Power in an electrical circuit represents the rate of energy transfer — how many joules of electrical energy are converted into other forms (heat, light, sound, kinetic) every second.
The equation P = I²R reveals something important: power increases with the square of the current. Doubling the current quadruples the power dissipated as heat in a resistor. This is why:
- High-current appliances (showers, kettles) are very expensive to run
- National Grid transmits electricity at very high voltage and low current to minimise energy losses in cables (P = I²R — lower I = far less heating loss)
- Fuses are rated by current — too much current means dangerous overheating
The equation P = V²/R shows that at a fixed voltage (like mains 230 V), lower resistance means more power. This is why a kettle (low resistance heating element) is much more powerful than an LED bulb (high resistance).
Quick Check: A toaster operates at 230 V with a current of 4 A. Calculate its power rating.
P = I × V = 4 × 230 = 920 W.
This how it works covers How It Works: Why High-Current Appliances Cost More within Electrical Power & Energy for GCSE Physics. Revise Electrical Power & Energy in Electricity for GCSE Physics with 15 exam-style questions and 30 flashcards. This topic shows up very often in GCSE exams, so students should be able to explain it clearly, not just recognise the term. It is section 6 of 15 in this topic. Use this how it works to connect the idea to the wider topic before moving on to questions and flashcards.
Practice questions for Electrical Power & Energy
What is the unit of electrical power?
A television has a power rating of 120 W and is left on for 3 hours. Calculate the energy transferred by the television in this time. Use the equation: energy transferred = power x time. Give your answer in joules.