Welcome to practical physicsPracticle physics - practical activities designed for use in the classroom with 11 to 19 year olds
 

Moving energy from one thing to another 2

Demonstrations

Examples of apparatus and demonstrations that can be used to illustrate different energy transfers and generate discussion.

Apparatus and materials

Various described below

Health & Safety and Technical notes


Procedure


The list below uses a wide range of apparatus and activities. Which ones you use will very much depend upon the apparatus that you have available and the interests of your students.


 
a Use a battery to light a lamp.

b Drive a bicycle dynamo by hand to light a lamp.
The dynamo can be driven at different speeds and gearing to a light a lamp.
 
c A thermocouple and galvanometer
Attach a piece of iron wire (about 24 SWG) about 75 cm long between two similar lengths of bare copper wire (also about 24 SWG) by twisting the ends together. Scrape the iron wire to obtain a good electrical contact with the copper. Connect the free ends of the copper wires to a demonstration galvanometer with, if necessary, a resistance box in the circuit. Keep one of the two junctions cool in a beaker of water at room temperature whilst the other is heated gently with a flame. (About 1,500 microvolts is the maximum likely to be reached.)
 
d Heat a piece of platinum wire with a Bunsen burner until it is white hot.
 
e Grind a handle round against a friction brake until it warms up. (An old-fashioned apparatus for measuring the 'mechanical equivalent of heat' (J) could be used.)

hit lead with hammer

f Hit a piece of lead with a hammer.
 
Wind a small piece of sheet lead about 3 mm thick round a thin piece of iron wire which will serve as a handle. If the sample is any more massive the temperature will not rise significantly. (A piece of lead pipe should not be used because it is too thick.) Place the lead on a sturdy base such as an iron kilogram mass to serve as an anvil. If necessary, you can place a felt pad or a newspaper under the anvil.
 
The hammer should be raised and allowed to fall (not hammered) on the lead a number of times. The temperature can be detected by holding the lead to the lips or cheek. It is also possible to use the thermocouple from c to measure the temperature rise. One junction is be inserted between the lead and its wire holder and the other connected to a sensitive galvanometer.
 
g Light a match.
 
h Light a firework rocket. Do it as the last thing in the lesson, outside, making sure that the class is standing well back.
 
i Add acid to alkali to produce thermal energy. Use an electrical thermometer or computer sensor.

coupled pendulums

j Coupled pendulums
Place two retort stands about 50 cm apart and fasten a light cord between them. Suspend two identical pendulums symmetrically from the cord so that they are about 25 cm apart. Set one of the pendulums swinging and let students watch the energy transfer from one to the other.

releasing a weight on a spring

k Releasing a weight on a spring.
Hang the compression spring from a retort stand firmly clamped to the bench. Hang a simple hardboard platform about 30 cm square from the end of the spring by four strings so that it is horizontal. Drop a brick onto the platform and observe the energy transfers.
 
l Torsional pendulum
Use the equipment in the last demonstration (k) as a torsional pendulum. Stop the vertical motion and give the platform a twist.
 
m Flywheel of variable inertia

flywheel of variable inertia

This consists of a rod symmetrically mounted so that it can rotate freely about an axis as shown and carrying two equal masses whose positions on the rod can be altered. Mount the whole on a stand so that the rod can rotate in a vertical plane. Tie a 1/2 kg mass to a string and wind the other end around the axle in such a way that in falling, the weight will cause the rod to rotate.
 
n Inertia operated toys, and clockwork toys.
There are many fun-to-operate, cheap toys which will motivate students to engage in energy transfer discussions
 
o Photographic exposure meter in which light produces an electric current.

 

Teaching notes


1 Step a The chemical energy stored in the cell is carried by the electric current to the lamp filament which then warms up. The hot filament glows, white-hot, and radiates energy to the surroundings.
 
