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Cambridge IGCSE Physics · 0625

Chapter 4: Electricity and magnetism — Part 6

Topic 4.2.5 · Electrical energy and electrical power

Energy transfer in a circuit

An electric circuit transfers energy from a source (cell, battery or mains) to the components, then into the surroundings (light, thermal energy, kinetic energy of a motor).

Energy flow in a circuit: electrical store in a cell or the mains, transfer to components (lamp, heater, motor), then to the surroundings as light, thermal energy or kinetic energy.
Diagram 1: Energy flow in a circuit: electrical store in a cell or the mains, transfer to components (lamp, heater, motor), then to the surroundings as light, thermal energy or kinetic energy.

Power and energy

Electrical power: P = IV with P in watts (W).

Electrical energy: E = IVt with E in joules (J) and t in seconds.

A lamp takes a current of 0.25 A from a 230 V supply. Calculate its power.

Worked example: electrical power is 0.25 times 230 equals 58 watts
P = IV = 58 W (2 s.f.).

A heater takes 2.0 A from a 12 V battery for 30 s. Calculate the energy transferred.

Worked example: electrical energy is 2.0 times 12 times 30 equals 720 joules
E = IVt = 720 J. Keep t in seconds for joules.

The kilowatt-hour

The kilowatt-hour (kWh) is the energy transferred when 1 kW works for 1 hour. It is the domestic energy unit on an electricity meter.

Energy in kWh = power in kW × time in hours. Cost = energy in kWh × price per kWh.

1 kWh = 3.6 × 106 J (because 1000 W × 3600 s).

A 2.0 kW heater is switched on for 3.0 hours. Calculate the energy transferred, in kWh.

Worked example: energy in kilowatt-hours is 2.0 times 3.0 equals 6.0 kilowatt-hours
E = 2.0 kW × 3.0 h = 6.0 kWh.

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