IGCSE Physics / Magnetism and electromagnetism
Making a uniform magnetic field
Edexcel International GCSE Physics (4PH1), chapter 20: Magnetism and electromagnetism, specification point 6.7. Free lesson.
Read the revision notes · IGCSE Physics past papers
A uniform magnetic field has the same strength and direction throughout a region. You can produce one in the middle of a narrow gap between two flat magnets, which is useful in motors and experiments.
What makes a field uniform
A uniform field is the same strength and has the same direction everywhere. Its field lines are straight, parallel and equally spaced. Equal gaps show that the field has the same strength throughout. To describe a uniform field for full marks, say the lines are parallel and equally spaced. Giving only the direction is not enough.
How it differs from a bar magnet’s field
A single bar magnet does not produce a uniform field. Its field lines curve around the magnet and crowd together near the poles. Since closer lines show a stronger field, the field is strongest at the poles and weaker further away. In a uniform field, equal spacing shows the strength is the same everywhere. Bar magnet: curved lines, closest near the poles, strongest at the poles. Uniform field: straight, parallel lines, equally spaced, same strength throughout.
How to make a uniform field
Use two flat magnets, also called slab magnets, with wide flat faces. Place unlike poles opposite each other, with the north pole of one facing the south pole of the other. Keep the magnets close, leaving a narrow gap. The field is uniform in the middle of this gap, where lines run straight from north to south. Unlike poles facing each other make the lines go straight across the gap. A narrow gap helps produce a uniform field. A wide gap lets the lines bulge and spread. Motors and experiments use the middle of the gap, where the field is uniform. In a motor, this means the force on a wire…
How to draw and describe the field
Draw the lines in the middle of the gap as straight, parallel lines with equal spaces between them. Add arrows from north to south. The field is not uniform right out to the edges: there, the lines curve outwards and spread apart. Show this change if you draw the edges. Use a ruler for the straight lines in the middle and keep the gaps equal. Do not claim that the field is uniform everywhere between the magnets. It is uniform only in the middle of the gap.