A level Physics · Edexcel 9PH0 · Topic 7C
Magnetic fields and forces
Key points
- A magnetic field is a region where a magnet, moving charge or current-carrying wire feels a force. Field lines go from N to S.
- Magnetic flux density is measured in tesla (T). Flux (Wb) for a field perpendicular to area .
- Force on a current-carrying conductor: ; direction from Fleming's left-hand rule.
- Force on a moving charge: . This force is always perpendicular to the velocity, so it makes charged particles move in circles, with radius .
- A straight wire carrying current has circular field lines around it (right-hand grip rule); a solenoid gives a field like a bar magnet.
- Uses include motors, mass spectrometers, cyclotrons and particle detectors that bend tracks to measure momentum.
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Magnetic fields push on moving charges and therefore on currents. These notes cover magnetic flux density, the force on a current-carrying wire, the force on a moving charge and why charged particles move in circles in a magnetic field.
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