Getting to know the joule and the watt
Practical Activity for 14-16
Students use a hand-turned generator to gain direct experience of measuring energy transfer and to get a ‘feel’ for the size of a joule and the size of a watt.
Apparatus and Materials
- SEP Energymeter and mains adaptor
- Hand-turned generator and small (low voltage) electric motor (e.g. SEP Energy transfer unit)
- 2 plug-plug leads, red
- 2 plug-plug leads, black
Health & Safety and Technical Notes
- Before using the energymeter, first connect the hand-turned generator to an electric motor. Turn the generator by hand, and note what happens to the motor as it is turned at different speeds.
- 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 transferred.
- Press the start/pause button of the energymeter and turn the generator. Watch the display to see how the value of energy transferred increases cumulatively as the handle is turned.
- Note how the value of energy transferred increases faster as the handle is turned faster. This could be done by seeing how long it takes to transfer 10 J of energy when the handle is turned at different speeds.
- A graph showing how the energy transferred changes over time can be made by measuring the total energy transferred after 10, 20, 30, 40 and 50 seconds. Two sets of measurements could be made by turning the handle first at a slow speed and then at a faster speed. This should give a graph showing two lines of different slopes.
- Now set the knob on the energymeter to measure power. Turn the handle again and note how the value of power increases when the handle is turned faster and decreases when it is turned slower.
- The key ideas that can be taught through this activity are that:
- Energy is measured in joules
- Power is the rate at which energy is transferred
- Power is measured in watts.
- A hand-turned generator is a good way to make a start on this, because it provides an experience through physical activity. Before making measurements, it is helpful to explore the effects qualitatively. Turning the generator by hand gives a very direct ‘feel’ for the way in which energy is ‘shifted’ from the generator to the motor; if the leads to the motor are removed so that there is no load, it is very striking how easy it is to turn the generator.
- The SEP Energy transfer unit is a convenient piece of equipment for carrying out this experiment, though any similar hand-turned generator and motor could be used. For the generator on the energy transfer kit, transferring 10 J of energy takes about 25 seconds when turned at moderate speed and 15 seconds at a fast speed.
- When students draw a graph plotting energy transferred against time, they should find that they get a steeper slope for the values when they turn the handle at a faster speed. This can form the starting point for a discussion of power (the slope of the line represents the rate at which energy is transferred, i.e. the power).
- It is worth making a comparison between the energy transferred in physical activity and the energy values of foods. A typical energy value for a 100 g chocolate bar is 2000 kJ. It would take 5 000 000 seconds (25/10 x 2000 x 1000) or about 1400 hours to transfer this amount of energy from the generator to the motor when turned at moderate speed.