Electrical Power

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CognitoElectrical power calculations

Key facts & equations

Electrical power formulas

P = IV (power = current × voltage). Most commonly used.
P = I²R (substituting V = IR). Useful when V is unknown.
P = V²/R (substituting I = V/R). Useful when I is unknown.
Units: watts (W). 1 W = 1 J/s.
Energy: E = Pt = IVt. Units: joules (J).

Worked examples

Kettle: 230 V, 8.7 A → P = 230 × 8.7 = 2000 W (2 kW).
LED: 3 W, 12 V → I = P/V = 3/12 = 0.25 A.
Resistor: R = 100 Ω, I = 2 A → P = I²R = 4 × 100 = 400 W.
To find energy: E = Pt = 2000 W × 120 s = 240,000 J.
Fuse rating: I = P/V. Choose smallest fuse above this value.

Energy transfer in circuits

Resistors transfer electrical energy to thermal energy (heat).
Motors: electrical to kinetic. Lamps: electrical to light (and heat).
Total energy = useful energy + wasted energy.
Efficiency = useful power output / total power input.
National grid uses high voltages to reduce current → reduces I²R losses in cables.

Exam questions — 4 questions · 10 marks · AQA 8463 style

3 marksPower calculationsAQA 8463 P2 style 🔢 Calculator

A vacuum cleaner operates at 230 V with a resistance of 52.9 Ω. (a) Calculate the current. (b) Calculate the power. (c) Calculate the energy used in 10 minutes. (3 marks)

Hint: (a) I=V/R. (b) P=IV. (c) Convert minutes to seconds, then E=Pt.

+30 XP

1 markHigh voltage in national gridAQA 8463 style

Why does the National Grid transmit electricity at high voltage?

A High voltage increases the power available to consumers

B High voltage increases the frequency of the AC supply

C High voltage reduces current, which reduces I²R power losses in the transmission cables

D High voltage is needed to operate transformers

3 marksEnergy in timeAQA 8463 style 🔢 Calculator

A toaster uses 1200 W for 3 minutes. (a) Calculate the energy used in joules. (b) Calculate the charge that flows if the current is 5.2 A. (2 marks)

Hint: (a) Convert minutes to seconds. E = Pt. (b) Q = It (charge = current × time).

+30 XP

Electrical Power

Electrical power formulas

Worked examples

Energy transfer in circuits

Module complete! 🎉