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Magnetic Field & Magnetic Field Lines

Key idea: O Level magnetism: what a magnetic field is, how to use a compass to find its direction, and how to draw magnetic field patterns and field lines.

  • SEC G3 Physics 2027
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Learning objectives

  • State the properties of magnets
  • Describe induced magnetism
  • Distinguish temporary and permanent magnets
  • Determine magnetic-field direction with a compass or bar magnet
  • Interpret bar-magnet field patterns
  • Draw the magnetic field pattern around a bar magnet and between the poles of two bar magnets
  • Interpret the field pattern around a straight current-carrying wire
  • Draw the magnetic field pattern around a straight current-carrying wire
  • Interpret the field pattern around a current-carrying solenoid
  • Draw the magnetic field pattern around a current-carrying solenoid
  • Relate current magnitude and direction to magnetic field
  • Describe electromagnet applications
  • Describe experiments showing the force on a current-carrying conductor in a magnetic field
  • Describe magnetic force on a charged-particle beam
  • Predict force reversal when current or field reverses
  • Use Fleming’s left-hand rule
  • Explain the turning effect on a current-carrying coil
  • Explain how current and turns increase the turning effect
  • Describe split-ring commutator action
  • Describe the effect of winding a motor coil on a soft-iron cylinder
  • Deduce that a changing magnetic field can induce an e.m.f.
  • Deduce that induced e.m.f. opposes the change producing it
  • Deduce factors affecting induced e.m.f. magnitude
  • Describe a simple a.c. generator and slip rings
  • Sketch a simple a.c. generator voltage–time graph
  • Describe a simple iron-cored transformer
  • Apply ideal-transformer equations
  • Explain cable loss and high-voltage transmission

1. Definition

A magnetic field is a region in which a magnetic pole experiences a force.

At a point, the magnetic field direction is the direction of the force on a north pole placed there. The north-seeking end of a small plotting compass indicates this direction.

What you need for this course

You should be able to use a bar magnet and plotting compass to determine field direction and draw magnetic field-line patterns.

2. Key Ideas

  • Outside a bar magnet, magnetic field lines go from N → S.
  • A plotting compass shows field direction: the compass north end points along the field line.
  • Field line rules:
    • arrows show direction
    • closer lines = stronger field (qualitative)
    • field lines never cross
  • Parallel, evenly spaced lines represent a more uniform field.

3. Detailed Explanations

A. Direction of the magnetic field

At any point, the north-seeking end of a small compass points tangentially to the field line. Outside a bar magnet, the arrows run from N to S; inside the magnet, they return from S to N, so field lines form closed loops.

B. Strength of the magnetic field (from line spacing)

The field is stronger near the poles, so a field-line diagram uses closer spacing there. The lines are a model: they show direction and relative strength, not physical threads in the space.

C. Plotting field lines using a compass (method)

You can plot one field line using this repeatable method:

  1. Place the compass beside the magnet and mark the positions of both ends of its needle.
  2. Move the compass so its south-seeking end is at the previous mark made by the north-seeking end.
  3. Mark the new north-seeking end and repeat in small steps.
  4. Join the marks with a smooth curve and add an arrow in the direction followed by the north-seeking end.
Bar-magnet field direction and plotting compassField lines curve from the north pole to the south pole outside a bar magnet. A compass on one field line is aligned tangentially, and a three-step plotting method is shown.Field direction around a bar magnetNSOutside: N → SArrows show direction.lines are densernear the polesNN end followsthe tangentPlot one line:1. mark both needle ends2. move S end to the previous N mark3. repeat and join smoothlyarrow follows the north-seeking end
Scroll diagram horizontally to read all labels.
Outside a bar magnet, field lines run from N to S. A plotting compass is tangent to the local field line, with its north-seeking end pointing along the field.
Simulation checkpoint

Open the Magnetism & Induction Lab. Show the bar magnet’s field and predict the compass north-end direction at three labelled probes before checking. Then explain why each compass is tangent to its local field line rather than simply pointing at the centre of a pole.

Link

Magnetic fields can also act on currents: Force on a Current-carrying Conductor.

4. Common Mistakes

  • Drawing arrows the wrong way (outside a magnet is N → S).
  • Drawing field lines that cross.
  • Saying “field lines push each other” (field lines are a diagram tool; use “like poles repel” instead).

