Mini Physics
Part of Mini EducationSwitch subject
SearchBookmarksMenu

Magnetic fields and field patterns

Key idea: A compass's north end points along the magnetic field. Outside a magnet, field lines run from north to south; closer lines represent a stronger field, and lines never cross.

Reading settings

On this page

Reading settings

Text size, line spacing and reading width for your lessons.

Text size
Line spacing
Reading width

Live preview

Take a moment to read at your own pace. Choose the text size and spacing that feel comfortable for you.

Pause after each idea, then explain it in your own words before moving on.

Narrower lines take effect when the screen has room.

Enlarged diagram

A magnet pushes or pulls on another magnet without touching it, and the push changes from place to place around it. A magnetic field describes that effect at every point: which way it acts and how strong it is. A small compass shows you the direction at any point, and field lines turn those directions into a picture you can draw.

Magnetic field direction

A magnetic field describes a magnet’s effect on other magnets and magnetic materials around it. Its direction at a point is the direction of the force on a small test north pole there.

A plotting compass is a tiny magnet that can turn freely. In a locally uniform field, forces on its N and S ends turn the needle until it lines up with the field. The N end of the needle points in the field direction. Outside a bar magnet, that is away from the N pole and towards the S pole.

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 the plotting method follows successive needle positions.Field direction around a bar magnetNSOutside: N → SFollow the arrows.Lines are closernear the poles.NN end followsthe tangent.1. Mark both needle ends.2. Move the S end to the previous N mark.3. Repeat, join smoothly and add arrows from N towards S.
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 the plotting method follows successive needle positions.Field direction around a bar magnetNSOutside: N → SFollow the arrows.Lines are closernear the poles.NN end followsthe tangent.1. Mark both needle ends.2. Move the S end to the previous N mark.3. Repeat, join smoothly and add arrows from N towards S.
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.

The compass reads the resultant external field: the combined effect of the magnets and Earth. Close to a strong bar magnet, its field usually dominates. Farther away, or where two magnets’ fields nearly cancel, Earth’s field can affect the reading.

Iron filings sprinkled round a magnet show the shape of the field, but not its direction: a filing has no labelled N end. Use a plotting compass when you need the direction.

Worked example 1

Mapping a field line with a plotting compass

Problem

Describe how to use a plotting compass to draw a field line around a bar magnet, with its direction.

Show full solution

  1. Set up

    Method

    Place the magnet on paper and draw round it. Label N and S.

    Reason

    The outline lets you replace the magnet in the same place and relate each line to the poles.

    Working

    Answer

    Keep other magnets, steel rulers and iron objects away from the paper.

  2. Mark the first direction

    Method

    Put a small compass near the N pole. Let it settle, then mark a dot at the N end of the needle.

    Reason

    The needle lies along the field, with its N end pointing along it.

    Working

    Answer

    Also mark a dot at the S end, so the two dots show the line’s direction at the start.

  3. Step along the line

    Method

    Move the compass until its S end is over the last N dot. Mark a new dot at the N end.

    Reason

    Each move traces the next short section of the field. A small compass gives a better local direction where the field curves rapidly.

    Working

    Answer

    Repeat until you reach the S pole or the edge of the paper.

  4. Draw and label

    Working

    Answer

    Join the dots with a smooth curve and add an arrow pointing the way the compass N end pointed: from N towards S. Start from other points near the N pole to draw more lines.

Check your understanding 1 Predict a compass at two positions

Assume the bar magnet’s field dominates Earth’s field at both compass positions below.

A compass sits on the axis of a bar magnet, just beyond its N pole. Where does the N end of the needle point?
The compass is moved to a point level with the middle of the magnet, a short distance to one side. Which way does the N end of the needle point?
Show answer

Just beyond the magnet’s N pole, the compass’s N end points away from the magnet. Beside its middle, it points parallel to the magnet towards the S end. Both answers use the local field direction and the stated assumption that the bar’s field dominates Earth’s field; a compass does not always point geographically north.

Drawing magnetic field patterns

Every field-line drawing follows the same rules:

  1. Outside a magnet, lines leave the N pole and enter the S pole. Put an arrow on every line.
  2. Lines never cross. At a point where the resultant field is non-zero, it has one direction.
  3. Lines are closest where the field is strongest, near the poles, and spread out where it is weaker.
  4. Draw smooth curves that leave N and enter S outside the magnet. The field continues through the magnet from S to N, so the complete lines form closed loops; they do not end in empty space.

For a single bar magnet, the lines loop from N round to S on both sides and look the same above and below the magnet. See the compass figure above.

With two magnets, find the facing poles first.

