MAGNETIC FORCE ON A CURRENT-CARRYING WIRE
OBJECTIVE:
To show that the magnetic force on a wire is proportional
to the current in the wire and to use that force to calculate
the magnetic field strength.
APPARATUS:
Large electromagnet with DC power supply
Two ammeters
U-shaped wire suspended from balance
DC power supply for wire
Gaussmeter with transverse probe
INTRODUCTION: The apparatus for this experiment is
set up as shown in Figure 1. A U-shaped wire forms part of a
balance and is also connected to a DC power supply so that a variable current may be passed through it. The
wire is placed in the gap between the pole faces of a large electromagnet which provides a uniform magnetic
field perpendicular to the plane of the U. The force on the horizontal wire forming the U is downward, while the
force outward on one vertical wire cancels the outward force on the other vertical wire. If the current in the U
were reversed, the upward force on the balance would still be measurable, but the inward forces on the vertical
parts would put the U in unstable equilibrium, making it impossible to keep squarely between the pole pieces.
After the weight of the U-shaped wire is measured with no magnetic field present, the magnet is
energized. The force due to the current in the wire interacting with the electromagnet can be found by
rebalancing, computing the force, then subtracting the weight of the U-shaped wire. At one electromagnet
current, Im, a series of force, F, versus U-shaped wire current, Iu, is found. This is repeated for a second
electromagnet current. For each of the two-electromagnet currents a graph is drawn to plot F versus Iu. From
these graphs and equation 1, the magnetic field strength B is found and compared with the directly measured
value.
F = IuBL
B
Equation 1
PROCEDURE:
1. Adjust the apparatus so that the horizontal portion of the U-shaped wire is centered between the pole
faces and parallel to them.
2.
Adjust the sliding weights on the balance to achieve a balance and record the balance reading as Mo on
the data table.
3. Turn on the electromagnet's power supply and adjust to 1 ampere of current through the electromagnet.
4. Turn on the power supply for the U-shaped wire and adjust for a current of 1 ampere through the wire.
Rebalance the apparatus: If the new balance reading is less than Mo found in step 2, the polarity is
wrong and the current in the U-shaped wire must be reversed now. If the current must be reversed, first
turn the current in the wire to zero, disconnect and reverse the wires at the power supply for the Ushaped wire, then increase the current to 1 ampere and rebalance the apparatus. Record the balance
reading on the data table M1.
5. Take balance readings for currents of 1, 2, 3, 4 and 5 amperes of current through the U-shaped wire.
Record these values on the data table as M2,M3,M4,M5. Return the wire current to zero.
6. Adjust the current in the electromagnet to 1.5 amperes and repeat step 5 for this new magnet current.
7. Turn off the power supply to the U-shaped wire after reducing to zero.
8. Use the Gaussmeter supplied to measure the magnetic field between the pole faces of the electromagnet
for both a current of 1.5 and 1.0 amperes and record these values on the data table.
9. Slowly reduce the current in the electromagnet to zero and turn off this power supply.
Record the horizontal length of wire. This number is posted near the apparatus.
MAGNETIC FORCE ON A CURRENT-CARRYING WIRE LAB SHEET 1
DATA TABLE: INCLUDE UNITS
Electromagnet
Balance
Electromagnet
Current 1.0A
Reading
Current 1.5A
Mo
M1
M2
M3
M4
M5
Mo
M1
M2
M3
M4
M5
Balance
Reading
Electromagnet
Current
1.0A
1.5A
Gauss Meter
Reading
CALCULATIONS: SHOW ALL WORK INCLUDE ANSWERS ON RESULTS TABLE
1. Multiply all balance readings by the acceleration due to gravity to compute the force exerted on the
balance for each measurement. Subtract the force due to Mo from each of the other computed forces.
This is the magnetic force on the wire for each trial.
2. Using the Graphical Analysis program, make plots of F versus Iul for each electromagnet current on
the same graph. Perform a linear fit. The slope from the linear fit will be the magnetic field strength B.
Record these value on the results table. The first data set should include the values for the U-shaped wire
and the current of 1 ampere through the wire. To create a second data set: click on the Data Menu ⇒
New Data Set. Make sure you put in correct units.
MAGNETIC FORCE ON A CURRENT-CARRYING WIRE LAB SHEET 2
3. Compare the experimental values of B to the values to the measured values using the Gauss meter. Note:
you should do the %Error twice, one for each of the values found in procedure 8.
RESULTS TABLE: INCLUDE UNITS
Electromagnet
Current 1.0A
FB at 1A
FB at 2A
FB at 3A
FB at 4A
FB at 5A
B
Percent
Error
Electromagnet
Current 1.5A
FB at 1A
FB at 2A
FB at 3A
FB at 4A
FB at 5A
B
Percent
Error
CONCLUSION:
TURN IN: LAB SHEET 1 AND 2, 1 GRAPH