Lesson 1 of 11 · 5 min

Magnets and their poles

NCERT §5.1

Nisha's tray starts with one bar magnet and a hacksaw. If she cuts the magnet in half, does she get a north pole in one hand and a south pole in the other?

The story this chapter follows: Nisha's magnet tray

Nisha's tray holds a bar magnet of moment 0.4 A m², a solenoid of 500 turns that matches it, an electromagnet giving a uniform 0.05 T, a compass, and a lab solenoid of 1000 turns per metre on 2 A. Beside it sit a soft-iron core of relative permeability 400, an alnico bar, a tube of water and an aluminium rod. Every idea in this chapter turns up on that tray.

The lesson in notes

In short

Magnetic fields are everywhere, from galaxies to atoms. The word magnet comes from Magnesia, a Greek island where magnetic ore was found as early as 600 BC.

The earth acts as a magnet whose field points roughly from the geographic south to the geographic north.

A freely hung bar magnet settles along the north–south line. The end that turns towards geographic north is its north pole; the other end is its south pole.

Like poles (N and N, or S and S) repel each other; unlike poles attract.

A pole cannot be isolated. Break a bar magnet in two and each half is a complete, somewhat weaker magnet with its own N and S. Isolated poles, called magnetic monopoles, are not known to exist, unlike separate positive and negative charges.

Iron and its alloys can be made into magnets.

Example 5.1: cutting a magnet across its length or along it gives two magnets either way, each with a north and a south pole.