Work, power and the National Grid

Work done

Mechanical work is done whenever a force moves an object through a distance in the direction of the force. The work done equals the energy transferred.

W = F × d, with W in joules (J), F in newtons (N), d in metres (m).

If you push a 50 N box 3 m across the floor, you do 50 × 3 = 150 J of work — that 150 J goes into the kinetic energy of the box and friction-heat.

Power

Power is the rate of energy transfer (or work done):

P = E / t, with P in watts (W), E in joules, t in seconds. 1 W = 1 J/s.

A 60 W bulb transfers 60 J of energy every second from electrical to light + heat.

Power in electrical circuits

For an electrical device on a steady pd, P = V × I (volts × amps). A kettle running at 230 V drawing 10 A uses P = 230 × 10 = 2300 W = 2.3 kW.

Energy in kilowatt-hours

Energy bills measure energy in kilowatt-hours (kWh): energy used (kWh) = power (kW) × time (h). A 2.3 kW kettle on for 0.1 hours uses 2.3 × 0.1 = 0.23 kWh.

Transformers and the grid

Transformers change ac voltage. Step-up transformers raise pd before transmission; step-down transformers lower it for homes. For an ideal transformer, V_p / V_s = N_p / N_s, where N is the number of turns. Higher transmission voltage means lower current and far less energy lost as heat (P_loss = I²R).

CCEA tip

W = Fd and P = E/t look almost the same. W is force × distance; P is energy ÷ time. Drawing a quick units box (J = N·m, W = J/s) on the exam paper avoids the confusion.