Electromagnetism
One question unlocks the whole topic: does your system have a battery? That single decision splits every electromagnetism question into two distinct paths — a current creating a magnetic field, or a changing field inducing a current.
The magnetic field forms concentric circles centred on the wire. The field is strongest close to the wire and weakens with distance — it never stops, it just keeps spreading out.
A single loop creates field lines that pass through the centre and bow outward on both sides — identical in shape to the field of a very flat bar magnet. The two faces of the loop behave like poles.
• Current flows anticlockwise → you're looking at the North pole.
• Current flows clockwise → you're looking at the South pole.
A solenoid stacks many loops in a row, creating a uniform magnetic field inside its core. Outside the solenoid, the field pattern is identical to a bar magnet — with a North and South pole at the two ends.
• Current flows anticlockwise → that end is the North pole.
• Current flows clockwise → that end is the South pole.
This also works in reverse: if you know the pole, you immediately know the current direction. The more turns (N) and the greater the current, the stronger the field.
As the magnet moves toward the loop, the flux through it increases. By Lenz's law, the induced current creates a field that opposes that increase — the near face of the loop becomes a North pole, pushing back against the approaching magnet.
2. Decide: is flux increasing or decreasing?
3. Lenz: induced B-field opposes the change.
4. The exam tells you which face to view from.
5. If induced B points toward you → anticlockwise current. Away → clockwise.
When the magnet moves into the solenoid, flux increases. The solenoid induces a current whose field opposes the entry — the near end becomes a North pole, repelling the incoming North pole of the magnet.
• Magnet exiting (N first) → near end = S (attracts, resisting exit) → current reverses.
• The solenoid always opposes whatever the magnet is doing.
Quick Reference
All five scenarios at a glance.
| Scenario | Battery? | What happens | Key rule | Formula |
|---|---|---|---|---|
| Straight wire | YES | Concentric circular field around wire | RHR — thumb = I, fingers = B | — |
| Current loop | YES | Field through centre; faces are N/S poles | Face rule — anticlockwise = N | — |
| Solenoid (battery) | YES | Uniform field inside; bar-magnet field outside | RHR + face rule | — |
| Flat loop (induction) | NO | Changing flux induces current in loop | Lenz + face rule | \(\varepsilon = -N\dfrac{\Delta\Phi}{\Delta t}\) |
| Solenoid (induction) | NO | Moving magnet induces EMF and current | Lenz + face rule + RHR | \(\varepsilon = -N\dfrac{\Delta\Phi}{\Delta t}\) |
| Magnetic flux | — | Amount of B-field through a surface | θ = angle between B and surface normal | \(\Phi = BA\cos\theta\) |