School Science Lessons
Please send comments to: J.Elfick@uq.edu.au
2.0.0 Scientific investigation
and experiments, design of experiments
Table of contents
2.1.4 Electrical appliances in the home
2.1.5 Heat insulation, properties of common materials
2.2.2 Energy transfer between pendulums, by resonance
2.2.3 Electric kettle heating efficiency
2.2.4 Light bulb brightness, Joly photometer, wax block
2.4.1 Make a photometer
2.1.4 Electrical appliances in
| See: Electrical
| See diagram 32.2.3
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:
1. Correct name and use of each electrical appliance
2. Normal or allowed working voltage and current
3. Working principles including a circuit diagram
4. Power input or useful power output, resistance and other properties
5. Operating method and points for attention
6. Safety characteristics, including the safety operating conditions
so that the operator will not be hurt and apparatus not to be damaged.
period, date of production, continuous operating time.
8. 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:
See 19.3.5: Microwave cooking
1. Be clear on how to switch off the power in an emergency and
the exact position of the appliance.
2. 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
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, such as 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.
3. 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 reading
not off the scale.
4. Plug the three pin plug into a normal three pin socket.
Do not change the pin and the socket.
5. Teachers should check all experiments involving electricity
no matter the voltage before they allow students to energize circuits.
6. Never allow students to work unsupervised on electrical experiments.
7. Ensure that no other appliances are working before starting
2.1.5 Heat insulation, properties
of common materials
See diagram 23.1.5
Organization and guidance: Learn the function of the heat insulation
of common materials.
Find out which of the materials is the best insulator and probably
help you when you are in state of emergency such as pouring a
spoon of boiling porridge into your dinner pail made from stainless
steel while you have dinner in your school dining room.
The simplest method of doing so is to feel a heated thing insulated
by these materials.
However, distinguishing the degree of their heat insulation in
detail is difficult.
The following way could distinguish their heat insulation in detail
and tell you the difference between their insulation.
To do this, set up four big beakers and four small ones, as shown.
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
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.
For example, 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.
Let the students think if there are other control factors in this
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
Continue to repeat this procedure if you have more materials to
2.2.2 Energy transfer between
pendulums, by resonance
See diagram 15.4.12
Study how 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.
Suspend a 100 cm strong string between two stands.
Attach two threads 2.5 cm each side of the centre of the strong
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
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
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.
2.2.3 Electric kettle heating
See diagram 32.2.3
Any kettle used to heat water can lose heat to its surroundings
and to the materials from which it is constructed.
The heat produced by the heat source does not only heat the water.
You can measure the heat efficiency of an electric kettle by doing
a simple experiment.
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
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
See 188.8.131.52: Luminous intensity,
candela, cp | See diagram 28.2.4: Make a photometer
Electric energy can be transferred not only into light energy but
also heat when light bulb works.
So its efficiency 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.
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)
It consists of two equal paraffin wax blocks separated by a thin
You can adjust the positions of two light sources to
be compared until the two wax blocks appear equally bright.
Also known as The Joly photometer is made from two identical blocks
of paraffin wax, B1 and B4, about 5 mm thick, separated
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
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.
2.4.1 Make a photometer
See diagram 28.2.4
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
Cut a piece of flat aluminium foil the same size and shape as the
Fit it between the blocks and fit the blocks and foil into the
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)