Magnetism » Practical: Plotting Magnetic Field Patterns

Why we need a practical

Remember, a magnetic field cannot be seen, and field lines are the way we draw it. In Magnetic Field Lines you learnt what the lines mean. In this practical you find out how to show the pattern for real.

This is a required practical. You must be able to describe the method and say what the results show. You can do it at home with a bar magnet, a sheet of paper and a small compass. If you cannot do it, you can still answer the exam question by knowing the steps below.

Key terms:

• Plotting compass: a small compass with a needle that turns freely, used to find the direction of a magnetic field at a point.
• Neutral point: a place where two magnetic fields cancel out, so the field is zero.

Method 1: iron filings

Iron filings are tiny splinters of iron. In a magnetic field each one becomes a very small magnet, so it turns to line up along the field.

1. Put the bar magnet on the table.
2. Lay a sheet of paper or thin card flat over the magnet.
3. Sprinkle iron filings evenly over the paper.
4. Tap the paper gently with a finger or pencil. The filings move and settle into a pattern.
5. Look at the pattern, then sketch it, or take a photo.

The filings form curved lines that leave one pole and loop round to the other. They are packed thickly at the poles, where the field is strongest, and spread out further away.

Filings show the shape of the field very quickly. They do not show its direction, because a filing lines up with the field but has no arrow on it.

Method 2: plotting compass

A compass needle is a small magnet. It lines up along the field, and its north-seeking end points in the direction of the field line at that spot.

1. Put the bar magnet in the middle of a large sheet of paper. Draw round it and label the poles N and S.
2. Place the plotting compass next to the north pole of the magnet.
3. Mark a dot on the paper at the tip of the needle, at each end of the needle.
4. Move the compass so that the end that was behind now sits exactly on the dot in front. Mark a new dot at the tip of the needle.
5. Keep doing this until the compass reaches the south pole, or the edge of the paper.
6. Join the dots with a smooth line. Put an arrow on it pointing the way the north end of the needle pointed. This is from N to S.
7. Start again from a new place next to the magnet and plot more lines.

Two magnets: what the patterns show

You can repeat either method with two bar magnets lying on the paper, a few centimetres apart.

Unlike poles facing (N facing S)

The magnets attract. The lines run from the north pole straight across the gap to the south pole. They are crowded together in the gap, which shows a strong field. In the middle of the gap they are straight, parallel and evenly spaced: a uniform field.

Like poles facing (N facing N, or S facing S)

The magnets repel. The lines from the two poles bend away from each other and never meet or cross. Halfway between the poles there is a gap with almost no lines. This is a neutral point, where the fields of the two magnets cancel and the field is zero. The filings do not line up there, and the two magnets do not turn a compass needle in any one direction.

Arrangement	Force	Pattern between the magnets
Unlike poles facing	Attract	Lines run from N to S across the gap, close together
Like poles facing	Repel	Lines bend away from each other, with a neutral point in the middle

Safety and good practice

Keep other magnets, steel objects and phones away from your paper, because they bend the pattern. Tap gently, because tapping too hard throws filings off the paper.

Exam-style question

A student investigates the magnetic field of a bar magnet and of two bar magnets.

(a) Describe how the student can use iron filings to show the field of one bar magnet. [3 marks]

(b) State what a plotting compass shows that iron filings do not. [1 mark]

(c) The student places two north poles facing each other. Describe the pattern of field lines between them. [3 marks]