Magnetic Fields Due to Currents

The force on a magnet due to a current running perpendicular to the magnetic field is proportional to the current, the length of the current element and the strength of the magnetic field.

For this experiment, we will use the following equipment to measure the magnetic field of a magnet: a current balance, a magnet, assorted current elements, a lab stand, a balance, a 5A DC power supply, wires, a 5 W rheostat and a 5A Ammeter.

Note that the red side of the magnet is the North pole, and the white side is the South. The direction of the magnetic field points from the North to the South pole.

Name:

Lab Partners:

When entering numeric data, use exponentials: ie., 1.6 * 10-19 = 1.6E-19.

Procedure

  1. Zero the balance. Place the magnet on the balance and re-zero the balance.

  2. Mount the current balance on the lab stand so that a current element plugged into the balance is centered in the magnet, but not touching it. Connect the current balance in series with the power supply, the rheostat and the ammeter.

  3. For the SF-42 current element (with an effective length of approximately 8.2 cm), measure the change in mass with the following currents. Record the actual current in each case.

    Be sure to enter the Dm values as positive even if you recorded them as negative, here and in the next step.

    target I (A)Actual I (A)Dm (g)
    0.5
    1.0
    1.5
    2.0
    2.5
    3.0
    Verify the measured sign of Dm, noting the direction of current flow and the orientation of the magnet relative to the current element. Note that the sign of Dm is the same as -F, because F = I l X B is the direction of the force on the current element, and the force we are measuring is on the magnet, which is in the opposite direction.

  4. Turning the power supply off each time you change the current element, measure the change in mass with a 3 A current with each of the following current elements. When you remove a current element, wiggle it out gently so that it does not break. Re-zero the balance for each element, and record the actual current in each case.
    ElementLength (cm)I (A)Dm (g)
    SF 401.1
    SF 372.1
    SF 393.1
    SF 384.0
    SF 416.2

Analysis

  1. Perform a least squares fit on the first set of data with x = I * 8.2 / 100 and y = Dm * g / 1000; the slope should be the strength of the magnetic field.

    Plot the points { x, y } and the line fit.

    B = T

  2. Perform a least squares fit on the second set of data with x = I L / 100 and y = Dm * g / 1000; the slope should be the strength of the magnetic field.

    Plot the points { x, y } and the line fit.

    B = T

©2004, Kenneth R. Koehler. All Rights Reserved. This document may be freely reproduced provided that this copyright notice is included.

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