Electromagnets · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Electromagnets: magnets you can switch on

Science · Forces & Motion · ages 12-13
Name ______________________   Date ____________

  1. An electromagnet keeps its full strength after the current is switched off.

    Circle one:   True   False

  2. Complete the sentence: winding a wire into a coil makes a stronger magnet because the small fields of every loop ___.

    • point in different ways and cancel
    • escape into the air before adding
    • point the same way and add up

  3. A student tests whether coil turns affect an electromagnet. Which plan is the fairest test?

    • Change turns, but keep current and core the same
    • Change turns and current, but keep the core the same
    • Change the core and turns, but keep current the same
    • Change the battery, core and turns together

  4. What creates the magnetic field around a wire?

    • Only a moving magnet nearby
    • Current flowing in the wire
    • The plastic coating on the wire

  5. Ben tests his electromagnet with the same battery and nail. A 10-turn coil lifts 4 paperclips, and a 20-turn coil lifts 7. The pattern keeps going: every extra 10 turns lifts 3 more paperclips. How many does a 30-turn coil lift?

    Answer: ______________

  6. Which core material makes the strongest electromagnet?

    • Iron
    • Wood
    • Plastic

  7. An electric bell's electromagnet pulls a hammer against a gong while its button is pressed. Why does the hammer stop striking when you release the button?

    • The hammer gets tired after hitting the gong several times
    • The electromagnet becomes permanent and holds the hammer in place without power
    • Letting go opens the switch, so current stops and the hammer springs back

  8. Ana rewinds a coil on the same nail, keeping its length and current the same. Only the number of turns changes. It lifts 4 clips before and 10 after. Which listed change explains the larger lift?

    Before4After10
    • She painted the nail to help clips stick
    • She added more turns around the nail
    • She removed turns, making a shorter coil

  9. A crane releases a car by switching off its electromagnet's coil current. A fridge magnet stays attached without a circuit. Why can a switch control the crane's powered pull but not the fridge magnet's?

    • The crane releases cars because its electromagnet is larger
    • Only the crane's powered pull needs coil current
    • The fridge magnet releases objects whenever its circuit current stops

  10. Ravi tests same-length coils and same-sized cores safely. Batteries are in series so voltages add. Two batteries give the same higher current in every coil; one gives the same lower current. More turns, current and an iron core each increase clip lift. Which build lifts most?

    • 45 turns on an iron nail with two batteries
    • 45 turns on a plastic rod with two batteries
    • 15 turns on an iron nail with one battery
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Answer key

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

Electromagnets: magnets you can switch on W1-mt_2qkn8Lhc8e-s1

  1. False · Stopping current removes the coil's field. A soft iron core may keep a little magnetism, but it does not keep the electromagnet's full powered strength.
  2. point the same way and add up · Inside the coil, the loop fields point the same way and add together, strengthening the field.
  3. Change turns, but keep current and core the same · A fair comparison changes one factor. To test turns, keep current and core material the same.
  4. Current flowing in the wire · Current flowing through any wire creates a magnetic field circling around it, even with no magnet nearby.
  5. 10 · Going from 20 turns to 30 turns adds one more step of 10 turns, which lifts 3 more paperclips: 7 plus 3 is 10.
  6. Iron · Iron has domains that line up and add magnetism, so it boosts the strength far more than wood or plastic.
  7. Letting go opens the switch, so current stops and the hammer springs back · Releasing the button opens the switch and stops current. The electromagnet loses its powered pull, so the hammer springs back and stops striking. The gong may keep sounding briefly.
  8. She added more turns around the nail · Only the turns change; current, coil length and nail stay the same. More turns add more loop fields, explaining the rise from 4 to 10 clips.
  9. Only the crane's powered pull needs coil current · Switching off coil current removes the crane's powered field, reducing its pull enough to release the car. Its core may retain some magnetism. The fridge magnet needs no circuit current.
  10. 45 turns on an iron nail with two batteries · Under the stated test conditions, the 45-turn iron-core build with two batteries combines more turns, an iron core and the higher current. Each comparison build lacks one or more of those increases.
Worksheet · LightMySky