The direction of the magnetic field at any point is given by the direction…

The idea

A magnetic field is a region where a magnetic force can act. To define its direction at a point, imagine placing a tiny free north pole there: the force it feels points along the field. Field lines map this: each line leaves a magnet's north-seeking pole and curves round to enter its south-seeking pole, so the arrow on a line runs N to S outside the magnet. Where lines are close, the field is strong; where they are spread out, it is weak. For example, around a bar magnet the lines form closed loops from N to S outside, and the arrow at any point shows the force direction on a north pole placed there. Inside the magnet the lines continue from S to N, completing the loop.

What earns the marks

• State that magnetic field direction at a point equals the direction of the force on a free north pole placed at that point.
• Draw field lines with arrows pointing away from the north-seeking pole and towards the south-seeking pole outside the magnet.
• Explain that field lines are a model: the tangent to a line at a point gives the field direction there.
• Use line spacing to compare field strength: closer lines mean a stronger field, wider spacing means a weaker field.
• Describe the field as three-dimensional around a magnet, not confined to the plane of the page.
• Recognise that inside a magnet the field lines run from south-seeking to north-seeking pole, completing closed loops.

Common mistakes and how to put them right

• Drawing arrows on field lines pointing from south to north outside the magnet.

  Put it right by remembering the convention that outside the magnet lines run from north-seeking to south-seeking pole.

• Treating field lines as paths that a north pole travels along.

  Put it right by explaining that a field line shows direction at each point, not a route.

• Assuming field lines can cross.

  Put it right by stating that at any point the field has one direction, so lines never intersect.

Exam tips

• When asked to draw a magnetic field, always add arrowheads and check they point away from N and towards S.
• If asked to compare field strength at two points, refer to the spacing of field lines rather than the length of the lines.
• Use the phrase 'force on a north pole' when defining field direction, as this matches the specification wording closely.