Power in circuits
- Power is the rate of energy transfer, measured in watts (W).
- P = V I: power = potential difference × current.
- P = I² R: power = current² × resistance.
Energy transferred
- The energy an appliance transfers depends on how long it is switched on and its power: E = P t.
- Work is done when charge flows in a circuit: E = Q V, energy (J) = charge (C) × potential difference (V).
- Everyday appliances transfer energy electrically to, for example, the kinetic store of a motor or the thermal store of a heating element.
Paying for electricity
- Energy used by appliances is often measured in kilowatt-hours (kWh): energy (kWh) = power (kW) × time (h).
- Cost = number of kWh × price per kWh.
- One kWh is 3 600 000 J.
Appliance ratings
- The power rating tells you how much energy the appliance transfers each second.
- Appliances with heating elements, such as kettles and heaters, have high power ratings.
Key terms
- Power
- The rate of energy transfer, measured in watts.
- Watt
- The unit of power; 1 W = 1 J/s.
- Kilowatt-hour
- The energy transferred by a 1 kW appliance in one hour.
- Power rating
- The maximum power at which an appliance transfers energy.
- Heating element
- A resistor designed to transfer energy to the thermal store of the surroundings.