Electric field concepts

CIE A-Level Physics· 15 min read

1. Core Definitions: Electric Field and Strength★★☆☆☆⏱ 4 min

An electric field describes the electrostatic influence a charged object has on other charges in the space around it. We quantify how strong the field is at any point using electric field strength.

📘 Definition

Electric field strength

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

The force per unit positive charge acting on a very small test charge placed at the point.

Example:

If a 1 C charge experiences 10 N force, $E = 10 N C^{- 1}$

📐 Worked Example

A test charge of $3.0 \times 1 0^{- 6} C$ experiences an electrostatic force of $1.2 \times 1 0^{- 2} N$ in an electric field. Calculate the electric field strength at that point.

1
Recall the definition of electric field strength:
2
$E = \frac{F}{Q}$
3
Substitute the given values: $F = 1.2 \times 1 0^{- 2} N$ , $Q = 3.0 \times 1 0^{- 6} C$
4
$E = \frac{1.2 \times 10^{-2}}{3.0 \times 10^{-6}} = 4000 \, \text{N C}^{-1}$
5
This can also be written as $4000 V m^{- 1}$ , an equivalent unit accepted by CIE.

Exam tip:

CIE mark schemes accept both $N C^{- 1}$ and $V m^{- 1}$ as correct units for electric field strength, but always match units to the calculation context.

2. Electric Field Lines★★☆☆☆⏱ 5 min

Field lines are a standard visual representation of electric fields that show both direction and relative strength of the field at any point.

📘 Definition

Electric field line rules

Key rules for drawing electric field lines for CIE exams

The tangent to a field line at any point gives the direction of force on a small positive test charge at that point
The spacing between lines shows relative strength: closer lines = stronger field
Field lines never cross each other (two force directions at one point is impossible)
Field lines start on positive charges and end on negative charges (or infinity for isolated charges)

📐 Worked Example

Describe the electric field pattern around an isolated negative point charge.

1
Field lines are drawn radially, pointing inwards towards the negative point charge
2
Lines are more closely spaced near the charge, and get further apart as distance from the charge increases
3
All lines end on the negative charge, no lines cross, and strength decreases with distance from the charge

3. Uniform vs Radial Electric Fields★★★☆☆⏱ 4 min

CIE A-Level Physics most commonly tests two types of electric field: uniform and radial (non-uniform). Both have distinct properties and field line patterns.

Property	Uniform Field	Radial Field
Source	Between two oppositely charged parallel plates	Around a point charge/spherical charged object
Field strength	Constant everywhere (edge effects ignored)	Decreases with distance from source
Field line pattern	Parallel, equally spaced straight lines	Radial lines, spacing increases with distance

📐 Worked Example

Compare the electric field strength between two oppositely charged parallel plates and around an isolated positive point charge.

1
Between the parallel plates, the electric field strength is uniform: it has the same magnitude and direction at all points between the plates (excluding edges)
2
Field lines are parallel and equally spaced, showing constant strength
3
Around the positive point charge, the field is radial: electric field strength decreases as distance from the charge increases
4
Field lines are radial, spacing increases with distance, showing decreasing strength

4. The Test Charge Requirement★★★☆☆⏱ 3 min

When we define electric field strength, we specify it uses a very small positive test charge. This requirement is commonly tested in explanation questions.

📘 Definition

Test charge

A positively charged object with very small magnitude of charge, used to measure the electric field at a point.

📐 Worked Example

Explain why electric field strength is defined in terms of a very small test charge.

1
A large test charge has its own electric field
2
This field will exert a force on the original source charge, causing it to move and alter the original electric field we want to measure
3
A very small test charge has negligible charge, so its own field does not distort the original field, giving an accurate measurement

Exam tip:

This explanation is a very common 2-mark question in CIE structured papers, learn the key point about not distorting the original field.

5. Common Pitfalls

Wrong move:

Drawing field lines starting at negative charges and ending at positive charges

Why:

Field lines show the direction of force on a positive test charge, which is attracted to negative charges and repelled by positive charges

Correct move:

Always draw field lines starting on positive charges and ending on negative charges (or infinity for isolated charges)

Wrong move:

Allowing field lines to cross each other in diagrams

Why:

If two lines crossed, this would imply two different directions of force at the same point, which is impossible

Correct move:

Draw lines that never cross; mark a null point where the field is zero between two like charges

Wrong move:

Writing units of $C N^{- 1}$ for electric field strength

Why:

Electric field strength is force per unit charge $E = F / Q$ , so the unit is newtons per coulomb, not coulombs per newton

Correct move:

Remember $E = F / Q$ , so units are $N C^{- 1}$ or the equivalent $V m^{- 1}$

Wrong move:

Assuming electric field is stronger closer to the plates in a uniform field

Why:

A uniform field by definition has constant strength everywhere between the plates, ignoring edge effects

Correct move:

Recognise equally spaced parallel field lines mean constant uniform electric field strength

6. Quick Reference Cheatsheet

Concept	Key Definition/Formula	Core Property
Electric field	Region where charge experiences force	Non-contact electrostatic force
Electric field strength	$E = F / Q$	Units: $N C^{- 1} = V m^{- 1}$
Field lines	Tangent = force on +ve test charge	Start +ve, end -ve, never cross
Uniform field	Constant $E$ everywhere	Parallel equally spaced lines, between plates
Radial field	$E$ decreases with distance $r$	Radial lines around point charges

When this came up on past exams

AI-estimated based on syllabus patterns — cross-check with official past papers for accuracy. Use only as revision-focus signals.

2022 · 2
Field drawing and definition question
2023 · 1
Multiple choice on field properties
2024 · 2
Field strength calculation question

What's Next

Mastery of electric field concepts is the foundation for all further topics in A2 electrostatics. Next you will learn Coulomb's law, which allows you to calculate the magnitude of electric field strength around point charges, followed by electric potential and potential energy in electric fields. These concepts are then applied to capacitance, energy stored in capacitors, and motion of charged particles in electric fields, all of which are common exam topics worth multiple marks in both paper 1 and paper 2.