Self-Inductance and Energy Stored in a Magnetic Field · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Coils fight change: inductance, stored energy, RL circuits

Science · Electricity & Magnetism · ages 20-21
Name ______________________   Date ____________

  1. An inductor of inductance L carries current I. How much energy is stored?

    • Half L I squared
    • L times I
    • Half C V squared

  2. Two solenoids are identical except that one has more turns. Which has the larger self-inductance?

    • Both are equal, turns do not matter
    • The one with fewer turns
    • The one with more turns

  3. When you close the switch in an RL circuit, the current jumps to E over R at once.

    Circle one:   True   False

  4. The magnetic field in a coil doubles while L stays fixed. What happens to the stored energy?

    • It quadruples
    • It doubles
    • It stays the same

  5. An RL circuit has L equal to 4 H and R equal to 2 ohm. What is its time constant?

    • 8 s
    • 2 s
    • 0.5 s

  6. Current through a coil starts rising. Which way does the induced emf push?

    • With the rise, boosting it further
    • It vanishes during any change
    • Against the rise, fighting the change

  7. Opening a switch on a live inductor throws a spark. Why?

    • The field energy dumps back into the circuit at once
    • The resistor releases stored charge
    • The battery surges when freed

  8. A student says that one tau after closing the switch, the current has fully reached E over R. What is the error?

    • The final value is E times R, not E over R
    • After one tau it has covered only about two thirds of the rise
    • Tau equals R over L, so the rise finishes instantly
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Answer key

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

Coils fight change: inductance, stored energy, RL circuits W1-mt_BLyLSJT-Fr-s1

  1. Half L I squared · The work of building the current parks in the field as half L I squared.
  2. The one with more turns · Solenoid L grows with turns squared, so more turns means bigger L.
  3. False · Back emf holds the rise back, so the current creeps up gradually.
  4. It quadruples · Energy density grows with B squared, so doubling B quadruples the energy.
  5. 2 s · Tau equals L over R, so 4 over 2 gives 2 seconds.
  6. Against the rise, fighting the change · The coil opposes the change, so its emf pushes against the rise.
  7. The field energy dumps back into the circuit at once · The parked field energy has to go somewhere, so it forces its way across the gap.
  8. After one tau it has covered only about two thirds of the rise · One tau covers about two thirds of the climb. Full arrival takes several tau.
Worksheet · LightMySky