Total Energy of a System
Energy and Thermal Physics

A steam engine

Practical Activity for 14-16 PRACTICAL PHYISCS

Demonstrations

Energy transfers using a model steam engine.

Apparatus and Materials

  • Steam engine unit (A model steam engine is included in the Malvern energy transfer kit)
  • Motor/dynamo unit
  • G-clamp, 5 cm
  • Line shaft unit
  • Mounted pulley and shaft light cord, about 1 m
  • Mass, 0.5 kg
  • Lamp unit
  • Driving belt

Health & Safety and Technical Notes

The Head of Department should ensure only staff that have been trained to use the school's steam engine(s) are allowed to do so.

Technicians also need instruction on maintenance.

All steam engines must be examined regularly as specified in the written scheme of examination (see CLEAPSS guidance).

Those models designed to use the laboratory gas supply should have been disposed of years ago. Methylated spirit burners should have been replaced with those designed for solid fuels.

Read our standard health & safety guidance


The steam engine must be operated using solid fuel, or where designed for it, liquefied petroleum gas.

When the steam pressure is high enough, turn the flywheel by hand until the condensed vapour has been expelled. The engine will now run freely.

Procedure

  1. Clamp the engine to the bench with a G-clamp and clamp the line shaft next to it. Join the small pulley on the engine to the large pulley on the line shaft with a belt. Attach a length of cord to a mass on the floor (about 1/2 kg ) with the other end attached to the line shaft. The engine will raise this load, increasing the energy stored gravitationally, and reducing the energy stored chemically in the gas supply or solid fuel and oxygen.
  2. Remove the load from the steam engine so that it accelerates.
  3. Remove the line shaft and clamp the motor/dynamo unit next to the steam engine. Connect the two with the drive belt. Connect the output of the dynamo unit to the lamp unit. Unscrew the lamps so that 1, 2, or 3 lamps are connected. This will produce a change in the mechanical load on the steam engine.

Teaching Notes

  • Avoid talking about a 'chain of energy transfers' and instead, the teacher needs to decide choose specific start and end points, and discuss the ways in which energy is stored.
  • When the fuel is burnt, energy stored chemically in the fuel is transferred. There is less energy stored chemically. One end point could be when water and cylinder have heated up, so that the energy is stored thermally in the water in the pressure cylinder.
  • Alternatively, the end point could be when the pistons are accelerating. The water vapour produced increases the pressure on the pistons, and the force produced makes the pistons accelerate. The energy stored chemically is now stored kinetically.
  • In procedure 1, the belt and line exert a force on the load, and a 1/2 kg load will be raised. The energy stored chemically is now stored gravitationally. When heavier loads are put on the line-shaft the steam engine will slow down because has a fixed power, which is the rate at which it can do work. If the load is removed all together as in 2, the steam engine speeds up.
  • In procedure 3, the dynamo produces an electric current which transfers energy to the lamps. The lamp filament warms up and light is radiated to the surroundings. The more lamps that are connected the slower the steam engine runs and the dimmer the lamps are. If too many lamps are connected the steam engine might stall. It is effective to use two or three low voltage bulbs in parallel. With all the lamps alight, the engine labours heavily; with none connected, it races.
  • It is worth pointing out that the overall energy analysis of the steam engine is one where the energy stored chemically is decreasing (as fuel and oxygen are used up), and the energy stored thermally in the surroundings is increasing (as the surroundings heat up).
  • This may be a good time to discuss the generation of electricity from various fuels and the reason why power cuts can happen when too many people try to run too many appliances at the same time. For example, on Christmas Day, it can take longer to cook lunch because everyone wants to do it at once. System managers must ensure that a power station does not stall and the a.c. frequency does not fall too low (this would damage sensitive equipment).

This experiment was safety-tested in November 2005

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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