Experiment 17

THE MAGNETIC FIELD

ANALYSIS

From equation (1) we see that the magnitude of the total magnetic field present at the position of the jar magnet is given by: (2)
where: c is a constant of proportionality, which, when known, will provide a calibration of the magnitude of the magnetic field in terms of the period of the jar magnet.

The field at the center of the Helmholtz Pair is a combination of the field produced by the current in the coils and the Earths field, which of course is always there. The field produced by the coils is directly proportional to the current that passes through them, , where k is the proportionality constant. We can therefore write the total field as: (3)
Combining with equation (2) leads to a straight-line relation between the reciprocal of the period squared and the current through the coils.

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(4)
The above relation illustrates that a straight line should be obtained if a plot of against the current I, is made.

1.    Plot against I. Draw the best straight line through the points and find the Slope S. Enter the value in cell D67. The value of k is automatically calculated by the spreadsheet in cell D56 from the coil properties that you entered. Using this value, and the value obtained for the Slope, compute , and enter the result in cell D69.

The jar magnet is now calibrated so that the magnitude of a magnetic field in which it is placed can be determined directly from the period of oscillation by using equation (2).

2.    From the period measured with the jar magnet in the Earths field alone in cell D63, use equation (2) to calculate the magnitude of the horizontal component of the Earth's magnetic field, and enter in cell D71. Compare the measured value with the value listed for your location in a handbook of physical measurements or as given by your lab instructor, and enter the percentage error in cell D73.

3.    Submit the completed Worksheet along with the graph as part of your lab report.