Electricity and magnetism · GCSE Physics
Magnetic fields
Teacher-written GCSE Physics revision on magnetic fields: north and south poles, field lines from north to south, plotting compasses, the Earth’s field, and the field around a current-carrying wire.
Field lines go from north to south, never cross, and are densest where the field is strongest. Like poles repel; unlike poles attract.
The important bits
What you need to know
- 1
Like magnetic poles repel; unlike poles attract. The force is non-contact: magnets can act through air and some materials.
- 2
A magnetic field is the region around a magnet where another magnet or magnetic material (iron, steel, nickel, cobalt) feels a force.
- 3
Field lines go from north to south, never cross, and are closest together where the field is strongest, usually near the poles.
- 4
A plotting compass aligns with the field: its north-seeking pole points along the field line, towards the magnet’s south pole if it is close enough to dominate.
- 5
The Earth has a magnetic field. A compass used for navigation aligns with that field. Geographic and magnetic poles are not exactly the same place.
- 6
A current-carrying wire has a magnetic field. Right-hand grip: thumb in the conventional current direction, fingers curl in the field direction (circles around the wire).
- 7
The field around a single wire is weaker than around a coil. Reversing the current reverses the field.
- 8
Induced magnetism: a magnetic material in a field can become a magnet while the field is there. Soft iron loses that magnetism when the field is removed; steel can stay magnetised.
Quotations worth analysing
Short evidence. Real method.
“Magnetic field lines go from north to south.”
Arrows on the lines are not optional. Crossing lines would mean two directions of force at one point, which does not happen.
“The magnetic field around a straight wire is a set of concentric circles.”
Circles, not a bar-magnet pattern. The right-hand grip sets the clockwise or anticlockwise sense.
Go deeper
Draw the field, then talk about the force
A bar magnet’s field leaves the north pole, loops through space, and returns at the south pole. Inside the magnet the lines continue, but GCSE sketches usually show the outside. Two north poles facing each other produce a null region between them where the fields cancel; two unlike poles produce a strong field in the gap. Iron filings map the pattern because each filing becomes a tiny induced magnet and lines up. A compass is cleaner for direction. Students draw lines that stop in space or that crash into each other. Every line that leaves a north should end on a south in the sketch, and the spacing should scream “strong here, weak there”. That picture is worth more than a paragraph that only says “magnets attract”.
Go deeper
The Earth’s field is why a compass works — and why it can lie
The Earth behaves like a giant bar magnet, but the magnetic pole near the geographic North Pole is a magnetic south pole, which is why a compass’s north-seeking end points north. Local iron, electric cables and speakers can distort the field, so a compass next to a steel table is a bad navigation tool. In exam diagrams of the Earth, field lines enter near the geographic north and leave near the geographic south. Link this to space physics if you meet it: the field also deflects charged particles from the solar wind. For Combined, the compass-and-Earth story is enough. Do not write that compasses point because “the North Pole is magnetic north” without the pole-type clarification if the question is precise.
See the idea in action
A plotting compass is placed beside the north pole of a bar magnet. The compass needle’s north-seeking end is repelled, so it points away from that pole, along a field line leaving N. Halfway along the magnet’s side the needle is parallel to the magnet, following the looping field. Far from the magnet the Earth’s field dominates and the needle points geographic north. A long straight wire carrying 4.0 A has circular field lines; reversing the 4.0 A current reverses the compass needles around it. No force number is needed here: the skill is direction, density of lines, and which field wins.
Exam technique
Turn knowledge into marks
Label N and S, draw lines from N to S that never cross, and pack them closer at the poles. For a wire, sketch circles and mark the current direction. Name iron, steel, nickel or cobalt as magnetic materials.
Common mistakes
Do not give these marks away
- 01
Drawing field lines that cross, or running them south to north.
- 02
Saying all metals are magnetic, or that a compass always points at a nearby magnet even when the Earth’s field is stronger.
- 03
Using the left hand for the field around a wire (right-hand grip is current to field).
Which statement about magnetic field lines is correct?
AThey go from south to north and often cross
BThey go from north to south, never cross, and are densest where the field is strongest
CThey only exist inside the magnet
DThey show the path of electric current through the air
Show the answer
They go from north to south, never cross, and are densest where the field is strongest. Direction, no crossing, and spacing for strength are the three rules examiners want on every field-line sketch.
Quick questions
If this is the bit you searched
Which materials are magnetic?
Iron, steel, nickel and cobalt (and some alloys). Copper, aluminium and most other metals are not magnetic, even though they may be conductors.
How do you plot a magnetic field with a compass?
Place a plotting compass near the magnet, mark the needle direction, move the compass, and join the marks into smooth curves from N to S.
What is the field pattern around a straight current-carrying wire?
Concentric circles centred on the wire. Right-hand grip: thumb in current direction, fingers show the field.
Why does a compass point north?
It aligns with the Earth’s magnetic field. The magnetic pole near geographic north behaves as a magnetic south pole, attracting the compass’s north-seeking end.