What is Electric Power?
Electric power measures how fast electrical energy is converted into other forms, like light or heat. It links voltage, current and resistance, and tells you how much energy an appliance uses each second.
Electric power is the rate of electrical energy transfer: P = V·I, measured in watts (W). It can also be written as P = I²R or P = V²/R using Ohm's law.
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Step-by-step worked examples
A lamp is connected to a 220 V mains supply and draws a current of 0.5 A. Find its power.
P = V × I P = 220 × 0.5 P = 110 W
An electric heater rated at 1000 W is plugged into a 220 V outlet. Find the current it draws.
P = V × I → I = P/V I = 1000/220 I ≈ 4.55 A
A motor coil has resistance 5 Ω and carries a current of 2 A. Find the power dissipated.
P = I²R P = 2² × 5 P = 4 × 5 = 20 W
Flashcards
Quick quiz
Q1.A device uses 220 V and draws 3 A. What is its power?
Q2.Which formula for power uses resistance directly with current?
Q3.What is the SI unit of electric power?
Q4.A 60 W bulb operates at 12 V. What current does it draw?
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Common mistakes
Thinking power and energy are the same thing. — Correct: Power is the rate of energy use (W); energy is power × time (J or kWh).
Using P = V×I when only resistance and current are known. — Correct: Use P = I²R instead, or first find V = IR.
Forgetting to square the current in P = I²R. — Correct: Current must be squared: doubling current quadruples power.
Mixing up volts and watts as the same unit. — Correct: Volts measure potential difference; watts measure power.
FAQ
What is the formula for electric power?
P = V × I, where power is voltage multiplied by current, measured in watts.
How do you calculate electric power from resistance?
Use P = I²R or P = V²/R, both derived from combining P = VI with Ohm's law.
What are some real examples of electric power?
A phone charger might use 5 W, a light bulb 60 W, and an electric kettle around 2000 W.
How is electric power different from electric energy?
Power is the rate of energy transfer (W); energy is power used over time, in joules or kWh.