Concept of a magnetic field

Physics A2 · Magnetic fields. The idea in plain words, the rules to remember, the mistake to avoid and a worked example, written and checked by Mega Lecture.

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The idea

A magnetic field is a region where a force acts on a magnet, on a current-carrying conductor or on a moving charge. It is a field of force, like electric and gravitational fields.

Magnetic fields are produced by permanent magnets and by moving charges, which includes any electric current. A charge at rest produces no magnetic field and feels no magnetic force.

We draw the field with field lines. The direction of a line at a point is the direction of the force on a free north pole there, so lines leave a north pole and enter a south pole. The closer the lines, the stronger the field.

Rules to remember

Magnetic fields come from permanent magnets and from moving charges (currents)
Field direction = direction of the force on a north pole = direction a plotting compass points
Lines run from north pole to south pole outside a magnet
Closer lines mean a stronger field (larger flux density)
Uniform field: parallel, equally spaced lines
Field lines never cross, because the field at a point has only one direction
A stationary charge feels electric and gravitational forces but no magnetic force

Common mistake

Students say a magnetic field acts on any charge. It acts only on a charge that is moving, and only if the motion is not along the field lines.

Worked example

On a field diagram the lines at point X are parallel and close together. At point Y they are parallel, equally spaced and twice as far apart. What does this tell you?

Parallel, equally spaced lines mean the field is uniform near each point.
The spacing of the lines shows the strength: closer lines mean a stronger field.
The lines are closer at X, so the field is stronger at X than at Y.
At both points the field direction is along the lines, the way a compass would point.

Answer: The field is uniform near each point and is stronger at X than at Y.

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