1), then (3), then (4).
See below:
(1) The ice absorbs most heat directly from the sun by radiation and lesser heat from its surroundings by conduction and radiation, but mainly by convection currents in the air.
Some of the ice has melts to form a layer of water over the ice.
Water is a bad conductor of heat so the ice will melt more slowly if the layer of water remains.
When the air is still, a layer of cold air forms round the ice and reduces the amount of heat received from the air by convection.
(2) The layer of water over the ice evaporates more freely than in (a) so that the ice is dried and so melts more quickly.
Unlike (1), new air is continually coming into contact with the ice so the amount of heat received by convection is not reduced.
(3) No heat is received by radiation from the sun or by wind convection.
Some heat is received by conduction and radiation from the shaded surroundings and some very small convection currents.
(4) No loss of heat by convection except from within the container.
When the temperature of the interior of the container falls below the room temperature it receives heat from the surroundings by convection, conduction and radiation at a rate depending on the temperature difference.
The temperature of the container falls until the heat received by the container equals the heat used in melting the ice.
The temperature difference for the container and air is less than for the ice and air, and the container is made of a bad conductor of heat, so melting in (4) is the slowest.

23.7.20Reduce heat loss with insulation
Use four large tin cans of equal size and four smaller tin cans of equal size.
Inside the first large can put a small can on two corks in a large can.
This is the control.
Select types of insulating material, e.g. sawdust, cork pieces, newspaper, plastic.
Put a small can inside each large can.
Pack one type of insulating material under and around each of the smaller cans.
Put a cardboard cover on each large can.
Make a hole in each cover for a thermometer.
Fill each small can to the same depth with water that is nearly boiling.
Record the initial temperature of the water in each can.
Record the temperature of the water in each can at five minute intervals.
Draw cooling curve graphs by plotting temperature against time for each can.
Note which material is the best insulator.

23.7.21 Relative conductivity
See diagram 23.1.7: Relative conductivity.
* Put matches on hot plates of different metals over burners.
* Use match head ignition when heating bars of metals attached to a common copper block.
* Hold one end of stainless steel, iron and aluminium rods in a Bunsen burner flame.

23.7.22 Touch wood and iron in the sun
An object made of iron may feel colder than an object made of wood at the same temperature even if both objects have been in the sunlight and the iron object appears hotter.
Iron is a good conductor of heat, so when the finger touches the iron, heat is transferred from the body to the iron, and is distributed over the whole of the piece of iron, so there is only a very slight rise in the temperature of the iron, and it seems cold.
Wood is a bad conductor of heat so when it is touched heat is transferred from the body to the wood, but the heat is concentrated in the touched region.
The temperature in that touched region rises and the wood appears comparatively warm.
However, heat has not spread throughout the bad conduction object made of wood so the other fingers may feel the wood to be cooler than the iron.
If the iron and wood are placed in the sun so that the same amount of radiant energy falls on both of them, the iron may become hotter than the wood, because the specific heat of the iron is much less than that of the wood so it is a better absorber of heat.

23.8.01 Convection
See diagram 23.2.8: Smoke moves up and down.
Convection is movement of heat energy through a liquid or gas that involves the flow of the medium itself.
Convection is caused by the expansion of the medium as its temperature rises, the expanded material being less dense, and rises above colder and denser material.
* Smoke from a fire rises, because the air above the fire is heated, expands, and therefore becomes lighter than the surrounding air, and hence is pushed up, carrying with it the particles of carbon, which constitute the smoke.
* Ice wrapped in a blanket melts slowly, because the blanket is a bad conductor of heat so little heat is conducted to the ice from the surroundings.
Also, the blanket prevents the outside air from coming into contact with the ice so little heat is conveyed to the ice by convection.

23.8.1 Convection smoke box
See diagram 23.2.6: Convection smoke box.
1. To make a convection box, cut away one side of a box and replace it with glass.
Cut two holes 2 cm diameter and 10 cm apart in the top of the box.
Attach two tubes above the holes to be chimneys.
Put a candle in the box under one chimney.
Light the candle.
Hold the smoking paper above each chimney.
See the convection currents through the glass side of the box.
2. Another way of showing air currents is by making use of the difference in refractive indexes of warm and cold air.
A car headlight bulb without a reflector will cast shadows of convection current from an electric heater.
Look at an object on the other side of a hot engine or a hot road.
The object will appear distorted, because the refractive indexes of warm and cold air are different.
This is one cause of mirages in the desert.

23.8.2 Convection cells
1. Cumulus cloud
Damp ground heated by the sun warms the air above it that expands, becomes less dense and rises as a thermal, carrying water vapour with it to form cumulus cloud.
The photosphere (shining surface), of the Sun consists of regions of hot larva that rise to the surface, cool, and drop back into the interior again.
2. Hadley cells, (George Hadley, 1685-1768, England) Hadley cells in the atmosphere occur at low latitudes where air warms and rises near the equator and descend towards the poles at about 30 degrees latitude.
They cause the tropical trade winds in the tropics and control low-latitude weather patterns.
They are single wind systems in each hemisphere.
They flow towards the West and the equator near the surface, and flow towards the East and North or South poles at higher altitudes.
3. Water and oil
Put water coloured with a vegetable dye in a tall beaker.
Add vegetable oil and baby oil.
Put the beaker on a low heat source.
Note the time taken by globs of oil to reach the surface and return to the bottom of the beaker.
Not the time taken again after increasing the heat from the heat source.
4. Boiler
Heat water in a large boiler heated by a ring of gas jets.
Note the convection cells in the water above the gas jets.
5. Lava lamps
A lava lamp contains coloured water, phenylamine or oil or wax slightly less dense than water and an incandescent light bulb surrounded by the oil at the bottom.
When the switch is turned on, the incandescent lamp becomes hot, heats the oil at the bottom and it becomes less dense and rises through the coloured water.
Near the top of the lamp, the rising oil cools, becomes more dense and sinks down towards the incandescent bulb.
So what you see is a convection current of oil in water.
6. Miso soup
Observe convection cells in heated soup containing small particles, especially the Japanese miso soup.

