Edexcel IAL Physics
Magnetic Fields
The big idea: a changing magnetic field can push on moving charges, push on current-carrying wires, and even create its own electric current — and that one idea powers motors, generators, and transformers.
Chapter SummaryEverything in this chapter, in one scroll
- Magnetic flux density (B) measures how strong a magnetic field is, in tesla.
- Magnetic flux (Φ) measures how much field passes through an area: Φ = BA.
- Flux linkage (NΦ) is flux × number of turns — used for coils.
- A moving charged particle in a field feels a force F = BQv sin θ — always perpendicular to its velocity, so it moves in a circle.
- A current-carrying wire in a field feels a force F = BIL sin θ.
- Fleming's Left Hand Rule gives the direction of that force from B and I (or v).
- Electromagnetic induction: a changing flux linkage induces an e.m.f in a circuit — this is how generators and transformers work.
- Faraday's Law gives the size of the induced e.m.f: ε = Δ(NΦ)/Δt.
- Lenz's Law gives the direction: the induced e.m.f always opposes the change that caused it (hence the minus sign).
Topic 1Magnetic Flux Density, Flux & Flux Linkage
Think of a magnetic field as invisible field lines spreading out from a magnet. Where the lines are packed tightly together, the field is strong. Where they're spread apart, it's weak. Magnetic flux density, B, is simply the official number that tells you exactly how strong the field is at a point — it's basically "how densely packed are the field lines here?" It's measured in tesla (T), and for reference, Earth's own magnetic field is a puny 0.032 mT, while an ordinary fridge magnet is about 5 mT — magnetic fields you actually notice tend to be much stronger than the one holding the compass needle north.
Quick way to remember it
"Magnetic flux density" is often just called
magnetic field strength in casual conversation — same thing, different name.
Flux density tells you how strong the field is at a point. Magnetic flux takes it a step further — it tells you how much of that field is actually passing through a specific area, like a hoop or a coil face. Picture holding a net in a stream of water: if you hold the net face-on to the flow, loads of water passes through it. Tilt the net so it's edge-on to the flow, and almost no water gets through, even though the stream itself hasn't changed at all. Magnetic flux works exactly the same way — same field, but how much of it "counts" depends entirely on the angle of the surface you're measuring through.