What is an electric field and how is it represented in Physics?

An electric field is a region around a charged particle where a force would be exerted on other charges. It is represented by electric field lines, which are imaginary lines that indicate the direction and strength of the field. The direction of the field is shown by the direction of the lines, and the density of the lines indicates the field's strength. This concept is crucial in the Cambridge International A Levels Physics curriculum.

How do electric field lines illustrate the strength and direction of an electric field?

Electric field lines provide a visual representation of the electric field's strength and direction. The lines point away from positive charges and towards negative charges, indicating the direction of the field. The closer the lines are to each other, the stronger the electric field in that region. This visualization helps students understand the concept of electric fields in the context of Cambridge International A Levels Physics.

What are the key properties of electric field lines?

Electric field lines have several key properties: they originate from positive charges and terminate at negative charges, they never intersect, and the density of the lines is proportional to the field's strength. These properties help in understanding the behavior of electric fields, a fundamental topic in the Cambridge International A Levels Physics syllabus.

How do electric field lines differ between point charges and uniform fields?

For point charges, electric field lines radiate outward in a radial pattern from positive charges and converge inward towards negative charges. In contrast, a uniform electric field, such as between two parallel plates, is represented by parallel, equally spaced lines. Understanding these differences is essential for mastering electric fields in the Cambridge International A Levels Physics course.

Why is the concept of electric fields and field lines important in Physics?

The concept of electric fields and field lines is crucial because it helps explain how charges interact at a distance without physical contact. This understanding is fundamental for analyzing electric forces and potential energy, which are key topics in the Cambridge International A Levels Physics curriculum. It also lays the groundwork for more advanced studies in electromagnetism and electronics.

Why is "Drawing field lines from negative to positive" a common mistake in Electric fields and field lines?

Why it happens: Confusion with current. How to avoid it: Field lines go from POSITIVE to NEGATIVE. Direction = force on POSITIVE test charge. Source: 9702 Examiner Reports 2022-2024

Cambridge International A Levels Physics (9702)

Electric fields and field lines

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Short Notes - Electric fields and field lines

Detailed Study Notes

Detailed notes on Electric Fields for Cambridge International A Levels Physics, covering key concepts, explanations, examples, and exam-focused revision points.

Electric Fields & Field Lines Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

$E = F/q$ . Field lines visualisation.

At a glance

$E = F/q$ .
Field lines from + to −.
Density = strength.
Units: N/C or V/m.
Direction: force on positive test charge.

What you’ll learn

Mapped to the Cambridge International A Level 9702 syllabus (2025-2027).

18.1.1 — Define electric field strength.
18.1.2 — Sketch field lines.

Field strength

$E = F/q$ .

Electric field. Region where a charge experiences a force.

Field strength $E = F/q$ where $q$ is test charge.

Direction. Force on POSITIVE test charge. Field points away from + charges, toward −.

Units. N/C = V/m.

Cambridge tip. Field strength is force per unit POSITIVE charge.

Figure 1 — Electric field strength is the force per unit positive charge

Both charges sit in the same field and feel forces of the same size, but in opposite directions. The field arrow always shows which way a positive charge would be pushed.

$E = F/q$ .
Direction: force on + charge.
N/C = V/m.

See the full worked example for electric fields and field lines →

Field lines

Visualise field.

Conventions.

Direction: force on + test charge.
Density: strength.
Start at + charge, end at − charge.

Positive point charge. Radial outward.

Two opposite charges (dipole). Curves from + to −.

Two like charges. Lines spread apart, meeting on perpendicular bisector.

Uniform field (parallel plates). Equally-spaced parallel lines.

Cambridge tip. Field lines NEVER cross.

Figure 2 — Radial fields around single point charges

The field of a single point charge is radial. The direction follows the convention: outwards from positive, inwards to negative, because that is the way a positive test charge would be pushed.

Figure 3 — Electric dipole field lines

Electric field lines start on positive charges, end on negative charges, never cross, and their density represents field strength.

Figure 4 — Two like charges and the neutral point between them

Two like charges repel, and their field lines do the same. Because no line can cross another, the lines from each charge curve away, leaving a neutral point on the perpendicular bisector.

The five field patterns you are expected to sketch.

Arrangement	What the lines look like
Single positive charge	Radial, pointing away from the charge
Single negative charge	Radial, pointing towards the charge
Two opposite charges	Curves running from the $+$ charge to the $-$ charge
Two like charges	Curves bending away from each other, with a neutral point between them
Parallel plates	Equally spaced parallel lines from the $+$ plate to the $-$ plate

In every case the lines start on positive charge, end on negative charge, and never cross.

- to − direction.
Density = strength.
Never cross.

See the full worked example for electric fields and field lines →

Quick recap

$E = F/q$ (force per + charge).
Lines + to −.
Density = strength.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

$E = F/q$ .
Field line conventions.

How it’s examined

E-field appears on every Paper 4. Most-tested: definition (4-5 marks), field-line sketches (5 marks).

Sources: Cambridge International A Level Physics 9702 syllabus (2025-2027); 9702 Examiner Reports 2022-2024; 9702/42 May/Jun 2024 question paper and mark scheme. Last reviewed 2026-05-11.

Take this whole topic with you

Step-by-step worked examples — Electric fields and field lines

Step-by-step solutions to past-paper-style questions on electric fields and field lines, written exactly the way a tutor would explain them at the board.

1 $E$ from force (4 marks)
Extended• Adapted from 9702/42 May/Jun 2024• E field
Question
A $+2.0 \times 10^{-6}$ C charge experiences 0.060 N force. Find $E$ . (4 marks)
Step-by-step solution
1. Step 1
  $E = F/q$ .
  $E = 0.060/(2.0 \times 10^{-6}) = 3.0 \times 10^4 \text{ N/C}$
Answer
$E = 3.0 \times 10^4$ N/C.
2Field lines (5 marks)
Extended• field lines
Question
Sketch field lines for (a) positive point charge, (b) two equal opposite charges, (c) uniform field. (5 marks)
Step-by-step solution
1. Step 1
  (a) Positive point charge: radial OUTWARD.
2. Step 2
  (b) Two opposite charges: curve from + to −, dense between.
3. Step 3
  (c) Uniform: parallel equally-spaced lines (e.g. between parallel plates).
Answer
(a) Radial outward. (b) Curves + to −. (c) Parallel.

Key Formulae — Electric fields and field lines

The formulae you need to memorise for electric fields and field lines on the Cambridge International A Level 9702 paper, with every variable defined in plain English and a note on when to use it.

Electric field strength
$E = \dfrac{F}{q}$
$F$
force on test charge $q$
When to use
Force per unit POSITIVE charge. Units: N/C = V/m.

Key Definitions and Keywords — Electric fields and field lines

Definitions to memorise and the exact keywords mark schemes credit for electric fields and field lines answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Electric field
Examiner keyword
Region where a charge experiences a force.
Electric field strength
Examiner keyword
Force per unit POSITIVE charge. $E = F/q$ . Units: N/C or V/m.

Common Mistakes and Misconceptions — Electric fields and field lines

The traps other students keep falling into on electric fields and field lines questions — taken from recent Cambridge International A Level 9702 examiner reports and mark schemes — and how to avoid them.

✕Drawing field lines from negative to positive
9702 Examiner Reports 2022-2024
Why it happens
Confusion with current.
How to avoid it
Field lines go from POSITIVE to NEGATIVE. Direction = force on POSITIVE test charge.