A charge of +2.0 nC placed at a point experiences a force of 6.0 × 10⁻⁵ N. What is the electric field strength there?

E = F/Q = 6.0 × 10⁻⁵ ÷ (2.0 × 10⁻⁹) = 3.0 × 10⁴ N C⁻¹.

A charge of −4.0 μC sits in a field of strength 5.0 × 10³ N C⁻¹. What is the magnitude and direction of the force on it?

F = EQ = 5.0 × 10³ × 4.0 × 10⁻⁶ = 2.0 × 10⁻² N, directed opposite to the field.

Two different charges are placed in turn at the same point in a field. Does the field strength at that point differ between them?

No, because field strength is a property of the field at that point and does not depend on the charge placed there.

What happens to the force on a fixed charge if the field strength at its position is doubled?

The force also doubles, since F = EQ and Q is unchanged.

How is the strength of an electric field shown on a field-line diagram?

By the spacing of the lines: the more closely packed the lines, the stronger the field.

What is meant by a uniform electric field?

One in which the field strength has the same magnitude and direction at every point, shown by parallel, equally spaced field lines.

Edexcel A-Level Physics 9PH0

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Electric and Magnetic Fields

Field strength calculations and field diagrams

2 things to know
Papers 1 and 3

Overview

Recall questions on finding force from field strength, whether the field depends on the charge placed in it, uniform fields, and how strength is shown by field-line spacing.

What you need to know

Answer each one before you reveal it.

Your recallNot started

Numbers

In what way is the definition E = F/Q analogous to the definition of gravitational field strength?

Numbers

How is field strength shown on a field-line diagram, does the field at a point depend on the charge placed there, and what happens to the force on a fixed charge if the field strength doubles?

Questions from this subtopic

Each one with the model answer underneath.

1
A charge of +2.0 nC placed at a point experiences a force of 6.0 × 10⁻⁵ N. What is the electric field strength there?
Model answer
E = F/Q = 6.0 × 10⁻⁵ ÷ (2.0 × 10⁻⁹) = 3.0 × 10⁴ N C⁻¹.
2
A charge of −4.0 μC sits in a field of strength 5.0 × 10³ N C⁻¹. What is the magnitude and direction of the force on it?
Model answer
F = EQ = 5.0 × 10³ × 4.0 × 10⁻⁶ = 2.0 × 10⁻² N, directed opposite to the field.
3
Two different charges are placed in turn at the same point in a field. Does the field strength at that point differ between them?
Model answer
No, because field strength is a property of the field at that point and does not depend on the charge placed there.
4
What happens to the force on a fixed charge if the field strength at its position is doubled?
Model answer
The force also doubles, since F = EQ and Q is unchanged.
5
How is the strength of an electric field shown on a field-line diagram?
Model answer
By the spacing of the lines: the more closely packed the lines, the stronger the field.
6
What is meant by a uniform electric field?
Model answer
One in which the field strength has the same magnitude and direction at every point, shown by parallel, equally spaced field lines.

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