23.8.3 Convection currents and ventilation
See diagram 23.128: Convection current ventilation.
1. Use a box with grooves for a lid and cut a glass window that slides in the grooves to make an airtight fit.
Bore four holes in each end.
Each end represents a window.
The top holes of each side are the top halves of each window.
Put four candles in the box, light them and close the sliding glass.
To study the best conditions for ventilation, put solid corks in the openings, close completely both windows, and note the candles.
Try the following different combinations of opening:
* One window open at the top and bottom, i.e. all four holes in one side open,
* One window open at the top and the other at the bottom,
* Both windows open at the top, one window open at the bottom,
* Both windows open at the bottom, one window open at the top.
Find which window openings provide the best ventilation.
2. To study the expansion of freezing water, use two identical drinking cups.
Fill the first cup with tap water at room temperature so that the water heaps up to form a meniscus.
Put the second cup in the freezing compartment of the refrigerator then add extra water to the cup to get the highest possible meniscus.
When the water in the cup is frozen, compare the meniscus of the frozen water with the meniscus at room temperature.
The frozen water heaped up, because it had expanded.
Water has a maximum density at 4 oC.
When water cools from room temperature to 4 oC, it contracts in volume.
When water cools from 4 oC to 0 oC, it expands in volume.
At 4 oC the density of water is 1000 kg m-3 (1 g per cc).
At 0 oC the density of water is 999.87 kg m-3 and the density of ice is 918 kg m-3, so ice floats on water.
3. Fill a large jar with cold water and weigh it accurately on a balance.
Empty the jar.
Fill the jar with exactly the same volume of hot water and weigh.
You will observe that the jar of warm water weighs less.
Volume for volume, cold water is heavier than warm water; so when water is heated convection current are set up, the warm water being lifted, because of buoyancy, by the cold surrounding water.
Hot water is less dense than cold water, and this is the cause of convection current in a liquid.
4. To study the expansion of freezing water, use two identical drinking cups.
Fill the first cup with tap water at room temperature so that the water heaps up to form a meniscus.
Put the second cup in the freezing compartment of the refrigerator then add extra water to the cup to get the highest possible meniscus.
When the water in the cup is frozen, compare the meniscus of the frozen water with the meniscus at room temperature.
The frozen water heaped up, because it had expanded.
Water has a maximum density at 4 oC.
When water cools from room temperature to 4 oC, it contracts in volume.
When water cools from 4 oC to 0 oC, it expands in volume.
At 4 oC the density of water is 1 000 kg m-3 (1 g per cc).
At 0 oC the density of water is 999.87 kg m-3 and the density of ice is 918 kg m-3, so ice floats on water.

23.8.4 Convection currents between jars of water
See diagram 23.2.2: Convection currents between jars of water.
Use four similar wide mouth jars with screw-on lids.
Fill jar 1 with tap water and jar 2 with hot water, 90 oC.
Add the same number of drops of red ink to each jar.
Close the jars and turn them upside down repeatedly to make the red colour even.
Stand the jars on the bench.
Fill jar 3 with tap water and jar 4 with hot water, 90 oC.
Cover the mouths of jar 3 and jar 4 with a card.
With your first two fingers pressing on the card, turn each bottle upside down to be ready to place them over the jars on the bench.
Put jar 3 over jar 2 and put jar 4 over jar 1.
Remove the cards between the jars and observe any change in colour of the water.
The less dense hot coloured water in jar 2 mixes with the more dense cold water in jar 3.
The more dense cold coloured water in jar 1 does not mix with the less dense hot water in jar 4.

23.8.5 Convection currents from an ink bottle
See diagram 23.126: Convection currents from an ink bottle.
1. Use a small ink bottle, fitted with a two-holes stopper.
Cut two pieces of glass tubing.
One piece should extend from the stopper almost to the bottom of the bottle.
The other piece should extend 5 cm up from the stopper.
Fill a large container with cold water.
Fill the small bottle with hot coloured water.
Put the small bottle in the bottom of the large container while holding the fingers over the ends of the tubing.
The hot coloured water rises in the large container as the cold water enters the bottle.
2. Repeat the experiment with one piece of tubing should be drawn out to a jet like the end of a medicine dropper.
This tube should be put just through the cork and should extend about 5 cm above it.
The other tube should be just level with the top of the cork and extend nearly to the bottom of the bottle.
Fill the bottle with very hot water that has been coloured deeply with ink.
Fill a very large glass jar with very cold water.
Rinse off the ink bottle and quickly place it on the bottom of the large jar.

23.8.6 Convection currents in a container
1. Weigh an empty container.
Fill a container exactly with cold water and weigh it again.
Empty the container and fill it exactly again with the same volume of hotwater and weigh it again.
The same volume of hot water weighs less than cold water.
When you heat water the lighter warm water nearer the source of heat displaces the heavier cold water and convection currents occur.
Hot water is less dense than cold water.
This is the cause of convection currents.
2. Fill two identical containers with water near 100 oC and near 0 oC.
Drop 5 drops of food colouring into the water in different places in the containers.
Observe the spread of the food colouring.
The food colouring mixes more quickly with the hot water, because its molecules are moving faster around each other.
In the cold water, the food colouring may just sink to the bottom to displace water by its own weight.

