# Mapping of Magnetic Field Experiment

## Experiment 1:

Aim: To map the magnetic lines of force surrounding a bar magnet placed in the magnetic meridian with its north pole pointing magnetic north of the earth and, hence to determine the moment of the magnet.

Apparatus: The given bar magnet, compass needle, brass fixing pins, drawing board, drawing paper etc.,

Procedure

(1) To draw the magnetic meridian

A sheet of drawing paper is fixed on a drawing board. A straight line is drawn symmetrically on the paper. The compass needle is placed on the line. The drawing board is slowly turned till the ling becomes parallel to the magnetic needle in the compass box. Since the magnetic needle always comes to rest along the magnetic north-south direction, the line is along the magnetic meridian. A dotted line is drawn perpendicular to the magnetic meridian. The outline of the drawing board is drawn on the table with a chalk. Hereafter the board should not be disturbed. A short straight line with an arrow head is drawn at the upper right hand corner of the paper to indicate the direction of the magnetic north of the earth.

(2) To draw the lines of force

The bar magnet is placed on the paper with its axis in the magnetic meridian; its north pole pointing earth's magnetic north. The dotted line should become the equatorial line of the magnet. The outline of the magnet is drawn on the paper. The compass needle is placed near the north pole of the magnet and a dot is put against the north pole of the needle. The compass box is moved-so that its south pole is against dot. Another dot is put against the north pole of the needle and the process is repeated to get a number of dots. The dots are joined with a smooth line. This is a line of force. A number of such lines of force are also drawn. The general pattern is shown in the figure. Arrow marks are put on the lines to show the directions of the lines of force. It is found that there are two points, N1 and N2, symmetrically situated on either side of the magnet on the equatorial line where there are no lines of force. The compass needle placed at these points will remain in any direction. These are null points. The distance 2d between the two null points is measured and d, the distance of a null point from the centre of the bar magnet is found out. The length 2l of the magnet is measured using a metre scale. From this, the half the length l of the magnet is determined. At null points, the horizontal intensity Bh of the earth's magnetic field and the field due to the bar magnet are equal and opposite.

i.e., Bh = 10-7 x   m/(d2 +l2)3/2

Therefore, m = Bh(d2 + l2)3/2 x 107;

The moment m of the bar magnet is calculated assuming the value of Bh.

Note:-The null points may be marked conveniently as follows:-

The compass box is placed on the equatorial line of the magnet. It is kept near the magnet. Since the field due to the bar magnet is greater than that of the earth, the magnetic needle in the compass box becomes parallel to the bar magnet with its north pole pointing the south pole of the bar magnet, which is towards the earth's magnetic south. The compass box is moved slowly along the equatorial line away from the bar magnet. When it just crosses the null point, the magnetic needle turns so that its north pole points the magnetic north pole. This is because the earth's horizontal field Bh becomes greater than that due to the bar magnet. The position of the compass box is marked. Now, the compass box is moved along the equatorial line towards the magnet. When it just crosses the null point, the magnetic needle turns as the field due to the bar magnet becomes greater. The position of the compass box is marked again. The midpoint between the two positions of the compass box marked on the drawing paper is marked as N1, the null point. In a similar way the null point N2 is noted along the equatorial line on the other side of the bar magnet.

## Experiment 2:

Aim: To map the magnetic lines of force surrounding a bar magnet placed in the magnetic meridian with its north pole pointing south and, hence to determine the moment of the magnet.

Apparatus: The bar magnet, compass needle, drawing board, drawing paper, brass fixing pins etc.

Procedure:
As in Experiment 1, the magnetic meridian is drawn on a drawing paper fixed on the drawing board. The bar magnet is placed on the meridian with its north pole pointing earth's magnetic south. The lines of force are drawn as explained above. The general pattern is shown in the figure. The null point N1 and N2 are obtained on the axial line of the-magnet. The distance 2d between the null points and hence d is found out. The length 2l of the magnet is determined using a meter scale. From this half the length 1 of the magnet is calculated. The moment of the magnet is calculated from the equation,

Bh = 10-7 x 2md/(d2l2)2

Therefore, m = Bh(d2l2)2 x 107/2d

Mapping of Magnetic Field Viva Questions with Answers

(i) What is a magnetic line of force?

Ans: It is a line in a magnetic field which indicates the direction along which the magnetic force acts. It is a continuous curve in a magnetic field such that the tangent at any point on it gives the direction of the resultant field at that point.

(ii) What is a neutral point in a magnetic field?

Ans: It is a point where the resultant magnetic field is zero.

(iii) No two lines of force intersect. Why?

Ans: If they intersect at any point it means that the magnetic field at that point are in two directions which is impossible.

(iv) What is the expression for the magnetic field at a point on the axis of a bar magnet?

Ans: B = (10-7 x 2md)T / (d2 — l2)2

(v) When a bar magnet is placed in the magnetic meridian with north pole pointing north we get two null points symmetrically situated. why?

Ans: There are two points on either side of the magnet on the equatorial line where the fields are equal.

(vi) What is the nature of the lines of force in a uniform magnetic field?

Ans: The lines of force are parallel.

(vii) Can you have a magnet with one pole?

Ans: No. A magnet has always two poles. Even if it is broken each piece becomes a complete magnet.

(viii) What is a magnetic dipole?

Ans: It is a current loop of moment  = iÂ Am

(ix) What is magnetic moment of a magnet?

Ans: It is numerically equal to the torque acting on it when it is placed at right angles to unit uniform field.

(x) What is magnetic equator?

Ans: It is the line joining points where the dip is zero.