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Class 8 Science — Magnetic and heating effects of current

Explain how a current makes a magnet and how it makes heat, and say what changes the strength of an electromagnet.

Name: ________________________

  1. 1.Why should a used battery go to an e-waste facility rather than into the household bin?

    • a) Household bins are too small for batteries
    • b) It will start working again once it has been sorted
    • c) A used battery is completely inert and so has no value
    • d) It can still contain acids and metals that may cause fires or harm the environment

  2. 2.Nichrome, rather than copper, is used in electric heating devices. Why?

    • a) Nichrome conducts electricity better than copper
    • b) For a wire of the same size and length, nichrome offers higher resistance and so gives more heat
    • c) Nichrome does not conduct electricity
    • d) Nichrome is cheaper than copper

  3. 3.A nichrome wire in a circuit gets warm. What is happening inside it?

    • a) The cell is hot, and the heat travels along the wire
    • b) The current rubs against the sides of the wire, and friction warms it
    • c) The wire is absorbing heat from the air around it
    • d) The wire resists the flow of current, and that resistance turns some electrical energy into heat

  4. 4.Match each part of a Voltaic cell to what it is.

    • Electrodes
    • Electrolyte
    • A dead cell
    • the liquid they are dipped in, usually a weak acid or salt solution
    • one whose chemicals have been used up, so it supplies no more electricity
    • two metal rods of different materials

  5. 5.The north pole of a magnetic compass is attracted towards end A of an electromagnet. Which pole is end A?

    • a) Both at once
    • b) South
    • c) North
    • d) Neither — an electromagnet has no poles

  6. 6.In the lemon cell built in the chapter, what plays the part of the electrolyte?

    • a) The iron nail
    • b) The LED
    • c) The lemon juice
    • d) The copper wire

  7. 7.Match each change you could make to an electromagnet with what it does.

    • Adding a second cell to the circuit
    • Winding more turns onto the coil
    • Reversing the direction of the current
    • Pulling the iron nail out of the coil
    • a larger current, so a stronger magnet
    • a stronger magnet without changing the current
    • the North and South ends swap over
    • a weaker magnet, since the core is gone

  8. 8.The chapter lists what the heat generated in a wire depends on. Which list is right?

    • a) The temperature of the room the circuit is in
    • b) The material, thickness and length of the wire, and how long the current flows
    • c) The colour of the wire and the shape of the switch
    • d) The material of the wire and nothing else

Answer key — Class 8 Science — Magnetic and heating effects of current

  1. 1. d) It can still contain acids and metals that may cause fires or harm the environment
  2. 2. b) For a wire of the same size and length, nichrome offers higher resistance and so gives more heat
  3. 3. d) The wire resists the flow of current, and that resistance turns some electrical energy into heat
  4. 4. Electrodes → two metal rods of different materials; Electrolyte → the liquid they are dipped in, usually a weak acid or salt solution; A dead cell → one whose chemicals have been used up, so it supplies no more electricity
  5. 5. b) South
  6. 6. c) The lemon juice
  7. 7. Adding a second cell to the circuit → a larger current, so a stronger magnet; Winding more turns onto the coil → a stronger magnet without changing the current; Reversing the direction of the current → the North and South ends swap over; Pulling the iron nail out of the coil → a weaker magnet, since the core is gone
  8. 8. b) The material, thickness and length of the wire, and how long the current flows