The short answer
Decide whether a changing magnetic set-up induces an EMF, which way it drives current, and what happens to the size of it when the conditions change.
Written and checked by GAMSAT tutors — not AI-generated.
Try the reasoning style
We treat forgetting as a failure — a lapse to be patched with reminders and records. Yet a mind that kept everything could not think; it would drown in the undifferentiated noise of every moment it had ever lived. To forget is not so much to lose information as to decide, mostly without our noticing, what was never worth keeping.
The author's argument relies most directly on which unstated assumption?
Pick an option to see how the tutor reasons to the answer — not just whether you were right.
Not quite — the answer is B.
Work backwards from the conclusion: a mind that ‘kept everything’ supposedly ‘could not think.’ That only follows if thinking means leaving most of experience out — so B is the premise the argument quietly rests on. A raises reliability, which the passage never weighs; C contradicts ‘mostly without our noticing’; D smuggles in a claim about intellect the passage never makes. The question rewards finding the hidden premise, not recalling a fact.
Section III hands you an apparatus you have never seen and asks: does anything happen, which way, and how big? Forces are the easy half — F = qvB sin θ and F = BIL sin θ, largest across the field, zero along it, always sideways.
Faraday in one line
EMF = N × (flux change) ÷ (time) — the rate, times the turns. Double the turns, double the EMF; halve the rate, halve it. Flux itself induces nothing: a coil motionless in a strong magnet gives zero.
| Set-up | Term that changes | What that tells you immediately |
|---|---|---|
| Loop dragged out of a uniform field | area in the field, A | an EMF only while an edge is crossing the boundary |
| Magnet falling through a coil | field through the coil, B | two pulses of opposite sign, with a zero between them |
| Coil rotating between the poles | orientation, θ | alternating EMF, largest edge-on, where the field skims across the face |
| Neighbouring circuit switched off | field through the loop, B | an EMF although nothing moves, lasting only while the current dies |
Moving fast is not the same as changing flux
A loop moving entirely inside a uniform field has constant flux and no EMF, however hard it is dragged: induction happens at the boundary. Nor need anything move — a loop beside a circuit switching off gets an EMF while its field dies.
Run any induction stem this way
Name which term of Φ = BA cos θ changes
θ is from the normal to the face: flux maximum face-on, zero edge-on. Only three things change it — B, the area in the field, or the orientation.
Growing or shrinking?
Ask whether flux that way is rising or falling. Everything downstream answers this.
The induced current opposes the CHANGE, not the field
Growing → the loop makes a field against it; shrinking → with it. Not against the existing field — half the wrong answers are that substitution. Right hand: thumb along the field the loop must make, fingers give the current.
Check the force
It fights what you are doing, so your work becomes the electrical energy. A coil helping the motion is a sign error.
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Worked example
A square loop of side 0.20 m is pulled at 0.50 m/s out of a uniform 0.40 T field, perpendicular to the loop. What EMF appears as it leaves, and once fully outside?
Check yourself
A hand-cranked generator spins a 200-turn coil in a fixed magnetic field and produces a peak EMF of 12 V. The coil is rewound with 400 turns of the same size and shape, and the handle is then turned at half the previous rate. The new peak EMF is closest to:
Key takeaways
- Φ = BA cos θ — only B, area, or angle can change it. Name which one first.
- EMF = turns × RATE of flux change. Steady flux induces nothing.
- Lenz: the current opposes the change, so the force fights the motion.
- No change, no EMF — induction happens at the boundary.
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