Concepts of Physics

IIT JEE Physics (1978-2018: 41 Years) Topic-wise Complete Solutions

Magnetization of a Needle

Introduction

It is known that rubbing the needle from one end to the other end of a bar magnet can magnetize the needle. However, it is generally believed that one has to rub the needle many times to bring reasonable magnetization in it.

Apparatus

a bar magnet, two sewing needles

Procedure

Put a sewing needle on a table. Press it by a finger at one end. Hold the bar magnet in the other hand. Press the portion of the needle close to finger by one pole of the magnet. Drag the magnet to the other end. Your needle has become reasonably strong magnet.

Hold the needle in your hand. Put a non-magnetized needle on the table. Bring the magnetized needle close to it and see the attraction. By properly moving your hand, you can make the needle on the table in desired way.

Discussion

Molecules of magnetic materials behaves like tiny magnets. Each of these tiny `magnetic particles' has two poles. Normally these tiny magnets are in random orientations. So their polarity neutralizes and material does not have any magnetic property.

When you rub it with a magnet, the tiny molecular magnets gets lined up with their north poles pointing in one direction and south poles pointing in other direction, making its magnetic property effective.

A permanent bar magnet is made of small regions or `domains'. Each domain acts as small magnet and contains atoms with their electrons acting as little magnets. All the little magnets in a domain point in the same direction. All the domains contribute to the strength of the magnet.