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. In a closed resistive coil, the induced current produces a field that helps the magnetic flux change that caused it.

    Circle one:   True   False

  2. A north pole approaches a closed resistive coil along its axis. What does the coil's near face become?

    • North, opposing its approach
    • South, attracting it inward
    • No pole, despite changing flux

  3. An ideal planar coil rotates at constant angular speed about an axis in its plane and perpendicular to uniform B. What sets its emf frequency?

    • The number of turns in the winding
    • The rotation rate of the coil
    • The resistance of the load circuit

  4. An ideal planar coil rotates steadily about an axis in its plane and perpendicular to uniform B. Its peak emf is N B A omega. Which change increases every factor in this product?

    • Fewer turns, smaller area, weaker B and slower rotation
    • Same turns, same area, same B and slower rotation
    • More turns, larger area, stronger B and faster rotation

  5. A moving magnet drives current through a resistive load. Why would an induced force that helped the motion violate energy conservation?

    • It would generate energy without an input
    • It would require a coil with no resistance
    • It would make every magnetic field vanish

  6. A north pole is withdrawn along the axis of a closed resistive coil. What does the coil's near face become?

    • North, repelling it farther away from the coil
    • South, attracting it back toward the coil
    • No pole, even though magnetic flux is changing

  7. A magnet and a closed resistive coil remain at rest, with constant flux linkage and no other supply. What happens after earlier transients die away?

    • Zero induced current after transients have died away
    • A steady induced current sustained by the stationary magnet
    • A growing induced current despite the unchanging flux

  8. A planar coil rotates steadily about an axis in its plane and perpendicular to uniform B. Its angular speed doubles between two runs; N, A, B and the axis stay fixed. What happens to its emf?

    • Frequency doubles; peak emf stays unchanged
    • Peak emf doubles; frequency stays unchanged
    • Both frequency and peak emf double
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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 · The induced field opposes the change in flux. It can reinforce an existing field when that field's flux is decreasing.
  2. North, opposing its approach · The approaching pole increases magnetic flux. A north face repels it and opposes that change.
  3. The rotation rate of the coil · Each revolution produces one emf cycle. Revolutions per second therefore equal cycles per second.
  4. More turns, larger area, stronger B and faster rotation · The stated change increases N, A, B and omega, so their product and the peak emf increase.
  5. It would generate energy without an input · The load receives energy. If the induced force also supplied mechanical energy instead of opposing motion, neither output would come from work done to move the magnet.
  6. South, attracting it back toward the coil · Withdrawal decreases flux. A south face attracts the departing north pole and opposes withdrawal.
  7. Zero induced current after transients have died away · Constant linkage gives zero induced emf. With resistance and no other supply, an earlier current dies away.
  8. Both frequency and peak emf double · Both f = omega / (2 pi) and peak emf = N B A omega are proportional to angular speed.
Worksheet · LightMySky