Using an energymeter to measure efficiency of energy transfer

Class practical

Students make measurements on the energy transfers involved when a falling mass drives an electrical generator, and calculate the efficiency of the process

Apparatus and Materials

For each student group

• SEP Energymeter and mains adaptor
• Pulley / generator and small (low voltage) electric motor (e.g. SEP Energy transfer unit)
• 1 slotted mass hanger (100 g) and 1 slotted mass (100 g)
• Thread for attaching slotted mass to pulley
• Metre rule
• 2 plug-plug leads, red
• 2 plug-plug leads, black

Note: The apparatus should be placed on top of a box on the lab bench so that it gives a 1 metre drop above the floor.

Health & Safety and Technical Notes

Read our standard health & safety guidance

Procedure

1. Before using the energymeter, first connect the generator to an electric motor and wind the thread attached to the slotted mass around the pulley. Let the slotted mass fall, and note that the motor turns around.
2. Now include the energymeter into the circuit as shown in the diagram. Plug the mains adaptor into the energymeter. Set the knob on the energymeter to measure energy transfer.

3. Turn the pulley to wind up the thread so that the 100 g mass is at the top of the metre rule. Press the ‘start/pause’ button on the energymeter and let go of the pulley. The mass should fall and the motor should turn round. From the energymeter display, make a note of the energy transferred from the generator to the motor. Repeat the experiment to see if you get consistent readings.
4. Calculate the energy transferred from the falling mass to the generator (work done by the falling mass) using the relationship: work done (J) = force (N) x distance (m)
5. From the values of the energy transferred from the falling mass to the generator and from the generator to the motor, calculate the efficiency of the process. efficiency = (energy transferred to motor / energy transferred from falling mass) x 100%
6. Repeat the experiment with different amounts of work done by the falling mass, for example a 100 g mass and a 0.5 m drop, a 200 g mass and a 1 m drop.

Teaching Notes

• The key ideas that can be taught through this activity are that:
• work done can be calculated from the force applied and the distance moved
• the efficiency of a process can be calculated from the energy input and the energy output.
• In most contexts, energy input means the energy transferred electrically from a power station or cell. Fuel or chemicals are depletd. Energy output means the work done in the device to do a job, such as heating, or lighting. Energy ends up being stored thermally in the surroundings.
• If students have not used this kind of arrangement before, then it is helpful if they explore the system qualitatively first before adding the energymeter. The SEP Energy transfer unit is a convenient piece of equipment for carrying out this experiment, though any similar generator and motor could be used.
• To avoid damage to the generator, the thread should be long enough for the masses to reach the ground to avoid jerking on the generator. Note that the thread needs to be wrapped clockwise on the pulley, so when the leads to the energymeter are connected with the correct polarity (i.e. red to red, black to black), the energymeter will record the energy transferred correctly. If the thread is wound the wrong way, the meter displays ‘Source current wrong direction’. The message ‘Source current wrong direction’ will also be displayed when the pulley is turned backwards to wind the thread back after the mass has fallen – students should ignore this.
• A mass of 1 kg has a weight of about 10 N, so a mass of 100 g has a weight of about 1 N, i.e. a 100 g mass is pulled towards the Earth with a force of 1 N. When it drops by 1 metre, the mass does 1 J of work on the generator. This is the energy input. Of course, we would expect the energy output measured using the energymeter to be less that the 1 J that was put in, but how much less will depend of the efficiency of the generator. Typically, the reading on the energymeter is about 200 mJ, so the efficiency is about 20%.