23.8.7 Convection currents in a test-tube
See diagram 23.1.8: Convection currents in a test-tube
Fill a test-tube with cold water.
When the water is still, add a very small crystal of potassium manganate (VII) and let it fall to the bottom leaving little colour trace.
Hold the test-tube in the bare fingers near the top, but not above water level.
Heat with a very small burner or candle flame at the bottom of the tube.
You can hold the warm test-tube with bare fingers.
Note the movement of the coloured dye from the crystal in the convection current.
Repeat the experiment, but heat very gently near the top of the water surface, while holding the test-tube near the bottom.

23.8.8 Convection currents in water
See diagram 23.2.1a: Convection currents in water.
1. Use two small plastic bottles.
Fill one bottle with cold coloured water and fill the other bottle with hot coloured water.
Completely cover the mouths of the bottles with plastic film or cling film, then fasten the plastic film under the mouths of the bottles with elastic.
Stand upright each plastic bottle in a large beaker.
Put tap water in each beaker to cover the plastic bottles standing in them completely.
Use a long straight wire or spike to make a hole in each film covering the mouths of the plastic bottles.
Observe the movement of coloured water in each beaker.
2. Use two 200 mL beakers.
Place one beaker on each pan of an adjusted beam balance.
Readjust the balance accurately to balance the two empty beakers.
Put 200 mL of tap water in one beaker and put 200 mL of hot water at 90 oC in the other beaker.
Observe whether the beakers still balance.
3. Use a large beaker full of tap water on a tripod stand.
Drop a few large crystals of potassium permanganate, potassium manganate (VII), from above the centre of the beaker.
Heat the beaker with a spirit burner placed under the centre of the beaker.
Observe the movement of the purple water.
4. Fill a big pot with icy water.
Put a few heavy objects in a small jar, e.g. glass marbles, lead sinkers, steel washers.
Pour hot water and a dye, e.g. black ink, into the small jar.
Drop the small jar into the big pot of cold water.
Observe the "undersea volcano" when the warm water from the small jar mixes with the hot water in the big pot to form convection currents.

23.8.9 Convection smoke box with two chimneys
See diagram 23.2.6: Convection smoke box
` 1. Use an open box and cut a pane of glass so that it just covers the opening of the box to make a window.
Cut two holes in the roof of the box.
Place two lamp chimneys or plastic tubes over the holes.
Place a short piece of candle on the floor of the box under one chimney.
Light the candle.
This represents a land area that the sun has heated.
Close the window.
Trace the air current in each chimney with a smoking piece of piece of paper.
Observe the movement of smoke inside the box.
Move the candle so that it is under the other chimney and repeat the experiment.
The smoke moves, because of convection currents.
2. The convection apparatus consists of a wooden box with clear plastic sides and two chimneys.
Place a lighted candle beneath one of the chimneys.
Hold an incense stick or smoke source over the other chimney.
The convection current is clearly visible.
3. A candle burns under one chimney in a two chimney convection box the use smoke to show convection in the two chimneys.
Use a box with a lid and glass wall.
Make two holes in the lid of the box to allow you to insert two cardboard cylinders A and B for chimneys.
Cut two thin pieces of thin paper and paste one piece on the top edge of cylinder B and the other piece on the lower edge of cylinder B.
Let both pieces of paper hang down.
Place a small birthday cake candle directly under the chimney A inside the box.
Light the candle and close the lid of the box.
Observe the direction of the moving pieces of paper on chimney B to show the direction of the flowing air inside the box.
4. Light a wad of newspaper then stamp out the flame to make smoke.
Hold the smoking newspaper above the chimney A then above chimney B and observe the movement of smoke.
The smoke moves up from over chimney A and down from over chimney B.

23.8.10 Convection tube
See diagram 23.29: Convection tube.
Fill a square tube with water.
Place a lighted Bunsen burner under one side.
Use a dropper to drop ink into the top hole.
The ink moves in the direction of the water flow.
Move the Bunsen burner to the other side to reverse the water flow.

23.8.11 Feel convection currents in a test-tube
See diagram 4.24: Feel a heated test-tube.
1. Fill a test-tube with cold water.
When the water is still, add a very small crystal of potassium manganate (VII) and let it fall to the bottom leaving little colour trace.
Hold the test-tube in the bare fingers near the top, but not above water level.
Heat the test-tube with a very small burner or candle flame at the bottom of the test-tube.
Hold the warm test-tube with bare fingers.
Note the movement of the coloured dye from the crystal in the convection current.
2. Empty, cool, and wash the above test-tube.
Fill the test-tube with cold water.
When the water is still, add a very small crystal of potassium manganate (VII) and let it fall to the bottom leaving little colour trace.
Hold the test-tube in the bare fingers near the bottom.
Heat the test-tube with a very small burner or candle flame at the bottom of the test-tube, below the water surface.
Continue heating for as long as the test-tube can be held, but do not hold the test-tube when it feels too hot for comfort.

23.8.12 Trace convection currents
See diagram 37.120: Convection currents.
1. Shield a burning candle to protect it from stray air currents.
Trace the air currents about it with smouldering paper.
Open a door a little way between a warm and a cool room.
With a piece of smouldering paper explore the air currents about the opening at various levels above the floor.
2. Explore the air currents in a room heated with a radiator or a stove.
3. Explore the air currents in a room ventilated with windows open at the top and the bottom.
4. Use an attached wire to lower a lighted candle into a milk bottle.
Note what happens to the candle.
Ventilate the bottle with fresh air.
Again place the lighted candle in the bottle, but this time separate the warm and cold air currents with a piece of cardboard cut in the shape of the letter "T ".
Use smoke from burning newspaper to see the air currents each side of the piece of cardboard.
5. Cut out the top of a drink-can to make a metal disc.
Punch a depression in the centre.
Cut along radial lines almost to the centre to make blades.
Twist all the blades in the same direction to make a wheel.
Mount the wheel on a pointed wire and hold it over a lighted candle to make the wheel turn.
6. Make a more sensitive wheel from the metal foil top of a milk bottle.
Place the top on a piece of absorbent paper with the flat side down.
Press the point of a ball pen into the middle to make a dent.
Cut "petals " in the turned-up edge to form the vanes of a turbine.
Pivot it on a pointed wire or a needle stuck into a cork.

