# Gravitational, electric and magnetic fields: Unit 3.4(b)

> define gravitational and electric field strength as force per unit mass or charge

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Specification statement (Unit 3.4(b)): define gravitational and electric field strength as force per unit mass or charge

## Explanation

1. **Gravitational field strength** g = force per unit mass = F/m (N kg⁻¹). **Electric field strength** E = force per unit positive charge = F/Q (N C⁻¹).
2. Field lines show the direction of the force on a test mass or positive test charge; their spacing shows the strength.
3. Between parallel plates the electric field is uniform: E = V/d.

## Worked example

An electron (charge −1.60 × 10⁻¹⁹ C, mass 9.11 × 10⁻³¹ kg) is in a uniform field of 5000 N C⁻¹. Find the force on it and its acceleration.

1. F = EQ = 5000 × 1.60 × 10⁻¹⁹ = 8.0 × 10⁻¹⁶ N, opposite to the field direction (the electron is negative).
2. a = F/m = 8.0 × 10⁻¹⁶ / 9.11 × 10⁻³¹ = 8.8 × 10¹⁴ m s⁻².

Answer: 8.0 × 10⁻¹⁶ N; 8.8 × 10¹⁴ m s⁻².

## Check

Two parallel plates 2.0 cm apart have a p.d. of 500 V. What is the field strength between them?

- 10 N C⁻¹
- 250 N C⁻¹
- 2.5 × 10⁴ N C⁻¹
- 1.0 × 10⁴ N C⁻¹

The answer and its explanation are shown in the app once you choose an option.

## Apply it

Explain why the gravitational field near the Earth's surface can be treated as uniform when a ball is thrown, but not for a satellite.

The model answer is shown in the app after you have tried it.

## Related

- [Gravitational, electric and magnetic fields lesson](https://revise-gray.vercel.app/lesson?subject=wjec-alevel-physics&topic=wjec-alevel-physics.fields)
- [Gravitational, electric and magnetic fields in the library](https://revise-gray.vercel.app/library?topic=wjec-alevel-physics.fields)