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

# Force law between two magnets as a function of their separation

## Objective

To explore force law between two magnets as a function of their separation.

## Introduction

The force between two magnets does not have a simple relation with distance. This is because both the magnets have two poles and so four forces are involved for each magnet. The resultant force depends on the geometry of the magnets. You will investigate the force between two cylindrical magnets in a particular geometry.

## Procedure

The spring balance is calibrated in newtons. On this scale 0.1 N has a physical length of 1 cm. Fix up the suspension rod horizontally and suspend the spring balance from it. At the lower end suspend the magnet A.

Fix the plastic tube around the lower part of the spring balance. The magnet A goes inside this tube.

Read the pointer on the balance. This actually gives the weight of the magnet A. Take this position as $$X$$ (force between the magnets A and B is zero). Insert the magnet B in the plastic tube from below by holding the long rod.

Gradually raise the rod and see when the spring balance pointer moves up by say 1 division so that you can read it. The plastic tube has a graph strip attached to it. Measure the facing ends of A and B on this scale. This gives separation between the magnets. Note the reading $$x_1$$ of the balance pointer. $$X-x_1$$ gives the force between the magnets.

Make table to note the position readings of magnet B on the graph strip and the corresponding balance reading. From these two readings you can find the decrease in separation between the magnets. Remember 0.1 N of graduation on the balance has physical length of 1 cm. The balance reading (with $$X$$) gives the force. Make 8 to 10 readings till the balance reading goes out of scale.

Draw a graph between the force between the magnets versus separation between their facing poles. Can you suggest an algebraic relation between the two.