Power and Energy Transfers

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CognitoPower and energy calculations

Key facts & equations

Power formula and units

Power = energy transferred / time. P = E/t. Units: watts (W).
1 W = 1 J/s. Kilowatt: 1 kW = 1000 W. Megawatt: 1 MW = 1,000,000 W.
Power = work done / time = force × velocity (for constant speed).
A device with higher power rating transfers more energy per second.
E = Pt (energy = power × time). E in joules if P in W and t in seconds.

Kilowatt-hours and electricity bills

Kilowatt-hour (kWh): energy transferred by a 1 kW device in 1 hour.
E (kWh) = power (kW) × time (hours). 1 kWh = 3,600,000 J = 3.6 MJ.
Cost of electricity = energy (kWh) × cost per kWh (pence).
E.g. 2 kW kettle, 0.1 hours (6 min): E = 2 × 0.1 = 0.2 kWh. At 30p/kWh: cost = 0.2 × 30 = 6p.
Reducing energy use = lower bills and lower carbon footprint.

Electrical power and current

P = IV (power = current × voltage). P = I²R (using resistance). P = V²/R.
E.g. toaster: 230 V, 4 A. P = 230 × 4 = 920 W.
Rearranged: I = P/V (used to find fuse rating). E.g. 2000 W kettle at 230 V: I = 2000/230 ≈ 8.7 A → use 13 A fuse.
Appliances with higher current require thicker wires (lower resistance).

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

3 markskWh calculationAQA 8463 P2 style 🔢 Calculator

A 3 kW electric shower is used for 12 minutes each day for 30 days. Calculate: (a) the total energy used in kWh, (b) the cost at 34p per kWh. (3 marks)

Hint: (a) Convert minutes to hours. E = P(kW) × t(hours). Multiply by number of days. (b) Cost = kWh × pence per kWh.

+30 XP

1 markPower definitionAQA 8463 style

Which of the following correctly defines power?

A The total amount of energy a device uses

B The rate of energy transfer (energy per second)

C Force multiplied by distance

D Current multiplied by resistance

3 marksFuse ratingAQA 8463 style 🔢 Calculator

A 1400 W toaster is connected to a 230 V mains supply. Calculate the current drawn and determine the appropriate fuse from: 3 A, 5 A or 13 A. (3 marks)

Hint: I = P/V. Choose the smallest fuse rated above this current. A fuse must be higher than the operating current but not excessively so.

+30 XP

1 markkWh to joulesAQA 8463 style

How many joules is 1 kWh?

A 1000 J

B 3600 J

C 3,600,000 J

D 1,000,000 J

Power and Energy Transfers

Power formula and units

Kilowatt-hours and electricity bills

Electrical power and current

Module complete! 🎉