# Magnetic field lines around a Bar Magnet Experiment

Experiment - 1

Mapping of Magnetic Field – I (North pole pointing south)

Aim: To map the magnetic lines of force around a bar magnet placed along the magnetic meridian with north pole pointing south and hence to find the moment and pole strength of the magnet.

Apparatus: Bar magnet, compass needle, drawing board, paper, pin etc

Principle:

Field along the axis of the magnet is given by B = µ0/4π . 2md/(d2-l2)2

Where, µ0 = 4π x 10-7 H/m

m = Dipole moment of the magnet

l = Half the length of the magnet

d = Half the distance between the null points

When north pole pointing south, neutral points are along the axial line of the magnet. At the neutral points (null points) the field due to the magnet is equal and opposite to the horizontal component of earth’s magnetic field (Bh).

ie, B = Bh

Therefore m = Bh(d2-l2)2 107/2d

Polestrength, P = m/2l

Procedure:

A sheet of paper is fixed on a horizontal drawing board using pins such that the plane of the paper is exactly parallel to the plane of the board. A line is drawn through the centre of the paper. The magnet and all magnetic materials are removed from the drawing board. A compass needle is placed with its centre on the line at the centre of the paper. The drawing board is rotated till the magnetic needle becomes parallel to the line. Then the line is in the magnetic meridian. The outline of the drawing board is drawn on the table using a piece of chalk. The board should be kept undisturbed throughout the experiment.

The north and south directions are marked on the paper. The bar magnet is placed symmetrically with its axis along the magnetic meridian and north pole pointing south. The outline of the magnet is also drawn on the paper. A dot is marked near to the north pole of the magnet. The compass needle is placed with its south pole above this point and the position of the north pole of the needle is again marked This is repeated till the compass needle reaches the south pole of the magnet .The dots are joined by a smooth curve This gives a magnetic line of force. A number of lines of force are drawn like this as shown in the figure. The directions of the lines of force from north pole to south pole are marked by marking arrow heads.

To locate the neutral points, the compass needle is moved slowly along the axial line. At the null points the needle just begins to rotate. The trace of the needle is taken. The distance (2d) between the null points and the length of the magnet (2l) are measured. Knowing Bh, the moment and the polestrength of the magnet can be calculated.

Observation Table and Calculations

Length of the magnet, 2l = ____cm

Therefore l = ____ x 10-2 m

Distance between null points, 2d = ____cm

Therefore d = ____ x 10-2 m

Horizontal component of earth’s magnetic field, Bh = 0.38 x 10-4T

Dipole moment, m = Bh(d2-l2)2 107/2d = _____A-m2

Pole strength of the magnet, P = m/2l = _____ A-m

Results

i. The combined magnetic field earth and magnet is plotted and the neutral points are located.

ii. The dipole moment of the magnet = ______ A-m2

iii. The pole strength of the magnet = ______ A-m

MODEL VIVA VOCE QUESTIONS AND ANSWERS

1. Define pole strength of a magnet. What is its unit?

Pole strength is the force experienced by a magnetic pole when it is placed on uniform magnetic field of unit intensity. Its unit is ampere-metre (A —m).

2. Define magnetic dipole moment.

The strength of the magnetic dipole is given by dipole moment. The dipole moment is a vector quantity whose magnitude is equal to the product of pole strength and length of the magnet. It is directed from south pole to north pole.

3. Define magnetic meridian.

It is a vertical plane passing through the magnetic axis of a freely suspended magnet at rest.

4. What is a neutral point?

In the magnetic field of a magnet, a neutral point is that point, where the field due to the magnet is completely cancelled by the horizontal component of earth's magnetic field.

5. How many neutral points are there for a magnet along its axial line when its north pole pointing south?

Two

6. How many neutral points are there for a magnet on its equatorial plane when its north pole pointing north?

Infinite number

7. What is a magnetic field?

The space around the magnet is called a magnetic field.

8. Define one tesla.

A magnetic field B is said to be of intensity I tesla if a charge of IC, moving with a speed of 1 ms-1 at right angles to the field, experiences a force of IN.

9. What are poles?

The regions of maximum magnetism in a magnet are called poles.

