Question

Read the excerpt from a story and answer the questions that follow:

On stage, a soft iron box sits atop a hidden copper coiled platform, conductive to electricity. I summoned a man who fancied himself a modern-day Hercules onto the stage and challenged him to lift the iron box. With ease, he lifted it up. Then, with a wave of my magic wand and a hidden switch activated, I said, “I’m going to take away your strength.” When I asked him to lift the box again, he struggled, unable to budge it.

1. Explain the secret behind the magic.
2. If you are asked to design the same magic in your science lab, give the necessary circuit diagram.
3. Give any one device that works on the same principle.

Answer 1

(a) The platform contains a hidden coil of insulated copper wire wound on a soft iron core, so it works as an electromagnet. The first time the man lifts the box, no current flows through the coil. The core is not magnetised, so he lifts the box easily. When the magician waves the wand, he secretly closes the hidden switch. Current now flows through the coil and the soft iron core becomes a strong electromagnet. It attracts the soft iron box and holds it firmly to the platform, so the man cannot lift it. His strength was not taken away; the magnetic force was holding the box down.

(b)

When the switch K is closed, the soft iron core becomes an electromagnet and holds the iron box.

(c) An electric bell works on the same principle (an electromagnet). An electromagnetic crane used to lift scrap iron is another example.