Electrical Energy and Electrical PowerCambridge IGCSE Physics: Revision notes

Section 1

How Do Circuits Transfer Energy?

Electric circuits transfer energy from a source of electrical energy (such as a cell, battery or mains supply) to the components in the circuit, and then out into the surroundings.

In a lamp, electrical energy is transferred to light and heat.
In a heater, electrical energy is transferred to thermal energy (heat).
In a motor, electrical energy is transferred to kinetic energy (and some heat).

Exam tip

Always describe energy as being 'transferred', not 'used up' or 'lost' — energy is conserved, just converted into other, often less useful, forms.

Section 2

Electrical Power

Electrical power (P) is the rate at which a component transfers energy.

P = I x V

where P is power in watts (W), I is current in amperes (A), and V is potential difference in volts (V).

A higher-power appliance transfers energy more quickly.

Key termselectrical power

Example

A kettle operating at 230 V and drawing 10 A has a power of P = IV = 230 x 10 = 2300 W.

Section 3

Electrical Energy

Electrical energy transferred (E) depends on the power of the component and how long it operates for.

E = I x V x t

where E is energy in joules (J), I is current in amperes (A), V is potential difference in volts (V), and t is time in seconds (s).

This equation can also be written as E = P x t, since P = IV.

Common mistake

Time must be in seconds when using E = IVt to get an answer in joules — convert minutes or hours to seconds first.

Section 4

The Kilowatt-Hour and the Cost of Electricity

For everyday billing, energy is often measured in kilowatt-hours (kWh) rather than joules, because household energy use involves very large numbers of joules.

One kilowatt-hour is the energy transferred by a 1 kW appliance running for 1 hour.
Cost is calculated as: Cost = number of kWh used x price per kWh.
Number of kWh = power (in kW) x time (in hours).

Key termskilowatt-hour

Example

A 2 kW heater running for 3 hours uses 2 x 3 = 6 kWh. At 30 cents per kWh, this costs 6 x 0.30 = 1.80 in cost units.

Must Know

Circuits transfer energy from the source to the components and then into the surroundings.
Electrical power: P = IV, measured in watts.
Electrical energy: E = IVt (or E = Pt), measured in joules.
The kilowatt-hour (kWh) is a practical unit of energy used for billing.
Cost of electricity = number of kWh x price per kWh; kWh = power (kW) x time (hours).

That's the notes covered.

Carry on to the next subtopic.