School Science Lessons
(UNPh23)
24-07-2024
Heat transfer
Contents
23.7.0 Conduction
23.6.0 Convection
23.8.0 Spirit burner, alcohol lamp
23.7.0 Conduction of heat, thermal conductivity, insulation
23.7.01 Conduction of heat, thermal conductivity
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, heat water in a paper bag
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, feel heat energy
23.7.8 Conduction of heat by metal bars, melt paraffin wax
23.7.23 Conduction of heat by metals, Davy lamp
23.7.10 Conduction of heat by wood, anisotropic conduction
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, properties of common materials
23.7.18 Heat paper without burning, coin on paper, hanging thread
23.7.19 Melt ice blocks
23.7.20 Reduce heat loss with insulation
23.7.21 Touch wood and iron in the sun
23.7.21 Touch wood and iron in the sun
23.7.22 Relative conductivity
23.6.0 Convection
23.6.01 Convection
23.6.1 Convection box, convection currents in air, mirage
23.6.2 Convection cells, lava lamps
23.6.3 Convection currents and ventilation
23.6.4 Convection currents between jars of water
23.6.5 Convection currents from an ink bottle
23.6.6 Convection currents in a container
23.6.7 Convection currents in a test-tube
23.6.9 Convection currents in water
23.6.10 Convection smoke box with two chimneys, smoke house
23.6.11 Convection tube
23.6.12 Feel convection currents in a test-tube
23.6.13 Hope's experiment, maximum density of water
23.6.14 Model heating system
23.6.15 Temperature of water at maximum density, 4 oC
23.6.16 Barnard cell
23.6.01 Convection
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.
23.6.1 Convection box, convection currents in air, mirage
See diagram 37.13: Convection box.
Experiments
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.
23.6.2 Convection cells, lava lamps, cumulus cloud, Hadley cells
1. 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.
2. 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.
3. 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 30o 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.
Experiments
4. 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.
5. Large 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.
6. Miso soup
Observe convection cells in heated soup containing small particles, especially the Japanese miso soup.
23.6.3 Convection currents and
ventilation
See diagram 4.30: Convection current ventilation.
Experiments
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,
*4. 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 0oC, 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 0oC, 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.
23.6.4 Convection currents between jars of water
See diagram 23.2.2: Convection currents between jars of water.
Experiment
Use four similar wide mouth jars with screw-on lids.
Fill jar 1 with tap water and jar 2 with hot water, 90oC.
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, 90oC.
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.
23.6.5 Convection currents from an ink bottle
See diagram 23.126: Convection currents from an ink bottle.
Experiments
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.
23.6.6 Convection currents in a container
Experiments
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.
23.6.7 Convection currents in a test-tube
Experiment
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.
23.6.9 Convection currents in water
See diagram 23.2.1a: Convection currents in water.
Experiments
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.
23.6.10 Convection smoke box with two chimneys, smoke house
See diagram 23.2.6: Convection smoke box
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See diagram 4.28: Convection bottle
Experiments
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.
23.6.11 Convection tube
See diagram 23.29: Convection tube.
Experiment
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.
23.6.12 Feel convection currents in a test-tube
See diagram 4.24: Feel a heated test-tube.
Experiments
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.
23.6.13 Hope's experiment, maximum density of water
See diagram: 23.2.7a: Hope's apparatus.
Water reaches a maximum density at 3.98 oC (39.16 F).
Experiment
Hope's apparatus (Thomas Hope, 1766-1844, Scotland), is a glass cylinder with a copper trough fitted around the middle of the cylinder.
Stand the cylinder vertically, fill it with water and put an ice / water freezing mixture in the trough.
Insert thermometers through holes in the top and bottom of the cylinder or put a small thermometer in the bottom of the cylinder.
Leave to stand until the more dense water collects at the bottom of the cylinder at a temperature of 4 oC, while ice form on the surface of the water in the cylinder.
A tall cylinder of water with a collar of salt / ice around the middle will freeze at the top and remain at 4 oC at the bottom.
In a jar of water 35 cm high with 15 cm of ice floating on top, the temperature at the bottom does not fall below 4 oC.
23.6.14 Model heating system
See diagram 23.29: Convection tube.
Experiment
Heat water in a loop of glass tubing.
Use a model of a heating system with an expansion chamber and radiator.