5. Exam Tips

  1. Always label N and S and add arrows on field lines.
  2. If asked about direction, mention the compass: “north end points along the field”.
  3. If asked about strength: “stronger where lines are closer”.

6. Worked Examples

Modelled example 1

Direction of field lines

Core

Problem

In the space outside a bar magnet, what is the direction of magnetic field lines, and what definition fixes that direction?
Study the worked solution

  1. Apply the direction convention

    Method

    Draw or state arrows from north to south outside the magnet.

    Reason

    Field direction is defined as the force direction on a north test pole.

    Working

    Outside the magnet: N → S.

  2. Complete the field idea

    Method

    Recognise that lines return through the magnet from south to north.

    Reason

    Magnetic field lines form closed loops.

    Working

    Inside the magnet: S → N.

Guided practice 2

Reading compass direction

About 4 min

Problem

A plotting compass near a bar magnet has its north-seeking end pointing right. State the local magnetic-field direction.

Use the labelled end, not the whole needle

Hints

Hint 1: indicator
The north-seeking end indicates field direction.
Hint 2: answer
Follow the direction in which that end points.
View solution step by step

  1. Read the local tangent

    Method

    State that the field points to the right.

    Reason

    The compass north end aligns along the local field direction.

    Working

    Compass N end → right; therefore vector B → right.

Common misconception 3

Field between unlike poles

Find and correct the mistake

Learner diagram

A learner draws arrows leaving both a north and a south pole that face each other across a small gap. Diagnose and describe the correct central pattern.

Use one direction convention throughout

View solution step by step

  1. Correct the arrows

    Method

    Direct lines across the gap from N to S.

    Reason

    Outside magnets, field direction is defined N to S.

    Working

    N → gap → S.

  2. Describe the central spacing

    Method

    Draw many nearly parallel, approximately evenly spaced lines in the middle.

    Reason

    Facing unlike poles produce a comparatively uniform field in the small central region.

    Working

    Parallel central lines represent similar direction and strength.

Examiner practice 4

Strong vs weak regions

2 marks

Examination question

Field lines are closer together in region A than region B. Compare the field strengths and explain how the diagram shows this. [2 marks]

Make a comparison and cite the representation

View solution step by step

  1. Infer relative strength

    2 marks

    Method

    State that the field is stronger in A.

    Reason

    Greater field-line density represents greater magnetic-field strength qualitatively.

    Working

    Lines closer in A → B_A > B_B.

Challenge 5

Field lines crossing

Minimal support

Diagram-validity transfer

A proposed field diagram contains two field lines crossing at point X. Use the behaviour of a plotting compass at X to evaluate the diagram.

Ask how many directions the compass can indicate

Hints

Hint 1: local tangent
A compass north end aligns with the field-line tangent.
Hint 2: at the crossing
Two tangents would assign two different directions to one point.
View solution step by step

  1. Interpret the crossing

    Method

    State that it would assign two field directions at X.

    Reason

    Each line’s tangent represents the local direction.

    Working

    Crossing lines → two tangents at X.

  2. Reject the diagram

    Method

    Conclude that magnetic field lines cannot cross.

    Reason

    A plotting compass at one point can settle in only one resultant field direction.

    Working

    One point → one resultant vector B direction.

7. Mind Stretchers

Mind stretcher 1: Very near the poleExtension

If a compass is placed very near the pole of a strong magnet, why can it be hard to draw neat field lines?

Show Answer

The field changes direction rapidly near the pole, so small movements of the compass can produce noticeably different directions. This makes the plotted line harder to draw smoothly.

Mind stretcher 2: Earth field ideaExtension

Why does a plotting compass work even though the compass is itself a small magnet?

Show Answer

The compass aligns with the resultant magnetic field at its position. Near a bar magnet, the bar magnet’s field is much stronger than Earth’s field, so the compass mainly aligns with the bar magnet’s field direction.

8. Practice and next step

Map and reverse fields in the Magnetism & Induction Lab, then continue to Induced Magnetism.

Continue with the next resource in this course.

Course and syllabus information
Course
SEC G3 Physics
Edition
SEC G3 Physics 2027