Magnetism and Electromagnetism
Magnetic fields
A magnetic field is a region where a magnetic force acts on magnetic materials or on current-carrying conductors. Magnetic field lines:
- Run from North pole to South pole outside the magnet.
- Never cross.
- Closer together → stronger field.
- Show direction of force on a free north pole placed there.
Permanent magnets (steel) retain magnetism. Temporary magnets (soft iron) are easily magnetised and demagnetised.
Plotting field lines: use a plotting compass or iron filings. Earth has a magnetic field (geographic North corresponds to the magnetic south pole of Earth).
The motor effect
A current-carrying conductor in a magnetic field experiences a force (the motor effect): F = BIL where B = magnetic flux density (T), I = current A, L = length of conductor in field (m).
Direction of force: use Fleming's left-hand rule (for conventional current):
- First finger → magnetic Field direction (N to S)
- seCond finger → conventional Current direction
- thuMb → Motion (force direction)
A rectangular coil of N turns in a magnetic field experiences a torque (turning force) — the basis of the DC electric motor. A split-ring commutator reverses the current every half-turn so the coil continues rotating in the same direction.
Electromagnetic induction
When the magnetic flux through a conductor changes, an e.m.f. (voltage) is induced — this is Faraday's law of electromagnetic induction.
Lenz's law: the induced current opposes the change that caused it (energy conservation).
Factors increasing the induced e.m.f.:
- Moving the magnet faster / increasing rate of flux change
- Stronger magnet (higher B)
- More turns on the coil
- Using a soft iron core (concentrates field)
AC generator (alternator)
A coil rotates in a magnetic field. As it rotates, flux through the coil changes → alternating e.m.f. is produced. Slip rings (not commutator) allow continuous rotation and produce a sinusoidal AC output. At positions parallel to B, rate of flux change is maximum → maximum e.m.f. At positions perpendicular to B (coil sides moving along field lines), rate of change = 0 → zero e.m.f.
Transformers
A transformer changes the voltage of AC. It has two coils (primary and secondary) wound on a soft iron core.
Transformer equation: V_p / V_s = N_p / N_s (also: V_p I_p = V_s I_s for 100% efficiency)
- Step-up transformer: N_s > N_p → V_s > V_p (voltage increases, current decreases).
- Step-down transformer: N_s < N_p → V_s < V_p (voltage decreases, current increases).
Why AC, not DC? A transformer requires a changing magnetic flux (from AC) to induce an e.m.f. in the secondary coil. A steady DC produces a steady magnetic field — no induction.
National Grid
Electricity is transmitted at very high voltage (e.g. 400 kV) to reduce current and therefore reduce power loss in cables: P_loss = I²R. Step-up transformer at the power station increases voltage; step-down transformers near homes reduce it to safe levels (230 V in UK/NI).
⚠Common mistakes
- Fleming's left hand vs right hand — left hand for motor (force on current in field); right hand rule for generators (predicting induced current direction) — or use Lenz's law for generators.
- AC not DC in transformers — DC produces no flux change → no induction.
- Transformer ratio mixing up p and s — label clearly and use V_p/V_s = N_p/N_s.
- Power loss equation: P = I²R not P = V²/R for the same cable (R is fixed).
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