10.1.3 Magnetic Fields — Edexcel GCSE Combined Science 1SC0 - Physics Revision Notes & Topical Questions

Section 1 of 9What is a magnetic field?

A magnetic field is a region of space in which a magnetic material or a moving charge or a current-carrying wire experiences a force. The field is invisible, but its effect can be seen: iron filings sprinkled on paper over a magnet line up along the field, and a small plotting compass points along it.

The magnetic field is a vector: it has both strength and direction, and so we use arrows to show it.

Section 2 of 9Field lines

We draw magnetic field lines to show a field. The rules are:

From north to south

Lines have arrows that show the direction of the force on a free north pole: from north to south outside the magnet.

Lines never cross each other.

The lines are closer together where the field is stronger (near the poles) and further apart where it is weaker.

Lines are continuous curves; outside the magnet they leave N and enter S.

Section 3 of 9The field of a bar magnet

Around a bar magnet the lines spread out from the north pole in curves and return to the south pole.

The field is strongest at the two poles, where the lines are most crowded, and weakens rapidly with distance.

Uniform field

In the space between the two poles of a horseshoe magnet the lines are almost straight, parallel and equally spaced,

which is a uniform field, one with the same strength and direction at every point.

Section 4 of 9Interaction of two magnets

1N facing S

N facing S the lines run straight from N to S between them; the magnets attract.

2N facing N (or S facing S)

N facing N (or S facing S) lines bulge away from each other and there is a point in the middle, called a neutral point,

where the fields cancel and the resultant field is zero.
The magnets repel.

Section 5 of 9Plotting a field

Place a bar magnet on paper and place a small plotting compass near one pole.

1

Mark the position of the needle's two ends with dots.

2

Move the compass so that its tail end is at the previous head dot, and mark again.

3

Repeat until the line returns to the other pole, then join the dots.

4

Repeat from different starting positions to get a set of lines.

5

Iron filings provide a quick picture but not the direction.

Section 6 of 9The Earth's field

The Earth behaves as if it had a huge bar magnet inside it.

The Earth's field

A compass needle (a small magnet) points along the Earth's field lines.

The needle's north pole points towards the geographic north,

so the Earth's magnetic pole in the northern hemisphere is in fact a magnetic south pole, since it attracts the north poles of compasses.

In the absence of any other field, a freely pivoted magnet settles in line with the Earth's field.

Comparing field strength at two points

To compare the field strength at two points on a field diagram, count the lines per unit area of an imaginary surface at right angles to the lines.

1

At the pole of a bar magnet the lines may be 2 mm apart; at the side, 10 mm apart.

2

The field at the pole is therefore about five times stronger in a drawing with this spacing.

3

Similarly, the force on a small piece of iron at each point is larger where the lines are crowded.

4

This is why iron filings cluster most densely at the poles of a magnet.

Remember that magnetic field lines form closed loops. Outside the magnet they go from N to S, and inside they continue from S back to N, so no line starts or finishes anywhere. The loop idea is more important at A level, but it helps explain why a magnet always has both poles.

Section 7 of 9Worked examples

A plotting compass is placed near the north pole of a bar magnet. In which direction does the compass needle point relative to the magnet?
Sketch description: Two bar magnets are placed with their north poles facing, 5 cm apart. Describe the field in the gap.

Section 8 of 9Key terms

Term	Meaning
Magnetic field	Region where a magnetic force acts on magnetic materials, magnets or moving charges.
Field line	A line showing the direction of the force on a free north pole.
Uniform field	Field with the same strength and direction everywhere, shown by parallel equally spaced lines.
Neutral point	Point where two fields cancel and the resultant is zero.
Plotting compass	Small magnetic compass used to find field direction.

Section 9 of 9Exam technique

1

Drawing field lines that cross or that stop in mid air.

2

Putting arrows pointing from south to north outside the magnet.

3

Thinking field lines are physical objects. They are a way to represent the field.

4

Drawing lines that are evenly spaced around a bar magnet.