School Science Lessons
(UNPh02)
2024-07-24
Scientific investigation and experiments, design of experiments
Contents
2.1.4 Electrical appliances report
2.2.3 Electric kettle heating efficiency
2.2.2 Energy transfer between pendulums, by resonance
2.1.5 Heat insulation, properties of common materials
2.2.4 Light bulb brightness, Joly photometer, wax block photometer
2.1.4 Electrical appliances report
Before preparing to teach this topic, select, examine and report on a useful electrical appliance, e.g., air conditioner, boiler, calculator, charger, clock,
dishwasher, doorbell, fan, freezer, fryer, hair dryer, heater, iron, mixer, motor, printer, radiator, refrigerator, roaster, shaver, telephone, television,
toaster, torch, toy, vacuum cleaner, Xmas tree lights.
Examine the nameplates and study the instruction manuals.
Report on the following:
* Correct name and use of each electrical appliance
* Normal or allowed working voltage and current
* Working principles including a circuit diagram
* Power input or useful power output, resistance and other properties
* Operating method and points for attention
* Safety characteristics, including the safety operating conditions so that the operator will not be hurt and apparatus not to be damaged.
* Relevant times and terms of use, e.g. guarantee period, scrap period, date of production, continuous operating time.
* Examine how the appliances convert electric energy to other forms of energy and think about how to design an experiment project to measure the
efficiency of energy transformation.
Points for attention before preparing to teach this topic:
* Be clear on how to switch off the power in an emergency and the exact position of the appliance.
* Any old or discarded appliance that requires mains power to operate should be inspected and repaired only by a qualified electrician.
If you have any doubt about the operating status or safety of any electrical appliance, do not under any circumstance connect it to mains supply.
As a rule, all appliances that require mains voltage to operate should be tested periodically by a qualified electrician.
Check with your local electricity supply authority about how often these checks should be done.
Be careful! Mains electricity can kill!
Other pieces of equipment contain high vacuum tubes, e.g. television sets and microwave ovens.
Breaking the glass container that is evacuated can cause injury from flying glass.
Do not use exposed wires to connect a circuit.
Pay special attention to whether the leads of the electrical appliance discarded for a long time are exposed or ageing.
You must wrap with electrical insulating tape or replace all exposed or ageing wires.
Check for damaged three pin plugs, exposed flex wire and exposed ends before the experiment.
* Use only ammeters, voltmeters and power meters authorized for use in schools.
Use only low voltage power packs up to 12 V.
Check the circuit before connecting the last lead to the source of power, especially if an ammeter is in the circuit.
Make the first connection to the source of power by switching on and off very quickly to check whether you have connected ammeters and voltmeters correctly with correct deflection
and scale reading.
* Plug the three pin plug into a normal three pin socket.
Do not change the pin and the socket.
* Teachers should check all experiments involving electricity no matter the voltage before they allow students to energize circuits.
* Never allow students to work unsupervised on electrical experiments.
* Ensure that no other appliances are working before starting the test.
2.1.5 Heat insulation, properties of common materials
See diagram 23.1.5 Four big and four small beakers
Method
* 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.
2.2.2 Energy transfer between pendulums, by resonance
See diagram 15.4.12 Pendulums.
* The time taken for energy transfer between pendulums depends on,
1. the distance between hanging points of the pendulums and
2. the length of the pendulums.
Method
Suspend a 100 cm strong string between two stands.
Attach two threads 2.5 cm each side of the centre of the strong string.
Attach 100 g weights to the end of each thread so that the length of the thread is 50 cm.
Pull one weight to the side through a 60o angle to the vertical.
While noting the time in seconds, release the weight so that it swings freely back and forth as a pendulum, but does not touch the stationary second pendulum.
* The energy of the first pendulum transfers to the second pendulum.
The first pendulum swings less until it stops swinging and the second pendulum swings more until it has the original swing of the first pendulum.
Note the time when the first pendulum stops.
The energy of the first pendulum transfers to the second pendulum.
Note the time when the second pendulum stops.
Note the times for five transformations of energy.
Calculate the average time needed for one transformation of energy.
* Repeat the experiment by increasing the distance between the hanging points of the pendulums.
Repeat the experiment by shortening the length of the thread.
Repeat the experiment by changing the initial angle of swing.
How does time of transfer depend on the following:
1. distance between pendulums,
2. length of pendulums,
3. original angle of swing of pendulums?
Note that the distance between pendulums affects the tension in the strong string.