23.9.0 Spirit burner
Spirit burner, alcohol lamp, methylated spirit burner, alcohol burner, alcohol lamp
See diagram 3.2.0.1: Alcohol lamp, spirit burner.
1. Use a small jar with a screw-on metal lid.
Invert the metal lid on a block of wood and use a large nail to punch a hole in the centre.
Note that when you replace the lid on the jar, the projecting metal around the hole will point upward.
Make a wick by tearing away a strip from old cloth.
Use a pencil to push the end of the wick through the hole in the lid, starting from the smooth side of the hole.
Half fill the jar with methylated spirits.
Screw the lid tightly on the jar and light the wick.
The metal lid will conduct heat away from the flame so that the flame does not go below the lid into the methylated spirits.
2. Use a small bottle with a screw metal cap as a simple spirit burner, (an alcohol lamp).
Punch a hole in the centre of the metal cap.
Enlarge the hole so that a metal tube 4 cm long fits into the hole.
Push the tube 1 cm into the bottle.
Make a wick from cotton waste or a cotton bath towel.
Put the wick in the bottle and pull it up through the tube.
Fill the bottle with methylated spirit.
Make a simple tripod stand with tin snips to cut away the sides of a tin can.
The wick should protrude about 3 cm from the cap and fit tightly into the wick holder.
The wick holder should fit tightly into the burner.
Use only methylated spirit or absolute alcohol (ethanol), as the fuel in the spirit burner.
3. Place the spirit burner on a metal tray or where it cannot be knocked over, i.e. not within "elbow radius" of the user.
Keep the container of methylated spirit stored in another room.
To fill the spirit burner, remove the screw cap containing the wick and use a filter funnel to three quarters fill the glass reservoir.
Replace the screw cap, screw it down tightly, and wipe the spirit burner dry of methylated spirit.
Wash the filter funnel.
4. To extinguish the flame, place a dry test-tube over it so that the rim of the test-tube touches the cap of the spirit burner, or use the glass / ceramic cap.
The spirit burner flame is almost invisible so be sure that the flame is really extinguished before handling or moving the spirit burner.
5. Students should not be allowed to lift the spirit burner or remove it from the bench.
However, they may move the spirit burner by sliding to move it to a safer or more convenient position.
6. Make an alcohol lamp, spirit lamp, from an ink bottle.
Use an ink bottle with a screw-on metallic cap, a metallic sheet of 2.5 cm × 4 cm, alcohol, and a wick made up of wasted cotton or cotton bath towel of length twice the height of the ink bottle.
Drill a hole with a nail in the centre of the cap of the bottle.
Use a file to enlarge the hole to diameter 10 mm and use a hard round object, e.g. a round file, to burnish the hole.
Roll the small metallic sheet into a cylinder.
The outer diameter of the cylinder is equal to the inner diameter of the hole on the cap of the bottle.
Push the cylinder about 1 cm into the hole on the cap.
If possible, solder the cylinder on the cap, and solder the cracks between the cylinder and the cap.
Insert the wick into the cylinder on the cap and leave a part of itslength outside of the cap and trim that part.
Fill fuel into the bottle, but not full.
Screw the cap on the bottle tightly to prevent evaporation.

23.9.1 Absorption of radiation
* Expose the lettered side of a white card with letters in India ink (China ink) to a hot source charring it where the letters are.
* Put a radiant heater midway between two junctions of a demonstration thermocouple and cover the junctions with black or white caps.