2 Step b Chemical energy (food and oxygen) stored in muscles is transferred to spin energy of the dynamo. When the dynamo rotates, the electric current carries energy to the filament of the lamp, which warms up and radiates energy to the surroundings as electromagnetic waves.
 
3 Step c When two dissimilar wires are twisted together and the junctions kept at different temperatures an e.m.f. (voltage) will be generated between the junctions, which can be measured by a sensitive galvanometer (Seebeck effect). The energy transfer is from thermal energy in the junctions, which is carried by the electric current, to the galvanometer causing the needle to rotate until the hairspring stores it as elastic energy.
 
4 Step d Chemical energy released by burning gas in air produces thermal energy of the flame. The flame transfers its energy to the platinum, which warms up until it is so hot that it radiates light into the surroundings. You can use cheaper iron wire.
 
5 Step e Turning the handle of a friction brake or even rubbing your hand along the table top will transfer energy from the chemical store in muscles to thermal energy wherever there is friction between contact materials. The temperature of the materials increases.
 
6 Step f Here the chemical store in muscles is transferred to uphill energy stored in the raised hammer. As the hammer falls the gravitational energy store of the hammer is carried by the moving hammer, to warm the lead, (gaining thermal energy).
 
This experiment is modelled on one carried out by Hirn in the 1850s. He used 350 kg hammer moving at 5 m/s to smash into 3 kg block of lead held against a 1 tonne anvil.
 
It is often said lead is used because it has a small specific thermal capacity. However, it also has a large density so that the thermal capacity per unit volume for lead is only a little less than for other metals. The real reason is that lead is inelastic and most of the energy of the falling hammer is transferred to it.
 
7 Step g When a match is struck, friction warms up the chemicals in the match head. This releases the chemical store of the match head. The atoms of the surrounding air move faster and spread-out away from the match taking their motion energy with them. The air gets hotter.
 
8 Step h The chemical energy stored in the firework plus oxygen is released when the chemicals are heated and an explosion occurs. The energy is transferred to the hot gases expelled from the back of the rocket and the rocket increases its motion energy.
 
9 Step i Adding an acid to an alkali causes a chemical reaction. Energy is released, warming up the resulting chemicals.
 
10 Step j When one of the pendulums is set oscillating its amplitude gradually dies down but its energy is carried along the connecting string to the other pendulum whose amplitude increases. (The pendulums must be the same length, as this is a resonance effect. Note that the two pendulums are not in phase.)
 
11 Step k When the brick is dropped onto the platform, uphill energy is transferred to the spring and the spring extends as it gains elastic energy. Elastic energy is then transferred back to the brick, which rises, regaining uphill energy. The brick overshoots its equilibrium position and as it rises further it gains uphill energy. The platform continues to oscillate until all the energy is dissipated as the system warms up, due to frictional forces.
 
12 Step l The energy is transferred between the spring's elastic energy and rotational kinetic (spin) energy.
 
13 Step m Compare the effects of the 1/2 kg falling when the masses are at the inner ends of the rod and at the outer ends. As the weight falls it transfers a gravitational energy store to spin energy of the rod and motion energy of the falling weight. As the moment of inertia is altered, the ratio of spin energy to motion energy will change.
 
14 Step o When light falls on the exposure meter, energy is transferred to the meter and an electric current is produced which affects the display.
 
15 In demonstrations (d), (e), (f), (g), (h), and (i) energy is transferred from various sources so that it ends up warming up the system. In most instances the temperature rise is very small and so the energy appears to have disappeared at the end of the process. These activities are examples of 'useful' energy resources being degraded so that it is 'difficult' to re-use it. It is also 'difficult' to reverse the process. For example, the chemical store of a match head, when struck, warms up the surrounding air and the atoms move a little faster and spread away from the match. In order to reverse the process the faster moving atoms would need to be collected together in order to warm up the chemical constituents of the match head and so reform the original match.
 
This experiment was safety-checked in August 2007