7.3.3

# Electric Field Strength 2

Test yourself

## A Uniform Electric Field

A uniform field is one which has the same strength at every point in the field.

### Uniform field

• A uniform field has the same strength everywhere, it can be made from parallel charged plates.
• If two charged plates are held a distance apart the field between them will be uniform
• The field lines will travel straight from the positive plate into the negative plate.

### Calculating the field

• If two plates are held apart with different charges they will have a voltage difference between them. The equation for the field strength is:
• Field strength = voltage drop ÷ distance between the plates
• $E=V/d$

## Work done in an Electric Field

Two oppositely charged parallel plates provide a uniform electric field. Uniform fields have field lines that are equally spaced, straight and parallel.

### Moving between charged plates

• The work done in moving a positively charged particle from the negative plate to the positive plate is:
• $W= F \times d$
• Where F is a constant force acting on the charged particle and d is the separation of the plates.

### Derivation

• The work done in moving a positively charges particle is also, by definition:
• $W = Q \times \Delta V$
• These two expressions are equal to each other:
• $F\times d=Q\times \Delta V$

### Derivation 2

• Rearranging this gives:
• $\frac{F}{Q}=\frac{\Delta V}{d}$
• Because $\frac{F}{Q}$ is defined as electric field strength, electric field strength between two parallel plates is:
• $E = \frac{\Delta V}{d}$