Electrical power and energy

The power of an appliance is the energy it transfers each second, in watts (W). You must recall both equations:

power = potential difference × current, or P = VI

power = current² × resistance, or P = I²R

Example 1: a 2300 W kettle runs on 230 V mains. Find the current.

1. Rearrange: I = P ÷ V
2. I = 2300 ÷ 230 = 10 A

Example 2: a current of 2 A flows through a 50 Ω heater.

1. P = I²R = 2² × 50
2. P = 4 × 50 = 200 W

Appliances transfer energy from the mains or a battery. A hair dryer fills the kinetic store of its fan motor and the thermal store of its heating element. The energy transferred depends on the appliance's power and how long it is on.

You must recall both equations:

energy transferred = power × time, or E = Pt

energy transferred = charge flow × potential difference, or E = QV

Example 1: a 2000 W hair dryer is used for 5 minutes. How much energy does it transfer?

1. Convert: 5 minutes = 300 s
2. E = Pt = 2000 × 300
3. E = 600 000 J (600 kJ)

Example 2: 30 C of charge flows through a 12 V battery. How much energy is transferred?

1. E = QV
2. E = 30 × 12
3. E = 360 J