Experiment – 2

Mapping of Magnetic Field – II (North pole pointing North)

Aim: To map the magnetic field around a bar magnet placed with its axis in the magnetic meridian and with its north pole pointing north and hence to determine its magnetic moment and pole strength.

Apparatus: Bar magnet, compass needle, drawing board, sheet of paper etc.

Principle:

The magnetic field at a point on the equatorial line of bar magnet is given by B = µ0/4π . m/(d2+l2)3/2

Where, µ0 = 4π x 10-7 H/m

m = Dipole moment of the magnet

l = Half the length of the magnet

d = Half the distance between the null points

In this case the null points are on the equatorial line of the magnet. At the neutral points the field due to the magnet (B) and the earth’s horizontal component of magnetic field (Bh) are equal and opposite.

ie, B = Bh

Therefore, m =  Bh (d2+l2)3/2 x 107

Pole strength, P = m/2l

Procedure

A sheet of paper is fixed on a horizontal drawing board. Two mutually perpendicular lines are drawn through the centre of the paper. The magnet and all magnetic materials are kept away from the drawing board. A compass needle is placed with its centre at the centre of the paper. The drawing board is roated till the magnetic needle becomes parallel in one of the lines. Then this line is in the magnetic meridian and the other equatorial line. The outline of the drawing board is marked on the table using a piece of chalk. The board should be kept undisturbed throughout the experiment.

The north and south directions are marked on the paper. A bar magnet is placed at the centre of the paper with its axis along the magnetic meridian and the north pole pointing north. The outline of the magnet is drawn. A point is marked near the north pole of the magnet. The compass needle is placed with its south pole coinciding this point and the position of the north pole of the needle is marked. This process is repeated till the compass needle reaches the south pole of the magnet or the end of the paper. These dots are joined by a smooth curve which gives a line of force. Similarly a number of lines of force are drawn like this to get a general form as shown in the figure.

To locate the null points, the compass needle is slowly moved along the equatorial line. At the neutral points, the needle just begins to rotate. These places are marked. The distance (2d) between the null points and the length (2l) of the magnet are measured. Assuming the value of the horizontal component of earth's magnetic field (Bh), the value of dipole moment and pole strength can be calculated.

Observations and Calculations

Length of the magnet, 2l = ____cm

Therefore l = ____ x 10-2 m

Distance between null points, 2d = ____cm

Therefore d = ____ x 10-2 m

Horizontal component of earth’s magnetic field, Bh = 0.38 x 10-4T

Moment of the magnet, m = Bh(d2+l2)3/2 x 107 = _____A-m2

Pole strength of the magnet, P = m/2l = _____ A-m

Results

i. The combined magnetic field of earth and magnet is plotted and the neutral points are located.

ii. Dipole moment of the magnet = _____ A-m2

iii. Pole strength of the magnet = _____ A-m

MODEL VIVA VOCE QUESTIONS AND ANSWERS

1. Two magnetic lines of force will never intersect.Why?

If they intersect at a point, then the field has two directions at that point, which is impossible.

2. The magnetic length of a magnet is smaller than its geometric length. Why?

The poles of a magnet are not situated at the ends of the magnet but a little inside it. Thus the magnetic length, which is the distance between the two poles, will be less than its geometric length.

3. What is meant by a magnetic field?

Magnetic field is the space around a magnet or the space around a current carrying conductor in which magnetic influence can be experienced.

4. Define magnetic field intensity.

It is the force experienced by a unit north pole placed at that point.

5. What is meant by uniform magnetic field?

A same force acts in the same direction of force as a unit of north pole is called uniform magnetic field.

6. Define neutral point.

Neutral point is that point in the region of the magnetic field of a magnet, at which the magnetic field intensities of the earth and the magnet have equal magnitudes and opposite direction. (The resultant magnetic field intensity is zero at a neutral point.)

7. Define magnetic lines of force.

The path along which the compass needles are aligned is known as magnetic line of force.

8. Why direction of magnetic line of force is from a north pole towards a south pole?

Because a free unit north pole (test pole) will move away from a north pole and towards a south pole.

9. Why does the magnetic compass needle suddenly reverse direction as it crosses the neutral point region?

Before neutral region, field of magnet is stronger. After neutral region, field of earth is stronger. So the direction of the resultant magnetic field becomes reverse. Hence magnetic compass needle suddenly reverse direction as it crosses the neutral point region.