4.7 Summary & Keep the Curiosity Alive

Key Points

  • When electric current flows through a conductor, it produces a magnetic field around it — the magnetic effect of electric current. The field disappears when the current stops. (Oersted, 1820.)
  • A current-carrying coil that behaves as a magnet is an electromagnet; an iron core makes it stronger. It has North and South poles.
  • An electromagnet’s strength increases with more current, more turns, and an iron core; its poles reverse when the current direction is reversed.
  • Lifting electromagnets on cranes lift and drop heavy metal by switching current ON/OFF. The Earth is a giant magnet because moving molten iron in its core creates currents.
  • The heating effect of electric current: resistance converts electrical energy to heat. Nichrome (high resistance) is used in heating elements of heaters, irons, kettles and lamps.
  • A cell/battery generates current through a chemical reaction between two electrodes and an electrolyte. Voltaic cells use a liquid electrolyte; dry cells use a paste and are single-use; rechargeable batteries (e.g. Li-ion) can be reused.

A Closing Challenge: Keep the Curiosity Alive

Think It Through

  1. Fill in: (i) The liquid in a Voltaic cell is the ______. (ii) A current-carrying coil behaves like a ______.
  2. True or false? (i) A coil becomes an electromagnet only when current flows. (ii) A single-cell electromagnet attracts more clips than a 2-cell one.
  3. Two lemon cells. In one, copper and iron are pushed into lemon juice; in the other, into pure water. In which case will the LED glow, and why?
  4. Slide out the nail. Neha removes the iron nail from her coil, leaving only the wire. Will the coil still deflect the compass — and will the deflection be more or less than before?
  5. Four coils of the same shape — made of iron, copper, aluminium and nichrome — each carry current with a compass nearby. In which of them will the compass deflect? (Think: does the magnetic effect need a special metal, or just a flowing current?)

You’ve now seen current do two kinds of work — pull like a magnet and warm like a flame. Next we step back from electricity to the broader idea behind all of it: force — the pushes and pulls that start, stop and steer everything that moves.