Electric field strength
Electric Fields - OCR A-Level Physics
Key Definition
Electric field strength
The force per unit positive charge experienced by a small test charge placed at that point. Measured in N C⁻¹ or equivalently V m⁻¹.
The force per unit positive charge experienced by a small test charge placed at that point. Measured in N C⁻¹ or equivalently V m⁻¹.
$$\begin{aligned}
E &= \frac{F}{Q} \\
&= \frac{V}{d}
\end{aligned}$$
$$E = \frac{F}{Q}$$
- Electric field strengthThe force per unit positive chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). at a point in an electric field. Measured in N C⁻¹ or V m⁻¹. is a vectorA quantity with both magnitude and direction (e.g. force, velocity, displacement). quantity - it has both magnitude and direction.
- The direction of E is defined as the direction a positive test chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C). would move if placed in the field.
- A 'small test chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).' is used in the definition to ensure it does not disturb the field it is measuring.
- Field lines point from positive to negative. Closer spacing indicates a stronger field.
Common Mistake
MEDIUM
Wrong: Electric field strengthThe force per unit positive charge at a point in an electric field. Measured in N C⁻¹ or V m⁻¹. is measured in N - it is just the force.
Right: E is force per unit charge (N C⁻¹). The force on a specific charge Q is $F = EQ$.
Right: E is force per unit charge (N C⁻¹). The force on a specific charge Q is $F = EQ$.