Concept of a magnetic field
Section: Magnetic Fields | Syllabus: Cambridge A Level Physics (9702)
Defining a Magnetic Field
A magnetic field is a region of space where a force is exerted on:
- A magnetic pole (e.g., a permanent magnet)
- A moving electric charge
- A current-carrying conductor
It is best defined as a field of force produced by moving charges or permanent magnets.
Sources of Magnetic Fields
Unlike gravitational fields (mass) and electric fields (charge), magnetic fields are fundamentally produced by moving charges.
- Permanent Magnets: Result from the spin and orbital motion of electrons within atoms (microscopic currents). Common magnetic materials include Iron, Nickel, and Cobalt.
- Currents: Any macroscopic flow of charge produces a magnetic field.
| Field Type | Source | Force Acts On |
|---|---|---|
| Gravitational | Mass | Mass |
| Electric | Charge | Charge |
| Magnetic | Moving Charge | Moving Charge / Magnet |
Field Lines Representation
Magnetic field lines visualize the strength and direction of the field.
- Direction: From North (N) to South (S) outside the magnet.
- Strength: Represented by the density of lines. Closer lines indicate a stronger field (higher magnetic flux density).
- Loops: Magnetic field lines always form closed loops (they continue from S to N inside the magnet).
- Interaction: Field lines never cross.
Do not confuse the direction of the field line with the direction of force. The field line shows the direction a "free" North pole would move.
Diagram: Field PatternsShow field lines for: 1. Bar magnet (loops N to S). 2. Uniform field (parallel, equally spaced lines). 3. Earth's magnetic field (similar to a bar magnet).
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