23.9.2 Black and white surfaces affect radiation
See diagram 23.8.7: Shiny cans and black cans.
1. To compare the influences of surfaces with different quality and colour in emitting and absorbing thermal radiation use three empty flat cans.
Remove their caps and clean them then dry them.
Paint their insides with white lacquer and outsides with black lacquer and white quicklime solution uniformly.
Choosing bright lacquers to paint them is better and do not paint the second layer of lacquer until the first one is fully dry.
Use a piece of white foam board for packing instruments.
Make three caps and pads for the three cans with the board.
Insert a thermometer into each cap.
Fill the 3 tin cans with the same volume of cold water.
Cover the cap on each can and put the pad under each can.
Place each can with the cap and the pad in the sunlight far from each another.
Record the original temperatures.
Then record their temperatures every 5 to 15 minutes.
Draw a temperature time curve with the 5 groups of data recorded.
The ideal distance between the bulb and cans is where your hand feels the heat from the bulb.
Repeat the experiment with hot water more than 80^oC and in a cool room.
Record the temperatures.
2. Use three same size metal drink-cans.
Paint the first can white.
Paint the second can black.
Let the third metal drink-can remain shiny.
Fill the cans to the same level with warm water at the same temperature.
Record the initial temperature.
Put cardboard covers with holes for thermometers over each can.
Put cans in a cool place.
Record the temperature of the water in each can at five minute intervals.
Describe the difference in the rate of cooling.
The second can cools fastest, because a black surface is the best radiator of heat.
3. Use a metal with width greater than a 100 W incandescent light bulb.
Paint one half of the inside of the can black.
Leave the other half shiny.
Put two blobs of petroleum jelly on opposite sides of the outside of the can.
Put one opposite the middle of the black interior and the other opposite the middle of the shiny interior.
Stick two coins on the blobs of petroleum jelly.
Fix a 100 watt light globe over the middle of the can and turn on the light.
The coin nearest the black interior falls first, because its blob of petroleum jelly melts first.
4. Wear a pair of old shoes.
Paint the left shoe black and the right shoe white.
Your left foot becomes your hot foot.
5. Wear a dark coloured sleeve and a light coloured sleeve of the same weave.
Your arm in the dark-coloured sleeve feels warmer.
If one sleeve is made of loosely woven material. it has many air spaces compared with a tightly woven material. and air is a heat insulator.
See diagram: Bichsel boxes.
A "black body" is a hypothetical perfect absorber and radiator of electromagnetic radiation.
Holes in black boxes are blacker than the boxes themselves, even though one box is painted white on the inside.
A box with a black interior and another box with a white interior show that the radiation coming from a cavity is determined by the temperature inside the cavity.
The two holes appear equally dark. although the inside of one box is painted white and the other is painted black.
So the radiation emerging from the holes is a function only of temperature.
Bichsel boxes demonstrates Wien's law of radiation:
The density energy of radiation Uv. which corresponds to the frequency of radiation v. depends on v and the absolute temperature T.
Bichsel boxes
1. A light tight box has two small identical holes in one face.
One of the holes is backed by a black piece of paper.
The other hole has no backing and appears much darker.
Open the box to show that the inside is painted white.
An ideal black surface absorbs. rather than reflects. all radiation that hits it.
If the hole in the box is small enough. most radiation that hits the hole enters the box. bounces around the box. most getting absorbed on each bounce, and never leaves the box through the hole.
So the hole is perfectly black.
The small hole also emits radiation. with a spectral distribution of frequencies that depends only on the temperature of the walls of the box.
The description of this radiation led directly to the discovery of quantum theory.
2. Two identical boxes are painted black on the outside, can be opened or closed, and have an identical small hole in the middle of the wall.
One box is painted black inside and the other box is painted white inside.
For both boxes, the holes appear blacker than the outside wall of the boxes.
This observation demonstrates that the radiation coming from a cavity is determined by the temperature inside the cavity.
3. A light tight oven with a tiny hole in the side. emits "cavity radiation" through the hole.

23.9.3 Black body radiation
Bichsel boxes
Use two identical teapots.
Fit a woollen tea cosy to one teapot.
Put the same volume of hot water and tea leaves in each teapot.
Put a thermometer in each tea pot and compare the loss of heat due to radiation.
Stefan-Boltzmann law. Wien's law. KircHoff's law
The Stefan-Boltzmann law states that the total energy radiated from a black body is proportional to the fourth power of the temperature of the body.
(Joseph Stefan 1835 - 1893. Ludwig Boltzmann 1844 -1906)
Heat can be transferred by wave motion. even across a vacuum.
This is called radiation.
Heat travels by radiation almost instantaneously.
A "black body" is an imaginary body that absorbs all the thermal radiation onto it and is a perfect emitter of thermal radiation as a continuous spectrum.
So the radiation includes all the wavelengths of electromagnetic radiation.
The intensity of the radiation is greatest at a wavelength that depends only on the temperature of the body.
Energy = σ × T⁴. ( σ (sigma) = 5.67 × 10^-8 Joules second^-1 metres Kelvin^-4. and T = absolute temperature.
Total energy radiated = E × surface area × time.
A full radiator would absorb all the radiant energy falling on it.
Lampblack is close to being a full radiator.
Wien's law (Wilhelm Wien. Germany 1864-1928) states the product of the absolute temperature of a body and the wave length of maximum radiation is a constant.
T × absolute temperature = constant.
The wavelength at which the maximum energy is radiated from a source is inversely proportional to the absolute temperature of the source.
So as temperature rises. maximum radiation decreases.
Hotter objects emit most of their radiation at shorter wavelengths and appear to be blue-white. white hot.
Cooler objects emit most of their radiation at longer wavelengths and appear red, because of infrared radiation, i.e. red hot.
KircHoff's law of radiation (Gustav KircHoff 1824-1887) refers to the observation that black clothes are good absorbers of heat and good emitters of heat.
However. on a hot day. the body wearing black clothes receives more heat than it can emit. so white clothes are preferred, because they are good reflectors of heat and poor absorbers of heat.
1. Hold your hand under an unlighted electric bulb. the palm upward.
Turn on the electricity.
Feel the heat almost as soon as you turn on the bulb.
The heat could not have reached your hand so quickly by conduction, because air is a very poor conductor of heat.
The heat could not reach your hand by convection, because convection carries the heat upward and away from your hand.
The heat came to your hand carried by short electromagnetic waves of wavelengths longer than light.

23.9.4 Colour temperature
Colour temperature is a measurement in degrees Kelvin that indicates the hue of a light source.
The colour temperature of a light source is the temperature of an ideal black body radiator that radiates light of comparable hue to the light source.
Colour temperatures over 5,000 K are called cool colours (bluish white) and lower colour temperatures (2000 K) are called warm colours, (yellow-white to red).
Within visible light, the shorter the wavelength the higher the colour temperature.
The hottest stars radiate blue (cool) light, and the coolest radiate red (warm) light.
Approximate ranges of colour temperature:
Infrared, candle 1800 K. tungsten incandescent bulb 2800 K. tungsten-halogen bulb (quartz bulb) 3200 K.
fluorescent lamp 4500 K. noon sunlight 5800 K. clear sky light 27,000-30,000 K, Ultraviolet
Trade descriptions used by the "Light Bulbs Direct" company:
Colour Temperature. "Designation". Application
2700 "Extra Warm White": Similar light to normal incandescent bulbs, giving a warm cosy feel.
3000 "Warm White": The colour of most halogen lamps.
Appears slightly "whiter" than ordinary incandescent lamps.
3500 "White": The standard colour for many fluorescent and compact fluorescent tubes.
4000 "Cool White": Gives a more clinical or high tech feel.
6000 "Daylight": Fluorescent or compact fluorescent lamps simulating natural daylight.
6500 "Cool Daylight": Extremely white light used in specialist daylight lamps.

