Electric Fields

Objectives

Lesson outcomes

Define electric field strength as force per unit positive charge.
Use F = qE for the force on a charge in an electric field.
Represent electric fields using field lines.
Use E = delta V / delta d for the uniform field between charged parallel plates.
Describe the motion of charged particles in a uniform electric field.

Syllabus

CIE 9702 syllabus points

5 linked

18.1.1 understand that an electric field is an example of a field of force and define electric field as force per unit positive charge
18.1.2 recall and use F = qE for the force on a charge in an electric field
18.1.3 represent an electric field by means of field lines
18.2.1 recall and use E = ∆V / ∆d to calculate the field strength of the uniform field between charged parallel plates
18.2.2 describe the effect of a uniform electric field on the motion of charged particles

Definitions

Required definitions

Electric field
a region where a charge experiences a force; electric field strength is force per unit positive charge.

Lesson Notes

Student guidance and lesson notes

Overview

This lesson introduces electric fields as the electric equivalent of a field of force. You will define electric field strength, use field-line diagrams, and describe how uniform fields between parallel plates affect charged particles.

What You Need to Know

An electric field is a field of force around charged objects.
Use electric field strength to compare the force on a positive test charge at different points.
The force on a charge in an electric field is F = qE.
Electric field lines show the direction of the force on a positive test charge.
A uniform field has constant field strength and parallel, equally spaced field lines.
Between charged parallel plates, E = delta V / delta d.
Positive and negative charges accelerate in opposite directions in the same electric field.

How to Work Through It

Start by comparing gravitational field strength with electric field strength.
Practise using F = qE with charge in coulombs.
Sketch field lines for single charges, pairs of charges, and parallel plates.
Use E = delta V / delta d for charged parallel plates and describe charged-particle motion.

Check Your Understanding

What does electric field strength measure?
Why are electric field lines defined using a positive test charge?
How does a negative charge move compared with a positive charge in the same uniform field?
What happens to field strength between parallel plates if the same p.d. is applied across a smaller separation?

Common Mistakes

Treating electric field strength as a force rather than force per unit charge.
Forgetting that q must be in coulombs.
Drawing field lines as if they show the path of a charge rather than the field direction.
Ignoring the sign of the charge when deciding force direction.

Next Steps

Keep field-line diagrams and uniform-field equations secure before using Coulomb’s law.
Be ready to compare the electric point-charge model with gravitational point-mass fields.