NCERT Solutions Curiosity Chapter 4 Think like a scientist — Electromagnets

Book page 51 Updated on2026-09-05

Q1.

Repeat Activity 4.3 with — (i) 2 and 4 cells with the same coil, (ii) 2 cells but different number of turns of the coil. What do you observe?

Answer

Both changes make the electromagnet stronger: the compass needle deflects more and the coil holds more clips.

What is changed	What you observe	Reason
1 cell	Small deflection; only a few clips	A single cell drives only a small current, so the field is weak
2 cells	Larger deflection; more clips	A battery of 2 cells drives a larger current, so the field is stronger
4 cells	Larger still	Still more current, still stronger field
2 cells, more turns on the coil	Stronger again, without changing the cells	Each extra turn adds its own field to the total

Why it happens: the magnetic field of the coil is produced by the current going round the turns. Increase the current and each turn contributes more; increase the number of turns and there are more contributions to add. Either way — or both together — the field inside the coil grows, so the needle swings further and more clips can be held.

Tip: keep the compass in exactly the same place each time, otherwise you are changing two things at once and cannot say which one caused the difference.

Q2.

Also, repeat Activity 4.4 by changing the direction of the current.

Answer

Reverse the connections to the cell and the deflection of the compass needle reverses too: the pole of the needle that used to be attracted to end A is now pushed away, and the other pole is attracted instead. In other words, the poles of the electromagnet have swapped over — what was South is now North.

Why it happens: the sense in which the current goes round the coil decides which end of the coil is North. Reverse the current and you reverse that sense, so the two poles exchange places. The size of the deflection does not change, because the amount of current is the same as before — only its direction has changed.

Putting the whole box together:

The strength of an electromagnet is changed by changing the current, the number of turns, or both.
The polarity of an electromagnet is reversed by reversing the direction of the current.

Did you know? A permanent bar magnet can do neither. Being able to change strength and polarity at will is exactly what makes electromagnets so useful in bells, motors, loudspeakers and cranes.

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