23.9.5 Different surfaces affect heat radiation and absorption
See diagram 4.36: Heat radiation and absorption.
1. Use three same size metal drink-cans.
Paint the first can white.
Paint the second can black.
Let the third metal drink-can remain shiny.
Fill the cans to the same level with warm water at the same temperature.
Record the initial temperature.
Put cardboard covers with holes for thermometers over each can.
Put cans in a cool place.
Record the temperature of the water in each can at five minute intervals.
Describe the difference in the rate of cooling.
The second can cools fastest, because a black surface is the best radiator of heat.
2. Fill the same metal drink-cans with very cold water and record the initial temperature.
Put cardboard covers with holes for thermometers over each can.
Put the cans in a warm place in the sun.
Record the temperature of the water at five minute intervals.
The black metal drink-can is the best absorber of heat.
3. Use a pair of old shoes.
Paint the left shoe black and the right shoe white.
Your left foot becomes your hot foot.

23.9.6 Feel radiation
Feel radiation through glass
1. Stand near an open window to feel the radiation from the sun onyour cheek.
Close the window.
You can still feel the radiation from the sun on your cheek.
2. Hold your cheek 25 cm from a hole in a wooden sheet placed in front of a heating element.
Feel the radiation on your cheek.
Put a piece of glass between your cheek and the hole.
Feel the radiation on your cheek.
Repeat the experiment using more sheets of glass.
Feel radiation with your hand and cheek
1. Hold your hand under an unlighted electric light bulb. the palm upward.
Turn on the electricity and feel the heat from the light bulb.
The heat could not reach your hand so quickly by conduction, because air is a very poor conductor of heat.
The heat could not reach your hand so quickly by convection, because convection carries the heat upward and away from your hand.
The heat came to your hand carried by short electromagnetic waves of wavelengths longer than light.
Radiation carries heat in every direction from the source.
2. Stand near an open window to feel the radiation from the sun on your cheek.
Close the window.
You can still feel the radiation from the sun on your cheek.
3. Hold your cheek 25 cm from a hole in a wooden sheet placed in front of a heating element.
Feel the radiation on your cheek.
Put a piece of glass between your cheek and the hole.
Feel the radiation on your cheek.
Repeat the experiment by using more sheets of glass.
4. Hold the palm of your hand very close to., but not touching. your cheek.
Feel the radiation from your hand.
Heat travels by radiation almost instantaneously.
5. Place a piece of glass between an incandescent light bulb and your hand to block any movement of air.
Feel the radiated heat.

23.9.7 Focus radiant heat waves
See diagram 23.3.7: Focus sun's rays.
1. Use a magnifying glass to focus the rays of the sun on a piece of paper tissue.
Observe that the tissue paper chars. then catches fire from the focussed heat rays.
2. Repeat the experiment with tissue paper blackened with Indian ink or soot.
The blackened tissue paper catches fire sooner than the white paper in the previous experiment.
3. Focus the sun's rays on your arm.
A bright spot forms and you can feel the hot spot.
Note the distance of the lens from your arm when the light spot is smallest and brightest.
This distance is the focal length of the lens.
Notice the distance of the lens from your arm when the spot feels hottest.
The two distances are different.

23.9.8 Heat radiation decreases with distance
Radiation shadow. radiation to and from the earth. clear cold night
See diagram 23.3.3: Radiation and distance.
Put 4 thermometers at two different distances as two groups of two with both thermometers in the same group at the same level distance from a heat source, but at different heights.
Group 1. (distance 1. from heat source) t1 upper, t1 lower
Group 2. (distance 2. from heat source) t2 upper, t2 lower
Different groups are at different level distances from the heat source., e.g. electric household radiator.
Measure the distances from the heat source.
Turn on the heat source.
Record the reading on the thermometer at each position when the reading stabilizes.
The intensity of thermal radiation from the heat source is dependent on the distance and independent on the direction.

23.9.10 Holes in carbon blocks
1. A carbon block with a hole bored in it is heated red hot with a torch.
The hole glows brighter.
Bore a hole in an old carbon arc rod and heat electrically.
The hole glows brighter.
2. Two holes are drilled in a carbon block.
One is filled with a porcelain insulator and the block is heated red hot with a torch.
Graphite and porcelain heated red hot look the same.
A pattern on a porcelain dish shows brighter when heated.

23.9.11 Leslie's cube
A Leslie's cube. invented by John Leslie. (1766 – 1832), Scotland. shows that surfaces at the same temperature do not radiate equally.
The cube has three different surface areas: black, white, and two are smooth brass. or one grey and one silvered.
The original was said to have side made of gold. silver. copper and white varnish.
1. Fill a Leslie's cube with water and heat with a Bunsen burner.
Compare the heat radiation from the surfaces with a thermopile or thermometer or just use your hand to feel the difference.
The heat energy radiated from the surfaces is at the same temperature,, but different surfaces emit different amounts of heat.
The black surface radiates the most energy. then the white surface, then the brass surface. or grey surface then silvered surface.
Move the thermopile to show the inverse square law. the magnitude of the quality is proportional to the reciprocal of the distance from the source.
2. Put a Leslie cube with opposite faces blackened between two bulbs of a differential thermoscope.
3. A brass container coated on half of one surface with a matte black paint and highly polished on the other.
Hot water is placed inside the cube and each surface presented in turn, at the same distance to a thermopile.
4. PASCO'S Apparatus consists of a cube internally heated by a temperature controlled light globe with black. white. shiny and matte surfaces.
5. Place a Leslie's cube on a turntable so that alternate faces emit radiation to a sensor to show that the radiant heat is greater from the black surface.

