Electrical power
The rate at which energy is transferred by a circuit. Power = potential difference x current (P = VI), or P = I squared x R. It is measured in watts (W).
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Analogy
A bigger push (voltage) and a bigger flow (current) both mean more energy delivered each second.
Example
V = 12 V, I = 2 AP = V x I = 12 x 2P = 24 WIn 10 s: E = 24 x 10 = 240 J
In practice
A 230 V hairdryer drawing 5 A has a power of 1,150 W. Using it for an hour transfers a lot more energy than an LED bulb.
Practice
Flashcards
- How do you calculate electrical power?Power = potential difference x current.
Interview questions
- A 230 V kettle draws a current of 8 A. Calculate its power and its resistance.
- Power = V x I = 230 x 8 = 1,840 W
- Resistance = V / I
- R = 230 / 8 = 28.75 ohms
Questions people ask
How do I calculate the energy used by an appliance?
Multiply its power by the time it is on. Energy (J) = power (W) x time (s). A 2,000 W heater running for 60 seconds transfers 120,000 J. Electricity bills use kilowatt-hours instead: a 2 kW heater running for 3 hours uses 6 kWh.
Why do power cables carry electricity at high voltage?
For a given power, a higher voltage means a smaller current, since P = VI. A smaller current causes less heating in the cables, because heat wasted depends on current squared times resistance. Using high voltage over long distances therefore wastes much less energy.