What is the concept of a magnetic field in Physics?

In Physics, a magnetic field is a vector field that describes the magnetic influence on moving electric charges, electric currents, and magnetic materials. It is represented by field lines that show the direction and strength of the magnetic force. The concept is crucial in understanding how magnets interact with each other and with other materials, and it is a fundamental topic in the Cambridge International A Levels Physics curriculum.

How is the direction of a magnetic field determined?

The direction of a magnetic field is determined by the direction a north pole of a compass needle points when placed in the field. By convention, magnetic field lines emerge from the north pole of a magnet and enter the south pole. This directionality is essential for understanding magnetic interactions and is a key concept in the study of magnetic fields at the Cambridge International A Levels.

What are the units used to measure magnetic fields?

Magnetic fields are measured in teslas (T) in the International System of Units (SI). Another common unit is the gauss (G), where 1 tesla equals 10,000 gauss. Understanding these units is important for quantifying the strength of a magnetic field, which is a significant aspect of the Cambridge International A Levels Physics syllabus.

How do magnetic fields interact with electric currents?

Magnetic fields exert a force on moving electric charges, which is described by the Lorentz force law. When an electric current flows through a conductor, it generates a magnetic field around it. This interaction is the basis for many applications, such as electromagnets and electric motors, and is a critical concept covered in the Cambridge International A Levels Physics curriculum.

What is the significance of magnetic field lines?

Magnetic field lines are a visual tool used to represent the direction and strength of a magnetic field. They help in understanding how magnetic forces are distributed in space. The density of the lines indicates the strength of the field: closer lines mean a stronger field. This visualization is an important part of learning about magnetic fields in the Cambridge International A Levels Physics course.

Why is "Treating magnetic field same as electric" a common mistake in Concept of a magnetic field?

Why it happens: Similar field-line concept. How to avoid it: Magnetic field acts only on MOVING charges or current. Electric acts on any charge. Source: 9702 Examiner Reports 2022-2024

Cambridge International A Levels Physics (9702)

Concept of a magnetic field

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Short Notes - Concept of a magnetic field

Detailed Study Notes

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

Magnetic Field Study Notes — Cambridge International A Level Physics 9702 (2025-2027 syllabus)

Field of magnet, current, solenoid.

At a glance

Field lines from N to S.
Wire: concentric circles, right-hand grip.
Solenoid: like bar magnet outside.
Acts on moving charge only.

What you’ll learn

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

20.1.1 — Describe magnetic fields from magnets, currents, solenoids.

Field patterns

Three sources.

Bar magnet. Lines exit N pole, curve around, enter S pole.

Long straight wire. Concentric circles around wire. Direction by right-hand grip rule (thumb along current, fingers curl in field direction).

Solenoid. Uniform parallel field INSIDE; field outside similar to bar magnet.

Acts only on moving charges/currents. Static charges feel no magnetic force.

Cambridge tip. Sketch arrows on lines to show direction.

Figure 1 — Magnetic field around a current-carrying wire (right-hand grip rule)

The field lines are circles drawn in a plane at right angles to the wire, so the wire must be shown end-on. Reversing the current reverses the sense of every circle.

Figure 2 — Field pattern of a bar magnet

Field lines never cross and never stop in mid-air — outside the magnet every line that leaves the north pole arrives at the south pole.

Figure 3 — Field pattern of a solenoid

The uniform interior field is what makes solenoids useful. Adding an iron core concentrates the same pattern and makes it much stronger.

Figure 4 — A magnetic field only acts on moving charge

An electric field would push the stationary charge; a magnetic field does nothing to it. Only once the charge moves across the field does a magnetic force appear.

The three field patterns to be able to sketch.

Source	Shape of the field	How to get the direction
Bar magnet	Loops from N round to S outside the magnet	Arrows always point away from N, towards S
Long straight wire	Concentric circles in a plane at right angles to the wire	Right-hand grip: thumb along $I$ , fingers curl with $B$
Solenoid	Uniform and parallel inside; bar-magnet shape outside	Right hand round the coil following $I$ ; thumb points to N

In every case the lines are closest together where the field is strongest, and they never cross.

Bar magnet: N to S.
Wire: circles by right-hand rule.
Solenoid: uniform inside.

See the full worked example for concept of a magnetic field →

Quick recap

Lines from N to S externally.
Right-hand grip rule for wire.
Solenoid uniform inside.

Memorise this

Verbatim phrases and definitions Cambridge mark schemes credit.

Field patterns for 3 sources.
Right-hand grip rule.

How it’s examined

Magnetic field on Paper 4 typically 5 marks. Standard sketches.

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.

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Step-by-step worked examples — Concept of a magnetic field

Step-by-step solutions to past-paper-style questions on concept of a magnetic field, written exactly the way a tutor would explain them at the board.

1Field line patterns (5 marks)
Extended• Adapted from 9702/42 May/Jun 2024• magnetic field
Question
Sketch field lines for (a) bar magnet, (b) long straight current-carrying wire, (c) solenoid. (5 marks)
Step-by-step solution
1. Step 1
  (a) Bar magnet. Lines from N out, curve around, into S.
2. Step 2
  (b) Long wire. Concentric circles around wire; direction by right-hand grip rule.
3. Step 3
  (c) Solenoid. Like bar magnet outside; uniform parallel field inside.
Answer
(a) N to S externally. (b) Concentric circles. (c) Like bar magnet but with uniform interior field.

Key Definitions and Keywords — Concept of a magnetic field

Definitions to memorise and the exact keywords mark schemes credit for concept of a magnetic field answers — sharpened from recent examiner reports for the 2026 Cambridge International A Level 9702 sitting.

Magnetic field
Examiner keyword
Region where a magnetic material or moving charge experiences a force.
Magnetic flux density $B$
Examiner keyword
Force per unit current per unit length on a conductor perpendicular to field. Units: tesla (T).

Common Mistakes and Misconceptions — Concept of a magnetic field

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

✕Treating magnetic field same as electric
9702 Examiner Reports 2022-2024
Why it happens
Similar field-line concept.
How to avoid it
Magnetic field acts only on MOVING charges or current. Electric acts on any charge.