Electric field strength (AQA A-Level Physics): Revision Notes

7.3.2 Electric field strength

Definition

Electric field strength can be calculated in three different ways depending on the type of field:

1. General formula for electric fields:

$$E = \frac{F}{Q}$$

Where:

• $E$ = electric field strength,
• $F$ = force on the charge,
• $Q$ = magnitude of the charge.

2. For uniform fields created by parallel plates:

$$E = \frac{V}{d}$$

Where:

• $V$ = potential difference across the plates,
• $d$ = distance between the plates.

3. For radial fields (fields that radiate from a point charge):

$$E = \frac{1}{4 \pi \varepsilon_0} \frac{Q}{r^2}$$

Where:

• $\varepsilon_0$ = permittivity of free space,
• $r$ = distance from the charge creating the field.

Types of Electric Fields

Electric fields, like gravitational fields, can be uniform or radial and are often represented by field lines:

• Uniform Field: A field with evenly spaced parallel lines, showing a constant electric force in one direction across the field.

• Radial Field: A field with lines radiating outward or inward from a point charge. The field strength decreases with distance from the charge, as represented by the spacing between lines increasing further from the centre.

Calculating Work Done in an Electric Field

When moving a charged particle between two points in a uniform electric field, you can calculate the work done using the formula:

$$\text{Work done} = Q \Delta V$$

Where:

• $Q$ = charge of the particle,
• $\Delta V$ = potential difference between the two points.