What is a uniform electric field and how is it represented?

A uniform electric field is a field in which the electric force is constant in magnitude and direction at every point. It is typically represented by equally spaced, parallel lines, indicating that the field strength does not change with position. This concept is crucial in Cambridge International A Levels Physics as it simplifies the analysis of electric forces and potential energy.

How is the strength of a uniform electric field calculated?

The strength of a uniform electric field (E) is calculated using the formula E = V/d, where V is the potential difference between two points and d is the distance between those points. This formula is essential for solving problems related to electric fields in the Cambridge International A Levels Physics curriculum.

What are some real-world applications of uniform electric fields?

Uniform electric fields are used in various applications such as capacitors, where they store energy, and in cathode ray tubes, which were used in older television and computer monitors. Understanding these applications helps Cambridge International A Levels students connect theoretical concepts to practical uses.

How does a uniform electric field affect a charged particle?

In a uniform electric field, a charged particle experiences a constant force, which results in uniform acceleration. This is because the force (F) on the particle is given by F = qE, where q is the charge of the particle and E is the electric field strength. This principle is often explored in Cambridge International A Levels Physics to understand motion under electric forces.

What is the relationship between electric field lines and uniform electric fields?

Electric field lines in a uniform electric field are parallel and equally spaced, indicating that the field strength is the same throughout. This visualization helps students in Cambridge International A Levels Physics to easily identify and analyze uniform electric fields in diagrams and experiments.

Why is "Using d in mm" a common mistake in Uniform electric fields?

Why it happens: Convenient unit. How to avoid it: d must be in METRES for E in V/m or N/C. Source: 9702 Examiner Reports 2022-2024

Cambridge International A Levels Physics (9702)

Uniform electric fields

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Short Notes - Uniform electric fields

Detailed Study Notes

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

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

$E = V/d$ . Charge motion in uniform field similar to projectile.

At a glance

$E = V/d$ between parallel plates.
Force on charge $F = qE$ .
Charged particle follows parabolic path in $E$ .

What you’ll learn

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

18.2.1 — Use $E = V/d$ .
18.2.2 — Analyse charge motion in uniform field.

Parallel plates

Uniform $E$ between.

Setup. Two parallel conducting plates with PD $V$ , separation $d$ .

Field. Uniform between, $E = V/d$ directed from + to − plate.

Force on charge. $F = qE$ .

Motion in field. Similar to projectile under gravity: charge accelerates in direction of field (positive) or opposite (negative).

Cambridge tip. Use SUVAT after computing $a = qE/m$ .

Figure 1 — Uniform electric field between parallel plates

Two parallel plates with PD V across gap d give a uniform field E = V/d running from + to −; a charge q in the field feels F = qE.

Figure 2 — Halving the plate separation doubles the field strength

Field strength depends on the p.d. and the separation. Bringing the plates closer with the same p.d. packs the same lines into a smaller gap, so the field is stronger.

Figure 3 — Forces on positive and negative charges between the plates

In a uniform field the force on a charge is the same wherever it sits between the plates. Only the direction depends on the sign of the charge.

$E = V/d$ .
Force $qE$ .
Parabolic motion analogous to projectile.

See the full worked example for uniform electric fields →

Electron deflection

CRT-style motion.

Setup. Electron enters horizontally between plates with uniform vertical $E$ .

Method.

Force $F = eE$ .
Acceleration $a = F/m_e$ .
Time through plates: $t = L/v$ (horizontal velocity unchanged).
Vertical deflection: $y = \frac{1}{2}at^2$ .

Use case. Cathode-ray tubes (CRT), oscilloscopes.

Cambridge tip. Always treat horizontal and vertical INDEPENDENTLY.

Figure 4 — Deflection of an electron in a uniform field

The electron keeps its horizontal velocity while accelerating sideways, exactly like a projectile under gravity. The result is a parabola inside the plates and a straight line after it leaves them.

Treat the two directions completely separately.

	Horizontal (along $v$ )	Vertical (along $E$ )
Force	None	$F = eE$
Acceleration	$0$	$a = eE/m_e$
Velocity	Stays at $v$	Grows from zero
Distance covered in time $t$	$L = vt$	$y = \frac{1}{2}at^2$

Combining a constant horizontal velocity with a constant vertical acceleration is what produces the parabolic path.

Constant horizontal velocity.
Vertical acceleration $= eE/m_e$ .
Parabolic trajectory.

See the full worked example for uniform electric fields →

Quick recap

$E = V/d$ .
Charge motion = parabolic.
Treat directions independently.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

$E = V/d$ .
$F = qE$ .

