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

A steam engine

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.

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


a 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, giving it potential energy which is drawn from the chemical energy of the gas supply or solid fuel.

steam engine setup with load dangling

b Remove the load from the steam engine so that it accelerates.
 
c 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.

dynamo clamped next to steam engine setup

 

Teaching notes


1 The steam engine exhibits a whole chain of energy transfers and the teacher needs to decide which are the important ones to highlight.
 
2 When the fuel is burnt, chemical energy stored in the fuel is transferred to the water in the pressure cylinder so that the cylinder and the water warm up. The water vapour produced increases the pressure on the pistons, so the energy stored as fuel + oxygen is eventually transferred to motion energy of the pistons.
 
3 In procedure a, motion energy from the pistons is transferred to spin energy of the flywheel and line-shaft, and a 1/2 kg load will be raised. The load will gain uphill energy. When heavier loads are put on the line-shaft the steam engine will slow down because it can only transfer energy at a given rate. If the load is removed all together as in b, the steam engine speeds up and increases its own motion energy.
 
In procedure c, the dynamo produces an electric current which carries 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.
 
4 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-checked in November 2005