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

Magnetic Field Lines for a Three Pole Magnet

Objective

To draw magnetic field lines for a given magnet.

Introduction

A magnet has two poles, north and south. The `three pole magnet' is an assembly of two bar magnets with similar poles connected end-to-end. The two sides of assembly have same pole but middle one has different. This can be used to dramatize the things. Take two strong magnets (preferably cylindrical) and tight-fit them inside a rubber/PVC pipe. Ask a student to find location and type of poles of this assembly.

Magnetic field line gives the direction of magnetic field at the points from which these lines are drawn. To get the magnetic field direction, one can use a small compass which rests in the direction of the resultant magnetic field (horizontal component). To get the field line due to magnet only, one should ensure that the compass needle rests along north-south direction. In this case, the torque due to the earth's magnetic field is zero.

Apparatus

a magnet, a drawing board or any board of size about 12 inch by 18 inch, a compass, two A4 size paper sheets or an A3 paper sheet, two wooden stands with a long thread tying both, cellophane tape

Procedure

You have to draw magnetic field lines for the given magnet. Place the paper sheet on the board and fix the corners using the tape. Put the two wooden stands with thread stretched between them at sufficient distance so that the drawing board can easily rotate under the thread. Place the magnet in the middle and tape it.

Draw 8 field lines starting from one end of the magnet, 4 on each side of it. Make sure, you make the compass in north south direction before marking the end positions to avoid torques from earth's magnetic field.

Note

Similar to three pole magnet, a mono-pole magnet assembly can be constructed by joining a strong and a weak magnet.