  • Unlike poles facing (N facing S): lines cross the gap from N to S. With broad pole faces close together, the central gap can have a roughly uniform field: nearly straight, evenly spaced lines. The lines curve and spread at the edges.
  • Like poles facing (N facing N): lines from each N pole curve away from the gap. They do not cross. For identical, aligned magnets, their fields cancel at the midpoint: a neutral point in the two-magnet model. This zero-field result assumes Earth’s field and other fields are ignored.
Magnetic fields between unlike and like polesThe left panel shows N-to-S field lines across the narrow gap and returning around the outside. The right panel shows selected field-line sections from two facing north poles bending apart. For identical aligned magnets and no other fields, their fields cancel at the midpoint.Unlike poles: N facing SLike poles: N facing NSNSNSNNSN → S across the central gap.Fields cancel at the midpoint.Arrowheads show field direction, not forces on the magnets.
Magnetic fields between unlike and like polesThe left panel shows N-to-S field lines across the narrow gap and returning around the outside. The right panel shows selected field-line sections from two facing north poles bending apart. For identical aligned magnets and no other fields, their fields cancel at the midpoint. Unlike poles: N facing SLike poles: N facing NSNSNSNNSN → S across the central gap.Fields cancel at the midpoint.Arrowheads show field direction, not forces on the magnets.
These are schematic patterns, not numerical field-strength measurements. Only selected outside sections are shown. Arrows follow the local field direction. The neutral point assumes identical aligned magnets, with Earth’s field and other external fields ignored.

Guided practice 2

Drawing the field between two S poles

About 6 min

Problem

Two identical bar magnets lie in a line with their S poles facing each other across a gap. Ignore Earth’s field and other fields. Sketch the field in and around the gap. Then answer the questions.

Try this before viewing the solution

Near the gap, which way do the arrows on the field lines point?
Where is the field zero?

Hints

Hint 1

compare with the N–N figure

The shape is the same as for two facing N poles.

Hint 2

fix the arrows

Only the arrows change: lines go into an S pole.

Show solution step by step

  1. Read the facing poles

    Method

    Identify like poles facing each other.

    Reason

    Like poles repel, so no line joins one S pole to the other.

    Working

    Answer

    The lines bend away from the gap, as for N facing N.

  2. Draw the lines and arrows

    Method

    Draw curves that come in from the sides and enter each S pole.

    Reason

    Outside a magnet, field lines run from N to S.

    Working

    Answer

    Every arrow near the gap points into the nearer S pole.

  3. Mark the neutral point

    Working

    Answer

    Mark the neutral point with a cross halfway between the poles. No line passes through it, and no two lines cross.

Check your understanding 2 A compass at the neutral point

Two bar magnets of the same shape and size lie in a line with their N poles facing each other. One magnet is more strongly magnetised. Ignoring Earth’s field, does the neutral point between the poles move towards the stronger or weaker magnet? Explain. Why might a real compass still turn at that point?

Show answer

Towards the weaker magnet. At the midpoint, the stronger magnet’s field wins. Moving towards the weaker magnet makes its field stronger and the other magnet’s field weaker, until the two fields are equal and opposite and cancel. Earth’s field is still present, so a real compass at that point can line up with Earth’s field.

Check your understanding 3 Draw the field for a reversed facing-pole arrangement

Two aligned bar magnets have broad facing poles close together. The left magnet’s facing pole is S and the right magnet’s facing pole is N. Ignore Earth’s field. On paper, sketch the field across the central gap and near its edges, with arrow directions. State which way a compass’s N end points at the gap’s midpoint. A learner copies an S–S drawing and marks a neutral point there. Explain why that does not fit this arrangement.

Show answer

Self-review: central-gap lines run from the right-hand N pole towards the left-hand S pole, roughly straight and evenly spaced away from the edges; lines fringe and curve near the edges. Put leftward arrows on the central lines and avoid crossings. The compass’s N end points left. At the midpoint the two magnets’ contributions point in the same direction, so they add rather than cancel. The S–S neutral-point construction concerns like facing poles, while these are unlike poles. The model assumes close broad faces for an approximately uniform central region, not uniformity everywhere.

Common mistakes

  • Saying a compass always points north. It lines up with the combined field of the magnets and Earth, and near a strong magnet the magnet’s field wins.
  • Drawing arrows from S to N outside a magnet, or leaving the arrows off. Outside a magnet, field lines run from N to S.
  • Letting field lines cross. The field has only one direction at each point.
  • Using iron filings to find the field direction. They show the shape of the field only; use a plotting compass for the direction.

Spotted an error? Report a correction

Syllabus and review details

Last reviewed:

Back to top