How it’s examined

Uniform field on Paper 4 typically 6-10 marks. Most-tested: $E = V/d$ (5 marks), deflection (8 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 — Uniform electric fields

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

1 $E$ between plates (5 marks)
Extended• uniform field
Question
Two parallel plates 5.0 mm apart have PD 200 V. Find $E$ between them. (5 marks)
Step-by-step solution
1. Step 1
  $E = V/d$ .
  $E = 200/(5.0 \times 10^{-3}) = 4.0 \times 10^4 \text{ V/m}$
Answer
$E = 4.0 \times 10^4$ V/m.
2Electron deflection (8 marks)
Extended• deflection
Question
An electron enters a uniform field of $1.0 \times 10^4$ V/m horizontally at $5.0 \times 10^6$ m/s. Plates 0.10 m long. Find vertical deflection on exit. Take $e = 1.6 \times 10^{-19}$ C, $m_e = 9.1 \times 10^{-31}$ kg. (8 marks)
Step-by-step solution
1. Step 1
  Force on electron. $F = eE = 1.6 \times 10^{-15}$ N.
2. Step 2
  Acceleration. $a = F/m_e \approx 1.76 \times 10^{15}$ m/s².
3. Step 3
  Time through plates. $t = L/v = 0.10/(5 \times 10^6) = 2 \times 10^{-8}$ s.
4. Step 4
  Vertical deflection. $y = \frac{1}{2}at^2$ .
  $y = \tfrac{1}{2}(1.76 \times 10^{15})(4 \times 10^{-16}) \approx 3.5 \times 10^{-1} \text{ m}$
Answer
$y \approx 0.35$ m (note: simplified — actual deflection limited by plate spacing).

Key Formulae — Uniform electric fields

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

$E$ between parallel plates
$E = \dfrac{V}{d}$
$V$
potential difference
$d$
separation
When to use
Uniform field between parallel plates.

Key Definitions and Keywords — Uniform electric fields

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

Uniform electric field
Examiner keyword
Field with constant magnitude and direction. Found between parallel plates.

Common Mistakes and Misconceptions — Uniform electric fields

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

✕Using d in mm
9702 Examiner Reports 2022-2024
Why it happens
Convenient unit.
How to avoid it
$d$ must be in METRES for $E$ in V/m or N/C.

Uniform electric fields — frequently asked questions

The things students keep getting wrong in this sub-topic, answered.

Setup. Two parallel conducting plates with PD $V$ , separation $d$ .

Field. Uniform between, $E = V/d$ directed from + to − plate.

Force on charge. $F = qE$ .

Motion in field. Similar to projectile under gravity: charge accelerates in direction of field (positive) or opposite (negative).

Cambridge tip. Use SUVAT after computing $a = qE/m$ .

Figure 1 — Uniform electric field between parallel plates

Two parallel plates with PD V across gap d give a uniform field E = V/d running from + to −; a charge q in the field feels F = qE.

Figure 2 — Halving the plate separation doubles the field strength

Field strength depends on the p.d. and the separation. Bringing the plates closer with the same p.d. packs the same lines into a smaller gap, so the field is stronger.

Figure 3 — Forces on positive and negative charges between the plates

In a uniform field the force on a charge is the same wherever it sits between the plates. Only the direction depends on the sign of the charge.

Horizontal (along

v

)

Vertical (along

E

)

Force

None

F = eE

Acceleration

0

a = eE/m_e

Velocity

Stays at

v

Grows from zero

Distance covered in time

t

L = vt

y = \frac{1}{2}at^2

Uniform electric fields

Short Notes - Uniform electric fields

Detailed Study Notes

At a glance

What you’ll learn

Parallel plates

Electron deflection

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1EEE between plates (5 marks)

2Electron deflection (8 marks)

EEE between parallel plates

Uniform electric field

✕Using d in mm

Uniform electric fields — frequently asked questions

Uniform electric fields

Short Notes - Uniform electric fields

Detailed Study Notes

At a glance

What you’ll learn

Parallel plates

Electron deflection

Quick recap

Memorise this

How it’s examined

Take this whole topic with you

1EEE between plates (5 marks)

2Electron deflection (8 marks)

EEE between parallel plates

Uniform electric field

✕Using d in mm

Uniform electric fields — frequently asked questions

1 $E$ between plates (5 marks)

$E$ between parallel plates

1 $E$ between plates (5 marks)

$E$ between parallel plates