23.6.15 Temperature of water at maximum density, 4 oC
Maximum density of water, negative expansion coefficient of water, "anomalous" properties of water
Experiments
1. Ice has a larger volume than the original volume of water before it freezes.
Water has a maximum density at 4 oC.
When water cools from room temperature to 4 oC, it is contracting in volume.
Most solids are denser than their liquids, but when water is cooled from 4 oC to 0 oC, its volume expands.
At 4 oC, the density of water is 1000 kg m-3.
At 0oC the density of water is 999.87 kg m-3 and the density of ice is 918 kg m-3.
2. The lower density of ice is caused by the formation of a hydrogen-bonded tetrahedral network of water molecules.
The temperature at which water remains liquid decreases with salinity.
Some people refer to the "anomalous" properties of water when its volume increases by about 9% on freezing.
The "anomalous" properties can be explained by how the size of the oxygen atom and its nuclear charge affects the electronic charge cloud of the hydrogen atoms bonded to it to form larger hydrogen bond aggregates.
Above 4 oC, the vibration of the O-H bonds is enough to move the water molecules apart.
Another "anomalous" property of water is its high boiling point.
Most molecules show an increase in boiling point with molecular weight, although this relationship is complicated by the shape of the molecule.
In water, hydrogen bonding keeps the molecules in a liquid state until 100oC.
However, if water behaved as a normal polar molecule, at its molecular weight it would boil at about -100oC.
In that case, the water around us would exist only as a vapour, not a liquid.
It takes a lot more kinetic energy at increased temperature to break the hydrogen bonds to free the water molecules as the gas.
This high boiling point effect is also seen with fluorine and nitrogen.
3. A freezing mixture of ice and common salt (sodium chloride), may drop to -20 oC.
In freshwater lakes, during the summer the upper levels are heated by the sun to form a less dense layer, called the epilimnion, above the cooler more dense layer, the hypolimnion, where anaerobic conditions may occur.
During autumn, the epilimnion cools and mixes with the hypolimnion causing overturn and churning up of nutrients towards the surface.
Algae may use these nutrients to cause algal blooms.
4. If you bore a hole through the surface ice of a frozen river and catch a fish through the hole, the fish will freeze to death when you pull it up, became it has been living at a temperature between 0oC and 4 oC.
5. During freeze thaw, erosion of rocks, water enters cracks in the surface of rocks, freezes in cold weather, then expands in warmer weather and splits off a piece of the rock, a form of weathering.
6. In countries where milk is delivered to front door steps overnight, on very cold nights the metal foil top of the milk bottle may lift up and the bottle may even shatter, because of the expansion of the milk as it freezes.
In very cold weather, freezing water can expand in water pipes and burst them.
23.6.15 Barnard cell
Experiment
Heat paraffin with aluminium dust in a small brass dish until convection cells are formed.
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
Experiments
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
Experiments
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.
Experiments
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, boil water in a paper cup, heat water in a paper bag
Experiments
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
Experiment
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 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.
Experiments
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.
23.7.7 Conduction of heat by different metals, feel heat energy
See diagram 23.1.4: Conduction of heat by different metals.
Experiments
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.
Experiments
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.
23.7.10 Conduction of heat by wood, anisotropic conduction
Experiment
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
Experiment
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.
Experiment
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 Conduction of heat by a coin on paper
Experiment
Hold a piece of paper above a candle flame: it will char if brought near.
Place a metal coin on the paper and repeat the experiment: the metal will conduct the heat away and leave a pattern on the paper.
23.7.14 Copper coil snuffer conducts heat
See diagram 4.20: Snuff out a candle flame with a copper coil.
Experiment
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
Experiment
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
Experiment
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
Experiment
The simplest method of testing insulators is to feel a heated thing insulated by these materials.
However, distinguishing the degree of their heat insulation in detail is difficult.
Set up four big beakers and four small ones, as shown in diagram 23.1.5.
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, e.g. polyester plastics, papers, 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 graphs on the one sheet of graph paper to see the conclusion clearly.
Continue to repeat this procedure if you have more materials to distinguish.
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.
Experiments
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
Experiments
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
Experiment
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 Touch wood and iron in the sun
Experiment
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.
23.7.22 Relative conductivity
See diagram 23.1.7: Relative conductivity.
Experiments
* 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.
23.7.23 Conduction of heat by metals, Davy lamp
See diagram 23.119: Davy lamp
1. 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.
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.
23.8.0 Spirit burner, alcohol lamp
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.