FINDING MAGNETIC FIELD AND FIELD LINES

FINDING MAGNETIC FIELD AND FIELD LINES

We are familiar with the fact that a compass needle gets deflected when brought near a bar magnet. A compass needle is, in fact, a small bar magnet. The ends of the compass needle point approximately towards north and south directions. The end pointing towards north is called north seeking or north pole. The other end that points towards south is called south seeking or south pole. Through various activities we have observed that like poles repel, while unlike poles of magnets attract each other.

Activity 13.2:

Figure 13.2: Iron filings near the bar magnet align themselves along the field lines

*  Fix a sheet of white paper on a drawing board using some adhesive material.

*  Place a bar magnet in the centre of it.

*  Sprinkle some iron filings uniformly around the bar magnet (Fig. 13.2). A salt-sprinkler may be used for this purpose.

*  Now tap the board gently.

*  What do you observe?

The iron filings arrange themselves in a pattern as shown Fig. 13.2. Why do the iron filings arrange in such a pattern? What does this pattern demonstrate? The magnet exerts its influence in the region surrounding it. Therefore the iron filings experience a force.The force thus exerted makes iron filings to arrange in a pattern. The region surrounding a magnet, in which the force of the magnet can be detected, is said to have a magnetic field. The lines along which the iron filings align themselves represent magnetic field lines.

Are there other ways of obtaining magnetic field lines around a bar magnet? Yes, you can yourself draw the field lines of a bar magnet.

Activity 13.3:

*  Take a small compass and a bar magnet

*  Place the magnet on a sheet of white paper fixed on a drawing board, using some adhesive material.

*  Mark the boundary of the magnet.

*  Place the compass near the north pole of the magnet. How does it behave? The south pole of the needle points towards the north pole of the magnet. The north pole of the compass is directed away from the north pole of the magnet.

Figure 13.3: Drawing a magnetic field line with the help of a compass needle

*  Mark the position of two ends of the needle.

*  Now move the needle to a new position such that its south pole occupies the position previously occupied by its north pole.

*  In this way, proceed step by step till you reach the south pole of the magnet as shown in Fig. 13.3.

*  Join the points marked on the paper by a smooth curve. This curve represents  a field line

Figure 13.4: Field lines around a bar magnet

*  Repeat the above procedure and draw as many lines as you can. You will get a pattern shown in Fig. 13.4. These lines represent the magnetic field around the magnet. These are known as magnetic field lines.

*  Observe the deflection in the compass needle as you move it along a field line. The deflection increases as the needle is moved towards the poles.

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Magnetic field is a quantity that has both direction and magnitude. The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole (note the arrows marked on the field lines in Fig. 13.4). Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves.

The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded (see Fig. 13.4).

No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.

Questions

1.  Why does a compass needle get deflected when brought near a bar magnet?

Source: This topic is taken from NCERT TEXTBOOK