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Magnetism & Electromagnetism

Magnetism & Electromagnetism · 1 question type

Practise

Exam Frequency Analysis

Past paper frequency (2018 to 2024)

This topic accounts for approximately 9% of your exam marks.

stable
Medium
Stable9%

Magnetic field patterns, the motor effect and Fleming's Left-Hand Rule tested in most series.

What a magnetic field is

  • A magnetic field is the region around a magnet in which a force would act on another magnet, or on a piece of magnetic material placed within it
  • Magnetic fields are visualised using magnetic field lines:
    • The direction of the field at any point is the direction the north pole of a small test compass would point if placed at that point
    • The strength of the field is shown by the spacing of the lines: close together where the field is strong, far apart where it is weak
  • Two important drawing rules:
    • Field lines always run from N to S outside the magnet (and from S to N inside it, completing a closed loop)
    • Field lines never cross or touch each other; they only meet at the poles themselves
Exam tip

Drawing magnetic field lines around a bar magnet

What comes up: draw or complete the field-line pattern around a bar magnet, or add arrowheads to given lines.

Write (two marks): (1) Lines run from the north pole and curve round to enter the south pole — arrowheads point from N to S on every line. (2) Lines are closest together at the poles (where the field is strongest) and spread out further away; no two lines cross or touch.

Watch out: if any arrowhead points the wrong way (S to N outside the magnet), the direction mark is lost entirely. Check every arrow on every line before moving on.

The field around a bar magnet

  • The field is strongest at the poles, so the lines are packed close together there and spread out further from the magnet
  • The pattern is symmetric: lines bow outward from the N pole, curve round, and re-enter the S pole
A bar magnet with the conventional field-line pattern drawn around it, the lines closer together at the poles and spreading out further away, with arrows running from the north pole round to the south pole
Source: Magnetic field definition by Save My Exams

Uniform fields

  • A uniform magnetic field has the same strength and the same direction at every point. On a diagram it is drawn as a set of equally spaced parallel field lines
  • A uniform field can be made by placing two flat magnets with opposite poles facing each other, separated by a few centimetres. Between the poles, the field looks uniform to a good approximation; outside that gap it is curved like a normal two-magnet field
A uniform magnetic field between the north pole of one magnet and the south pole of another, shown as equally spaced parallel field lines pointing from N to S
Source: Magnetic field definition by Save My Exams