Total Energy of a System
Energy and Thermal Physics

Moving energy from one thing to another 2

Practical Activity for 14-16 PRACTICAL PHYISCS

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

Read our standard health & safety guidance


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.

  1. Use a battery to light a lamp.
  2. 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.
  3. 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.)
  4. Heat a piece of platinum wire with a Bunsen burner until it is white hot.
  5. 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.)
  6. 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.
  7. 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 3 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.
  8. Light a match.
  9. Light a firework rocket: Do it as the last thing in the lesson, outside, making sure that the class is standing well back.
  10. Add acid to alkali to produce a temperature change: Use an electrical thermometer or computer sensor.
  11. 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 observe the motion.
  12. 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 what happens.
  13. Torsional pendulum: Use the equipment in the last demonstration (11) as a torsional pendulum. Stop the vertical motion and give the platform a twist.
  14. 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.
  15. Inertia operated toys, and clockwork toys: There are many fun-to-operate, cheap toys which will motivate students to engage in energy transfer discussions.
  16. Photographic exposure meter in which light produces an electric current.

Teaching Notes

  • In these discussions it is important to choose start and end points carefully when talking about the way or ways that energy is stored. Separate out the processes producing the transfer (an electric current flows, a force does work etc.) from the energy analysis.
  • Step 1, the cell produces an electric current which heats the lamp filament. The hot filament glows, white-hot, and radiates light and infrared radiation to the surroundings. Energy stored chemically in the cell is now stored thermally in the surroundings.
  • Step 2, you apply a force to rotate the dynamo, which produces an electric current. The current flows in the filament of the lamp, which warms up and radiates energy to the surroundings as electromagnetic waves. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
  • Step 3, 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 current produces a force on the hairspring inside the galvanometer, which moves the needle to show the current. Energy stored thermally in the junctions is now stored elastically in the hairspring.
  • Step 4, the gas burns and heats the platinum and the surroundings. The platinum warms up until it is so hot that it radiates light into the surroundings. Energy stored chemically in the gas and oxygen is now stored thermally in the surroundings. (You can use cheaper iron wire.)
  • Step 5, turning the handle of a friction brake or even rubbing your hand along the table top will produce a temperature increase wherever there is friction between contact materials. The temperature of the materials increases. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
  • Step 6, here you use a force to lift the hammer, and the force of gravity pulls the hammer down. The hammer exerts a force on the lead, heating it up. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
  • 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 the collision between the hammer and the lead is inelastic, and most of the energy of the falling hammer is transferred to it.
  • Step 7, when a match is struck, friction warms up the chemicals in the match head and the match lights. The atoms of the surrounding air move faster and spread-out away from the match. The air gets hotter. Energy stored chemically in the match head and the oxygen is now stored thermally in the surroundings.
  • Step 8, when the chemicals in the firework ignite they heat the air, and produce a force on the fragments which move. The firework emits light, and the heating of the air produces sound. The air slows down the fragments of firework. Energy stored chemically in the firework head and the oxygen is now stored thermally in the surroundings.
  • Step 9, adding an acid to an alkali causes a chemical reaction, and the temperature of the product increases. Energy stored chemically in the acid and alkali is now stored thermally in the product and the surroundings.
  • Step 10, when one of the pendula is set oscillating (by using muscles to displace it) its amplitude gradually dies down but the string exerts a force on the other pendulum whose amplitude increases. (The pendula must be the same length, as this is a resonance effect. Note that the two pendulums are not in phase.) Eventually the pendula will be at rest. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
  • Step 11, you use a force to lift the brick, and the force of gravity pulls the brick when it is dropped onto the platform. The platform exerts a force on the spring, which extends. The spring exerts a force on the platform, which moves up. The air exerts a force on the platform and brick, reducing the amplitude of the oscillation. Eventually the platform and brick are at rest. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
  • Step 12, you use a force to twist the pendulum, which deforms the wire. The force produced by this deformation twists the wire back, and it continues to oscillate. The aire exerts a force on the platform and brick. Eventually the platform and brick are at rest. Energy stored chemically in food and oxygen is now stored thermally in the surroundings.
  • Step 13, 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 exerts a force on the rod, which causes it to rotate. The way that it rotates depends on the distribution of the masses. Energy stored gravitationally is now stored thermally in the surroundings.
  • Step 15, when light falls on the exposure meter, an electric current is produced which affects the display. The display emits light which heats the surroundings. Energy stored chemically (e.g. in the fuel in the power station and oxygen) is now stored thermally in the surroundings. If you are using sunlight, then the energy wasstored in the nuclear fuel in the Sun.
  • In demonstrations (4), (5), (6), (7), (8), and (9) 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 being dissipated so that it is difficult to use it. It is also difficult to reverse the process. For example, the energy stored chemically in a match head (and oxygen), 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-tested in August 2007

Total Energy of a System
appears in the relation dU=dQ+dW
is used in analyses relating to Thermal Equilibrium
IOP DOMAINS Physics CPD programme

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