Electric Field Strength (AQA A Level Physics)

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7 November 2024

An electric field is a region of space in which an electric charge experiences a force
The electric field strength at a point is defined as:
The force per unit charge experienced by a small positive test charge placed at that point
The electric field strength can be calculated using the equation:

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

Where:
- E = electric field strength (N C⁻¹)
- F = electric force on the charge (N)
- Q = magnitude of the charge (C)

Note that the definition specifies that a positive test charge is used
This sets a clear convention for the direction of an electric field, for example, in a field of strength $E$ :
- A positive charge $+ Q$ experiences a force $E Q$ in the direction of the field
- A negative charge $- Q$ experiences a force $E Q$ in the opposite direction

Hence, electric field strength is a vector quantity and is always directed:
- Away from a positive charge
- Towards a negative charge

A charged particle experiences a force of 0.3 N at a point where the magnitude of electric field strength is 3.5 × 10⁴ N C⁻¹.

Calculate the magnitude of the charge on the particle.

Answer:

Step 1: Write down the equation for electric field strength

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

Step 2: Rearrange for charge Q

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

Step 3: Substitute in the values and calculate:

$Q = \frac{0.3}{3.5 \times 10^{4}} = 8.571 \times 10^{- 6} = 8.6 \times 10^{- 6}$ C (2 s.f.)

the (exam) results speak for themselves:

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