Question

Describe two methods of determining the arrangement of the lines of force in the field close to a bar magnet. Give a brief explanation of each method.

Answer 1

First method: Place cardboard on the top of a bar magnet and scatter some iron filings uniformly over the whole of cardboard. Now tap it with a pencil. The filing is magnetized by induction and arrange themselves in curved lines as shown in the figure. The curved lines are called the magnetic lines of force which may be defined as the lines in a magnetic field along which free magnetic poles tend to be driven if free to do so.

Second method: Lines of force can be traced on a paper by using a tracing needle or small magnetic compass needle.
The bar magnet is placed on a sheet of paper and its boundary is drawn with a sharp pencil. A point X is marked along the boundary towards the north of the magnet.
The tracing needle is then placed at point X in such a way that its one endpoints towards the point X. With the help of a pencil, the direction of the other end of the needle is marked on paper. Let it be point Y.
Now, shift the needle from the point X and place it in such a way that its one end points towards the point Y. The direction of other end of needle is marked by pencil. Let it be point Z. The process is continued till a closed curve is obtained.

This curve is called a magnetic line of force.

If we plot a number of such curves around the magnet, starting from different points then space so enclosed is called magnetic field.