In this this demonstration students predict which kettle boils first. You can use it to illustrate that power is the rate at which energy is transferred and to introduce the relationship P = IV.
- 3 kW electric kettle
- Small low-power electric kettle (eg 1 kW travel kettle of type found in hotel rooms)
- 2 plug-in energy meters with voltage, current and power functions
- 500 ml measuring jug or beaker
- Thermometer (optional)
A 3 kW kettle draws a large current. Don’t use an extension lead. Plug each kettle into its own wall outlet and check that the RCD circuits in your lab do not trip when you switch on both at the same time.
The energy functions on your meters are likely to be calibrated in kilowatt-hours. They are not needed for this activity. If you do decide to use them remember to reset meters to zero before you start and explain how to convert to joules (1 kWh = 3 600 000 J).
- Plug the large kettle into a power meter and into the mains supply. Do the same for the small kettle.
- Set both meters to the voltage function and note values on board. They should see that both kettles give the same reading (240 V).
- Pour 0.5 litre of water into each kettle. Measure water temperature (optional).
- Switch on both kettles at the same time.
- Switch both meters to the current function and note values on board.
- Switch to the power settings and leave both kettles switched on.
- Which kettle will boil first?
- The kettles are plugged into the same mains supply. Which kettle transfers energy to the water more quickly?
- Can you see a relationship between power, current and voltage?
Students may think that the smaller kettle will come to the boil first because there is less casing material to warm. Remind them that inside there is a large mass of water. It is not possible to work out which kettle will boil first from its appearance alone.
The mass of water poured into each kettle is the same. The starting temperature for the water is also the same and so is the end temperature because both kettles switch off automatically when the water reaches 100°C. The energy required to raise the temperature of the water in each kettle is equal.
The power reading in watts is the energy in joules transferred electrically in one second by the heating element circuit. For the small kettle the rate is about 1,000 joules per second. For large the kettle it is close to 3,000 joules per second.
The current readings reveal the reason that the larger kettle will warm up more quickly. The current is three times as great and so energy is transferred at three times the rate. We’d expect the kettle with a 3 kW heating element to boil about three times as fast as one with a 1 kW element.
Explain that the electrical power depends on both voltage (ie energy transferred per charge) and current. Multiply current (I) and voltage (V) to introduce the relationship for electrical power (P). Show that P = IV for both kettles.
If students ask why the larger kettle has a higher current, calculate resistances by dividing voltage by current. The large kettle’s heating element has a lower resistance and so for the same ‘push’ (voltage) from the mains the current inside it will be larger.
Students define power as the rate at which energy is transferred and can use the relationship P = IV to calculate power for an electrical appliance.
This experiment was safety-checked in March 2020.