23.9.12 Light a match with reflectors
See diagram 28.3.6: Radiation.
Two reflectors are set at opposite ends of the lecture bench.
One contains a heater controlled by a variac.
The other has a match at the focal point of the reflector.
Turn the variac all the way up and wait.
The match will light in about 1 minute.
If it takes longer. something is wrong.
Alignment is critical!

23.9.13 Plate in a furnace
A white plate with a black pattern on it appears as a dark plate with a grey pattern on it when placed in a furnace.
Good reflectors are bad absorbers and bad radiators.
The white plate is a good reflector. therefore when heated in the furnace it is a poor radiator compared with its surroundings and so appears dark.
The black pattern will. as it is a bad reflector. be a good radiator compared with the plate at the same temperature. so appears relatively white.
But being a glazed black, it is probably a better reflector. and hence a slightly worse radiator. than all other black portions of the furnace.

23.9.14 Radiant heat passes through glass
1. Hold your cheek about 25 cm away from the hole in a plastic sheet fixed in front of a heating element or the sun's rays.
The hole should be level with the glowing part of the heating element.
Insert a glass plate between your cheek and the hole.
Take it out and put it back. noting what you feel.
Repeat the experiment using two sheets of glass plate held together.
2. On a sunny day. feel the warmth from the sun through a clear glass window.
However. if you light a fire and place a sheet of glass between you and the fire. you cannot feel the warmth of the fire.
The energy distribution curve for the sun has its maximum in the visible region. and the sun emits also considerable energy in the infrared and in
the region of short heat waves.
This energy passes through glass with little absorption.
The energy radiated by the fire mainly consists of heat rays of long wavelengths. and these wave lengths are almost completely absorbed by the glass.

23.9.15 Radiant heat using parabolic reflectors and a thermopile
1. Use a heat source at the focal point of one concave reflector to direct heat at a thermopile mounted at the focus of a second concave reflector
2. A thermopile mounted at the focus of a parabolic mirror detects radiation differences from different coloured beakers of water.
3. Show transmission of radiant heat with a match at the focus of one parabolic reflector lit by a heating element placed at the focus of another reflector.
Use two parabolic mirrors to transmit radiation to light matches.

23.9.16 Radiation from shiny surface and black surface
See diagram 23.36: Dull and bright cans.
See diagram 23.8.7: Shiny can and black can.
1. Paint one side of a copper sheet black, so that it has a dull black surface, and polish the other side until it is bright and shiny.
Attach the copper sheet horizontally to a retort stand.
Heat the copper sheet with Bunsen burners until it is very hot.
Remove the Bunsen burners and turn the copper sheet vertical.
Hold the back of the hand near the bright and shiny side of the copper sheet, then near the dull black side of the copper sheet.
Be careful! The back of the hand must not touch the hot copper sheet!
You can bring the back of the hand much closer to the shiny side than to the black side.
The two sides have the same temperature,, but more radiation comes from the black side of the copper sheet.
2. Hold a sheet of paper near a stove heating element painted half white and half black.
Paper held close to a stove element is not scorched where the element is painted white.

23.9.17 Reflection of radiant heat waves
Heat tissue paper with a magnifying glass.
Note the distance from the reading glass to the tissue paper.
Put a tilted mirror half way between the lens and the paper.
Feel with your hand above the mirror until you find the point where the heat waves are focussed.
Hold a piece of paper tissue at this point.
The paper ignites.

23.9.18 Surface colour and the heat absorbed
See diagram 23.8.8: Surface colour and the heat absorbed.
Cut two vertical slits opposite each other on the side of a cylindrical tin can. so that the surface of the tin can is divided into two parts.
Blacken inside one half with ink or "dead black" paint. or paste apiece of black paper.
Leave the other half shiny.
Put a lighted candle inside the tin can at the centre.
The surface of the two parts of the tin can will have different temperatures.
Test by touching them with hands.
Fix matchsticks with wax on the outer surface of the tin can so that the matchstick on the half that has a black surface inside the tin falls first.

23.9.19 Surface radiation from an engine
A paper-covered tin can cools faster than a shiny can.
In the radiator of a water-cooled engine, the water, heated by passing round the engine, passes through a hollow metal tubes.
They conduct the heat through them to be taken up by the surrounding air, which is usually forced through the metal tubes by a fan.
The radiator should be black to radiate best.
The water is either pumped through the metal reticulation. or passes round it by convection currents.
In a motor cycle. heat flows out through the metal cylinder to the metal flanges. whence it is radiated. or passed on to the surrounding air currents.
The flanges ensure a big radiating surface. and big area of contact with the cooler air.
They should be kept black.

23.9.20 Teapot Use two identical teapots.
Fit a woollen tea cosy to one teapot.
Put the same volume of hot water and tea leaves in each teapot.
Put a thermometer in each teapot and compare the loss of heat due to radiation.

23.9.21 Thermopile
Electromotive force, emf
A thermopile is a set of joined thermocouples.
A thermopile is made of thermocouple junction pairs connected electrically in series.
The absorption of thermal radiation by one of the thermocouple junctions.
called the active junction. increases its temperature.
Differential temperature between an active junction and a reference junction at fixed temperature produces an emf proportional to the differential temperature.
This effect is called a thermoelectric effect.
For more sensitivity. thermocouples are joined in series to make a thermopile.
Thermopiles do not respond to absolute temperature., but generate an output voltage proportional to a local temperature difference or temperature gradient.
A thermopile consists of four iron and four copper wires twisted to form seven junctions to produce a thermoelectric emf.
The end terminals connect to a galvanometer.
Use a hand held hair drier is a suitable source of heat to activate the thermopile.

