# Electric power

Rate of energy use in a circuit, P = IV = I squared R = V squared / R. The unit is the watt.

Physics (university) · Part 08: Circuits and magnetism · Card 4 of 12 · Last updated 2026-10-19

## Analogy

How fast a battery drains.

## Example

**Power use**

P = I V
P = 3 x 12
P = 36 W

## In practice

A 12 V, 3 A device uses 36 W, or 0.036 kWh per hour.

## Practice

### Flashcards

- **Write the power formulas for a resistor.** P = IV = I^2 R = V^2/R.

### Interview questions

**A 12 V battery is connected to a 6 ohm resistor in series with a parallel pair of 6 ohm and 3 ohm resistors. Find the current from the battery and the power used.**

- Parallel pair is 2 ohm
- Total is 8 ohm
- I = 1.5 A, P = 18 W

## Questions people ask

### How do I work out the cost of running a device?

Energy in kilowatt-hours equals power in kilowatts times hours. A 2 kW kettle used for 0.1 hours uses 0.2 kWh. Multiply by the price per kWh to get the cost. Power companies bill in kWh, and 1 kWh equals 3.6 million joules, so it is a large unit.

### Why are power lines high voltage?

Power lost in a cable is I squared R. For the same delivered power P = IV, raising V lowers I, which cuts losses sharply. Doubling the voltage halves the current and reduces heating loss by a factor of four. Transformers then step the voltage down for homes.

## Related concepts

- [Ohm's law](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-2)
- [Series and parallel resistors](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-3)
- [Electric current](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-1)
- [Kirchhoff's junction rule](https://studyvisually.com/learn/science/physics-university/physics-university-part-8/physics-university-part-8-concept-5)

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