Table 2.2.2
| Experiment |
distance
d |
length
l |
angle
a |
Transfer
(seconds) |
Total
(seconds) |
Average
(seconds) |
| 1 (control) |
4.5 |
50 |
60o |
. |
- |
- |
| 4 (distance) |
10 |
50 |
60o |
. |
. |
. |
| 3 (length) |
4.5 |
100 |
60o |
. |
. |
. |
| 4 (angle) |
4.5 |
50 |
30o |
. |
. |
. |
2.2.3 Electric kettle heating efficiency
See diagram 32.2.3 Electric kettle
Any kettle used to heat water can lose heat to its surroundings and to the materials from which it is constructed.
So the heat produced by the heat source does not only heat the water.
Measure the heat efficiency of an electric kettle.
BE CAREFUL! Be sure that water cannot come into contact with the power supply.
Some simple heating elements are bare wire and should not be used for this experiment!
Do not operate an electric kettle with wet hands!
Be sure that students and teachers cannot be scalded by steam.
1. Record the power rating of the heater element.
2. Measure and record the temperature of 500 mL of water and pour it into a kettle.
3. Switch on the power supply to the kettle and start timing how long it takes the kettle to bring the water to boil.
4. Switch off the power supply when the water boils, and record the time it took for the water to come to boil.
5. Empty the kettle and allow the element to cool to room temperature then repeat steps 2, 3 and 4 and find the average time to bring the water to boil.
* To calculate the efficiency of the kettle you need to find how much energy the water absorbed to bring it to boiling point.
Use the formula Q = mc (Tf - Ti), where m = mass of water, c = specific heat of water, Tf = final temperature and Ti = initial temperature.
Then divide this value by the time it took to bring the water to boiling and you get the power consumed in boiling the water.
Finally you divide this value by the power rating of the element to give the efficiency of the kettle.
The following example is based on a kitchen kettle with an element rating of 2, 200 watts:
m = 0.5 kg, c = 4186 J / kgoC, Tf = 100oC, Ti = 22oC.
So Q = 0.5 × 4186 × (100 - 22) = 163, 254 Joules
The time taken to bring the water to boil was 94 seconds.
Therefore the power consumed to boil the water = 163, 254 / 94 = 1, 737 Watts
* To find the efficiency of the kettle divide the power used to boil the water by the power output of the element and multiply by 100 to give a percentage value, i.e. (1, 737 / 2, 200) × 100 = 79%.
The efficiency of the kitchen kettle is 79%, or 21% of the power output is wasted.
2.2.4 Light bulb brightness, Joly photometer, wax block photometer
Electric energy can be transferred not only into light energy, but also into heat energy when light bulb works.
So the efficiency of a light bulb can be expressed as the ratio of luminous intensity to consumed electric power.
Light intensity at distance s from a light source varies inversely with the distance squared, s2.
It can be measured with a light meter or a photometer.
If the light meter is calibrated to the size of camera length apertures, it is called an exposure meter.
1. Using a Joly photometer (wax block photometer)
The Joly photometer consists of two equal paraffin wax blocks separated by a thin opaque sheet.
Adjust the positions of two light sources to be compared until the two wax blocks appear equally bright.
The Joly photometer is made from two identical blocks of paraffin wax, B1 and B4, about 5 mm thick, separated by a sheet of aluminium foil.
Luminous sources of light, intensity I1 and I4, are placed each side of the blocks at distance S1 and S4 from the aluminium sheet,
so that B1 receives illumination only from S1 and B4 receives illumination only from S4.
By viewing from the side, i.e. in the plane of the aluminium sheet, the intensity of the diffused light from the paraffin blocks can be compared.
If the photometer is moved between two light sources so that the light intensity seen in each block is the same,
then I1 / S14 = I4 / S44.
2. Make a photometer
See diagram 28.2.4: Make a photometer.
Use a rectangular cardboard box, e.g. a school chalk box.
Cut four identical rectangular windows in the sides of the box.
Make two paraffin blocks each 5 mm thick and half the area of the window so that the two blocks can just fit side by side in the window.
Make sure that the upper and lower surfaces of the paraffin blocks are smooth.
Cut a piece of flat aluminium foil the same size and shape as the paraffin blocks.
Fit it between the blocks and fit the blocks and foil into the window.
Fix two globes in lamp holders each side of the box.
One globe of known light intensity, e.g. 40 watt frosted bulb, luminous intensity about 32 candelas.
The luminous intensity of the other globe is unknown.
Darken the room and turn on the power for the two globes.
Slide the photometer to a position where the two sides of the paraffin blocks are equally bright.
Record the distances from the aluminium foil sheet to each globe.
If I1 = known intensity, e.g. 32 candelas and I4= unknown intensity then as I1 / S14 =
I4 / S44, I4 = (32 / S14)
/ S44.