23.9.22 Thermoscope to compare absorption of radiation
See diagram 23.116: Simple thermoscope
Practise with different materials before doing this experiment, because for most cloths the absorption of infrared is almost independent of colour.
The amount of surface area pointing towards the source is also a variable.
Use two identical clear plastic bottles.
Put a dark coloured piece of cloth or plastic in one bottle.
Put an identical amount of white cloth or shiny metal foil in the other bottle.
Fit the bottles with one-hole stoppers with 20 cm of glass tubing.
Into each glass tube introduce a bead of water or oil.
Place each bottle in the sun. or about 50 cm from a bright light bulb or 1 metre from a fire or 20 cm from a burning lamp or candle.
Note the rate at which the beads of water or oil rise in the tubes.
1. Paint two thermoscopes. one thermoscope white the other black and illuminate both by a lamp.
2. Two thermoscopes have a black surface and a white surface.
The pressure inside the thermoscopes is less than atmospheric pressure and is equalized by the connecting tubes and valve A.
Close valve A so that the two thermoscopes are independent.
Shine a 150 W spotlight on the thermoscopes and observe the difference in fluid height showing a difference in pressure between the two thermoscope tubes caused by different temperatures.
So the black surface has absorbed more heat than the white surface.
3. Use flasks. or cut off light bulbs.
Fit both flasks or bulbs with corks and tubes about 15 cm in length.
Make holes 22 cm apart in a base board.
Pass the lower ends of the tubes through flat corks. glue the tubes in a vertical position and connect the open ends by rubber tubing.
Remove one bulb and blacken the other bulb in a candle flame.
Pour liquid into the U-tube so formed until the level is about 7 cm above the baseboard.
Replace the clear bulb and slide the tube in or out so that the liquid remains level.
Place a candle equidistant between the bulbs and note the levels of the liquid in the U-tube.
4. Experiment with different materials before doing this experiment, because for most cloths the absorption of infrared is almost independent of colour.
The amount of surface area pointing towards the source is also a variable.
Use two identical clear plastic bottles.
Put a dark coloured piece of cloth or plastic in one bottle.
Put an identical amount of white cloth or shiny metal foil in the other bottle.
Fit the bottles with one-hole stoppers with 20 cm of glass tubing.
Into each glass tube introduce a bead of water or oil.
Place each bottle in the sun. or about 50 cm from a bright light bulb or 1 metre from a fire or 20 cm from a burning lamp or candle.
Observe the rate at which the beads of water or oil rise up the tubes.
5. Show selective absorption of radiation using a thermoscope.
Place different screens between a heat source and a thermopile detector.
Focus a large light on a blackened match head the clear glass bulb of a thermoscope and the bulb covered with black paper.
6. Thermoscope to compare absorption of radiation
Experiment with different materials before doing this experiment, because for most cloths the absorption of infrared is almost independent of colour.
The amount of surface area pointing towards the source is also a variable.
Use two identical clear plastic bottles.
Put a dark coloured piece of cloth or plastic in one bottle.
Put an identical amount of white cloth or shiny metal foil in the other bottle.
Fit the bottles with one-hole stoppers with 20 cm of glass tubing.
Into each glass tube introduce a bead of water or oil.
Place each bottle in the sun. or about 50 cm from a bright light bulb or 1 metre from a fire or 20 cm from a burning lamp or candle.
Note the rate at which the beads of water or oil rise in the tubes.

23.9.23 Transfer heat by radiation
Hold the palm of your hand very close to., but not touching. your cheek.
Feel the radiation from your hand.
Heat travels by radiation almost instantaneously.
Hold your hand under an unlighted electric light bulb. the palm upward.
Turn on the electricity and feel the heat from the light bulb.
The heat could not reach your hand so quickly by conduction, because air is a very poor conductor of heat.
The heat could not reach your hand by convection, because convection carries the heat upward and away from your hand.
The heat came to your hand carried by short electromagnetic waves of wavelengths longer than light.
Radiation carries heat in every direction from the source.
Put a piece of glass between a light bulb and your hand to block any movement of air.
Feel the radiated heat.

23.6.0 Heat experiments (Primary)
See diagram 23.33: Air oven.
See diagram 23.4.9: Air thermometer.
See diagram 23.105a: Ball and ring.
See diagram 23.107: Bimetallic strip.
See diagram 23.3.7: Burn with a magnifier.
See diagram 23.7.00: Celsius temperature scale.
See diagram 23.24: Charcoal burner.
See diagram 23.7.00: Clinical thermometer.
See diagram 23.3.16: Compensated balance wheel of a watch.
See diagram 23.127: Convection disc.
See diagram 23.4.10: Expansion gauge.
See diagram 23.110: Expansion of air.
See diagram 23.4.6: Expansion of a solid.
See diagram 23.106: Expansion of a solid when heated.
See diagram 23.108: Expansion of liquids.
Heat
See diagram 23.11.0: Heat required to vaporize a liquid.
3.39 Heat snake
See diagram 23.116: Heat snake.
See diagram 23.1.1chd: Heat transfer.
See diagram 23.4.6: Heat water in a sealed flask.
See diagram 23.4.2: Heated liquids expand.
See diagram 23.1.3: Metal can heater.
See diagram 23.105: Ring and plug apparatus.
See diagram 23.7.00: Science experiments thermometer.
See diagram 23.1.6: Simple heating devices.
See diagram 23.30: Simple stand for heating.
See diagram 23.103: Temperature rise from heat energy intake.
See diagram 23.6.4: Test heated rubber band.
See diagram 23.116: Thermoscope.
See diagram 23.104: Transfer kinetic energy to heat energy.
See diagram 23.7.00: Wall thermometer.