Lenz's Law and the Alternating-Current Generator · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Fighting change, spinning power

Science · Energy · ages 17-18
Name ______________________   Date ____________

  1. The induced current flows so as to help the change that produced it.

    Circle one:   True   False

  2. You push a north pole into a coil. What does the near face become?

    • North, to repel it
    • South, to welcome it
    • Neutral, nothing happens

  3. What sets the frequency of a generator output?

    • The wire colour
    • The spin rate of the coil
    • The room temperature

  4. Which set raises the peak emf of a generator?

    • Thinner wire and longer cables
    • Fewer turns and slower spin
    • More turns, bigger area, stronger field, faster spin

  5. Why must the induced current oppose the change?

    • Otherwise energy comes from nothing
    • Otherwise the wires melt
    • Otherwise the magnet stops

  6. You pull the north pole back out of the coil. What does the near face do?

    • Stays north to push it out
    • Flips south to hold it back
    • Goes neutral at once

  7. A magnet sits perfectly still inside a coil. What current flows?

    • None, nothing changes
    • A steady large current
    • A current that grows daily

  8. Spin rate doubles with the same coil and field. What happens to the output?

    • Only the frequency doubles
    • Only the peak doubles
    • Frequency doubles and peak doubles
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Fighting change, spinning power W1-mt_69u-fTOSku-s1

  1. False · It always fights the change. Helping would build free energy.
  2. North, to repel it · The induced field fights the incoming pole, so the near face turns north.
  3. The spin rate of the coil · Each turn gives one cycle, so faster spinning means faster alternation.
  4. More turns, bigger area, stronger field, faster spin · Peak emf scales with turns, area, field, and spin rate together.
  5. Otherwise energy comes from nothing · Helping the change would suck the magnet in while lighting bulbs for free.
  6. Flips south to hold it back · The change is now leaving, so the coil flips south to hold the pole back.
  7. None, nothing changes · Induction needs change, and a still magnet changes nothing.
  8. Frequency doubles and peak doubles · Spin sets both the cycle rate and, with the rest, the peak height.
Worksheet · LightMySky