Magnetic fields
Moving charges and electric currents create magnetic fields, and a magnetic field in turn exerts a force on any moving charge that crosses it.
Practise this conceptMagnetic fields are created by moving charge — a current-carrying wire, a spinning electron, or the churning molten iron deep inside the Earth all generate one. A stationary charge, sitting perfectly still, creates no magnetic field at all, only an electric one.
That same asymmetry shows up in reverse: a magnetic field only pushes or pulls on a charge that is itself moving. A stationary charge sitting in even a very strong magnetic field feels nothing from it, but the moment it starts moving, it feels a force perpendicular to both its velocity and the field.
Unlike electric field lines, which start on positive charges and end on negative ones, magnetic field lines never start or end anywhere — they always form closed loops. This reflects a deeper fact: nobody has ever found an isolated magnetic north or south pole on its own. Cut a bar magnet in half, and you don't get a separate north piece and south piece — you get two smaller magnets, each with its own north and south pole.
Key ideas
Requires: Electric fields
Unlocks: Electromagnetic induction
Formulas
| Symbol | Name | Unit |
|---|---|---|
| force | N (newton) | |
| magnetic field | T (tesla) | |
| electric current | A (ampere) | |
| distance | m (metre) |
Common misconceptions
- A stationary electric charge creates a magnetic field, just like a moving one.
- Magnetic field lines begin and end at magnetic poles, the way electric field lines begin and end at charges.
- A magnetic field exerts a force on any charge, whether it is moving or stationary.