Electric power

The idea

Electric power, the rate at which electric energy is dissipated or consumed in a circuit, is P = VI = I²R = V²/R.

Fast or slow

Two heaters are switched on for one minute. One uses a lot of energy in that minute and the other uses only a little. Work done in a second is called power.

Which statement describes electric power?

• The rate at which electric energy is consumed in a circuit
• The total energy stored in the battery
• The charge of one electron

Three formulas, one answer

Pick a battery and a resistance. Check the three values of the power.

Power

However you worked it out, the answer was the same. The heat produced each second in the resistor is the electric power.

Electric power is the rate at which electric energy is dissipated or consumed in a circuit: P = VI = I²R = V²/R.

Notes

Electric power is the rate at which electric energy is dissipated or consumed in a circuit: P = VI = I²R = V²/R.

Check yourself

The potential difference across a bulb is 12 V and the current through it is 3 A. What is its power, in W (the unit of power, explained next)?

Answer: 36 W

P = VI = 12 V × 3 A = 36 W.

A current of 3 A flows through a resistor of 4 Ω. What power is used in it, in W?

Answer: 36 W

P = I²R = 3 × 3 × 4 = 36 W.

A potential difference of 12 V is applied across a 4 Ω resistor. What power is used in it, in W?

Answer: 36 W

P = V²/R = 12 × 12 divided by 4 = 144 ÷ 4 = 36 W.

Which of these is the meaning of electric power?

• The total number of electrons in the wire. Power is about energy, not a count of electrons. It is the rate at which electric energy is consumed.
• The size of the battery. Power is the rate at which electric energy is dissipated or consumed in a circuit.
• The rate of consumption of electric energy — correct. Yes!