Magnetic Flux, Flux Linkage and Faraday's Law · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Magnetic flux and induced emf

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

  1. B equals 0.5 T and the coil's area is 0.2 square metres. The coil is face on to the field. What is the flux in webers?

    Answer: ______________

  2. A flat loop of area A is in a uniform magnetic field of strength B. Theta is the angle between the field and the loop's normal, which is perpendicular to its surface. What is the magnetic flux through the loop?

    • B A cos theta
    • B over A times theta
    • B plus A plus N

  3. A coil edge on to a strong field still has zero flux through it.

    Circle one:   True   False

  4. A coil's flux linkage falls from 12 to 3 weber-turns in 3 seconds. What is the magnitude of the average induced emf, in volts?

    Answer: ______________

  5. A flux of 0.1 webers threads a 100 turn coil. What is the flux linkage?

    Answer: ______________

  6. A coil rotates in a steady field so that the angle between its normal and the field changes. Why is an emf induced?

    • The wire warms up and pushes charge around the coil
    • The field gradually strengthens as the coil rotates
    • Its cos theta changes, so linkage changes

  7. Rosa reads emf off the linkage value instead of its slope. What is her error?

    • Flux linkage values must always remain zero
    • Emf depends on linkage's rate of change
    • Emf is always equal to the coil's turn count

  8. A flux linkage against time graph shows a straight fall of 8 weber-turns over 2 seconds. What is the magnitude of the induced emf, in volts?

    Answer: ______________

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Answer key

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

Magnetic flux and induced emf W1-mt_ZoN5JePCpt-s1

  1. 0.1 · Face on means cos theta is 1: 0.5 times 0.2 is 0.1 webers.
  2. B A cos theta · With theta measured from the loop's normal, the field component perpendicular to the loop is B cos theta. Multiplying by area A gives B A cos theta.
  3. True · Edge on, the tilt reduces the flux to zero even when the field is strong.
  4. 3 · The signed linkage change is 3 - 12 = -9 weber-turns. Average emf is -(-9/3) = +3 V, whose magnitude is 3 V.
  5. 10 · Linkage counts flux once per turn: 100 times 0.1 is 10.
  6. Its cos theta changes, so linkage changes · N, B and A stay fixed, but the changing angle changes cos theta. The flux linkage N B A cos theta therefore changes and induces an emf.
  7. Emf depends on linkage's rate of change · Faraday's law uses the rate of linkage change, not the linkage value. Signed emf is the negative gradient; its magnitude is the gradient's absolute value.
  8. 4 · The gradient is -8/2 = -4 Wb/s. Signed emf is the negative gradient, +4 V. Its magnitude is 4 V.
Worksheet · LightMySky