Displacement Current and Maxwell's Equations · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Fixing Ampere's law: displacement current and Maxwell's set

Science · Electricity & Magnetism · ages 20-22
Name ______________________   Date ____________

  1. Which Maxwell equation says there is no magnetic charge?

    • Gauss for magnetism
    • Faraday's law
    • The Ampere-Maxwell law

  2. A loop circles the gap of a charging capacitor. What plays the role of current through the loop?

    • Nothing: the loop rule breaks and cannot be repaired
    • The changing electric flux between the plates
    • The static charge sitting on the plates

  3. Magnetic poles always come in north-south pairs, and no lone pole has been found.

    Circle one:   True   False

  4. Changing magnetic flux makes a curling electric field around it. Which equation says this?

    • Gauss for E
    • Faraday's law
    • Gauss for magnetism

  5. Which term does Maxwell add to the right side of Ampere's loop rule?

    • mu0 times the conduction current through the gap
    • eps0 times the static electric field between plates
    • mu0 eps0 times the rate of change of electric flux

  6. How did Maxwell's equations predict light?

    • They combine into a wave with speed 1 over sqrt(mu0 eps0)
    • They forbid changing fields, so no wave can leave a source
    • They show light needs a material medium such as air

  7. An antenna current oscillates, and a distant receiver with no wire link picks up the signal. How does the signal get there?

    • Electrons leave the antenna and fly through the air to the receiver
    • Accelerating charges launch E and B waves that carry the signal
    • The battery pushes a steady current across the empty space

  8. A student says the equations are fully symmetric, with lone north poles sourcing fields as usual. What is the error?

    • Symmetry is perfect: lone north poles source the field
    • The equations forbid electric charge as well
    • No magnetic charge is in the set; poles come in pairs
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Answer key

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

Fixing Ampere's law: displacement current and Maxwell's set W1-mt_YR8m8TNfyy-s1

  1. Gauss for magnetism · Gauss for magnetism reports zero magnetic source.
  2. The changing electric flux between the plates · Changing E flux acts exactly like a current in the loop rule.
  3. True · Every known magnet carries both poles together.
  4. Faraday's law · Faraday's law links changing B flux to a curling E field.
  5. mu0 eps0 times the rate of change of electric flux · The added term is mu0 eps0 times the rate of change of electric flux. Eps0 times that rate alone is the displacement current.
  6. They combine into a wave with speed 1 over sqrt(mu0 eps0) · The predicted speed matched the measured speed of light, so light is an electromagnetic wave.
  7. Accelerating charges launch E and B waves that carry the signal · The antenna's charges keep accelerating, so they launch linked E and B waves. At the receiver the passing wave pushes on electrons and drives a current.
  8. No magnetic charge is in the set; poles come in pairs · The magnetic Gauss law has zero source, which spoils the mirror.
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