🧲 Opposite poles facing
Straight, parallel, evenly spaced lines in the gap, from N to S. This is a uniform field.
Magnetism » Magnetic Field Lines
What you'll learn this session
Study time: 30 minutes
Edexcel spec: 6.4, 6.7
- What a magnetic field line is and what its arrow tells you
- How to read field line spacing to judge the strength of a field
- The field pattern around a bar magnet, in words
- How two permanent magnets make a uniform field
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Unlock This CourseMagnetic field lines
Remember, a magnetic field is the region around a magnet where a magnetic force acts. You cannot see it, so physicists draw it. The drawing is made of magnetic field lines.
A magnetic field line shows the direction of the force on a north pole placed at that point. Picture a tiny, free north pole sitting in the field. A magnet would push it away from a north pole and pull it towards a south pole. The path it would follow is the field line.
Because of this, field lines always have arrows, and the arrows follow one rule:
Learn this
Outside a magnet, magnetic field lines run from the north pole to the south pole.
Key terms:
- Magnetic field line: a line drawn to show the direction of the force on a north pole at each point in a magnetic field.
- Uniform field: a field that has the same strength and the same direction everywhere in it.
Three rules for field lines
🎯 Arrows
Every line has an arrow. It points the way a north pole would be pushed: away from N and towards S.
📏 Spacing
Lines close together mean a strong field. Lines far apart mean a weak field.
❌ Never cross
Field lines never cross. A north pole at one point can only be pushed one way, so only one line can pass through it.
The spacing rule is the one students forget. The lines are not just decoration. Where the lines are crowded together, the magnetic force is bigger. The lines are most crowded at the poles of a magnet, so that is where the field is strongest. The lines spread out as you move away, so the field gets weaker.
The field of a bar magnet
Here is how to draw the pattern for a bar magnet, step by step, as you would in an exam:
- Draw the bar magnet as a rectangle. Label one end N and the other end S.
- Draw lines that leave the north pole, curve round the outside of the magnet and enter the south pole. Draw several on each side.
- Put an arrow on every line pointing away from N and towards S.
- Make the lines crowded near the two poles and more spread out further away.
- Check that no two lines touch or cross.
The result is a set of smooth loops. Each one starts at the north pole and finishes at the south pole. The lines that leave from the end of the magnet are close together, and the outer loops are wider and weaker. You will learn how to find this pattern by experiment in Practical: Plotting Magnetic Field Patterns.
A uniform magnetic field from two magnets
Sometimes you need a field that is the same strength everywhere in a small region. This is a uniform field. Its field lines are straight, parallel and evenly spaced.
You can make one using two permanent magnets:
- Take two bar magnets, or two magnets with flat faces.
- Place them with opposite poles facing each other: a north pole faces a south pole.
- Leave a short gap between them. Keep the gap small compared with the size of the faces.
In the gap, the field lines run straight across from the north pole to the south pole. They are parallel and the same distance apart, so the field is the same strength all through the gap. Near the edges of the gap the lines bulge outwards a little, so the field is only uniform in the middle.
🔄 Single bar magnet
Curved lines that are crowded at the poles and spread out further away. This is not a uniform field, because the strength and direction change from place to place.
Worked example
Question: Describe how the field lines look in the gap between a north pole and a south pole that face each other, and what this tells you about the field. [3 marks]
Step 1: In the gap the lines are straight and parallel to each other, with arrows pointing from the north pole to the south pole.
Step 2: The lines are the same distance apart all the way across the gap.
Step 3: Equal spacing means the field has the same strength everywhere in the gap.
Answer: The lines are straight, parallel and evenly spaced, going from north to south. This shows a uniform field, with the same strength and direction everywhere in the gap.
Common mistakes
- Leaving out the arrows, or drawing them from south to north. Outside the magnet the arrows always go from N to S.
- Letting two field lines cross or touch. They never do.
- Drawing evenly spaced lines around a single bar magnet. The lines are crowded at the poles.
- Placing two like poles opposite each other to try to make a uniform field. You need opposite poles facing.
- Thinking that the lines are real threads. They are a way of drawing the field.
Exam-style question
(a) State what is meant by a magnetic field line. [2 marks]
(b) A bar magnet has a magnetic field. Describe what the field lines look like outside the magnet, and say where the field is strongest. [3 marks]
(c) Describe how two permanent magnets can be arranged to make a uniform magnetic field, and describe the field lines in the gap. [3 marks]
Model answer
(a) A line that shows the direction of the force (1) on a north pole placed at that point in the field. (1)
(b) The lines are curves with arrows, running from the north pole round to the south pole. (1) They never cross. (1) The field is strongest at the poles, where the lines are closest together. (1)
(c) Place the two magnets with opposite poles facing each other, a short distance apart. (1) In the gap the lines are straight and parallel. (1) They are evenly spaced and run from the north pole to the south pole. (1)
Exam tip
If a question asks you to draw field lines, check three things before moving on: an arrow on every line, N to S, and no crossing lines. These are easy marks to lose.