Phys 112L
Spring 2008
Laboratory 8: Currents and Magnetic Forces
1 Forces between Two Currents
The force exerted by one current carrying wire on another parallel current carrying
wire of the same length is
F=µ0I1I2
2πd L(1)
where I1and I2are the currents in the two wires, dis the distance between the wires
and Lthe length of the wires.
This relationship can be investigated using a current balance, whose essential working
parts are two rigid parallel wires through which the same current is passed. The current
balance is designed so that one of the upper wire is free to pivot and the lower wire is
fixed. When currents pass through the wires in opposite directions the lower wire exerts
force on the upper wire which causes it to rotate upwards. This can be counteracted
by adding masses to the small balance pan on the upper wire, eventually returning
the upper wire to its equilibrium position. When the balance is in equilibrium, the
gravitational force due to the additional mass equals the force exerted by the lower wire
on the upper wire.
a) Predict the current required to balance the current balance arm when there is
10 mg masspiece in the balance pan. Repeat this for the case where there is 20mg
masspiece in the balance pan.
b) Set up the current balance with no additional mass in the pan. Record the equilib-
rium point by marking the position of the reflected laser beam on the wall. Measure
the distance between the two wires and the length of the upper wire.
c) Place a 10 mg masspiece in the balance pan. Adjust the current until the arm
balances and measure the current. Reverse the direction of the current through the
apparatus (this should partially cancel the effects of the earth’s magnetic field) and
again balance the arm and measure the current. Determine the average current
and calculate the difference between this and the predicted current.
d) Repeat the previous part using a 20 mg masspiece in the balance pan.
2 Force Exerted by Current Carrying Coil on a Magnet: Determining the
Earth’s Magnetic Field
This experiment uses a circular current-carrying coil with Ncomplete loops of wire.
The magnetic field at the center of the loop points along the axis of the loop in a direction
