Lab: Strength of magnetic field

The compass needle assumes an equilibrium position along the resultant of fields M and B. ? is the angle between the compass direction and magnetic north.

tan ? = B

M

If you know the value of the horizontal component of the earth's magnetic field in your area, the actual value of the field B can be computed. If the value of the earth's field is not known, you can use tan ? as a measure of the field. Since the strength of the earth's field M in any particular area is constant, tan ? is proportional to B.

Procedure:

. Rule a line down the centre of a long sheet of paper. This line will mark the axis of the bar magnet. Centre the bar magnet along this axis at the left side of the paper, with the N-pole facing to the right. Trace the outline of the bar magnet on the paper.

2. Erect a line perpendicular to the axis of the bar magnet at the point that is farthest from the magnet. This line will mark the N-S line of the earth's magnetic field.

3. Place the centre of the compass over the intersection of this N-S line and the axis line. Rotate the paper under the compass until the N-S line coincides with the direction of the compass needle. Fix the paper in this position with masking tape.

4. Place the compass with its center on the farthest point on the axis line. Note the direction taken by the compass by marking a dot on the paper at each end of the compass needle. Remove the compass and draw a line between the two dots. With the protractor, measure the angle between the compass direction and the N-S line. Record this as angle ?. Measure the distance d of this point from the N-pole of the bar magnet.

5. Move the compass closer to the N-pole, and record the angle and distance. Repeat this step until you reach the end of the bar magnet.

Data Collection:

Distance d

(cm)

?

(?)

tan ?

/d2

(cm-2)

61.6

68.0

2.48

2.64*10-4

51.7

65.0

2.14

3.74*10-4

42.5

52.0

.28

5.54*10-4

30.9

33.5

0.66

.05*10-3

7.8

1.5

0.20

3.16*10-3

5.70

0

0

3.08*10-2

Observations:

As the compass is moved further away from the bar magnet, the magnet's magnetic field gradually decreases.

At the point for which ? = 45?, the horizontal component of the earth's magnetic field is the same as the field strength of the bar magnet. The distance from the N-pole of the magnet where the two field intensities are equal is found by drawing a graph with tan ? as the ordinates against the distance d as abscissas. Find the d corresponding to tan 45? = 1 on the graph.

Conclusion:

The relationship between B and the distance from the pole is like this: As distance d increases, the magnetic field intensity B at that point decreases.

Diana Herwono D 0861 006

2