Physics Lab
Magnetic Forces on Current-Carrying Conductors
Background: A current-carrying wire in a magnetic field experiences a force that is
usually called the magnetic force. The magnitude of the magnetic force is given by
the formula
F = I L B sin θ
(1)
where I is the current in the conductor, L is the length of the conductor segment, B
is the external magnetic field strength, and θ is the angle between the conductor (or
current) direction and the magnetic field direction. In this lab you will test whether
the force obeys this formula.
Part I: Force vs. Current
Set up the apparatus as demonstrated by your instructor, use the SF42 loop.
Record the masses read by the scale with currents of 0, 1, 2, 3, 4, and 5 amps
running through the circuit; make a table of the results in your lab report.
Assuming the length, magnetic field, and angle all stayed constant, equation
(1) predicts that the magnetic force will vary linearly with current. This means that
every one amp increase in current should cause an equal change in the magnetic
force. Calculate the mass changes for each increase in amperage.
(Q1) Does your data show support for equation (1)? Explain. Would you
characterize your results as excellent, good, fair, poor, or terrible? Why?
Part II: Force vs. Conductor Length
By swapping loop tiles, the length of the conductor segment within the field
can be varied. The available lengths are shown in the following table. Do not vary
the current during this part. Equation (1) predicts that the magnetic force should
vary linearly with conductor length if all other variables are held fixed.
Get mass measurements for four different conductor lengths. For each
additional trial, calculate ∆m/∆L, the change in mass divided by the change in
length. If the masses vary linearly with conductor
length, these ratios should all be the same.
(Q2) Show your data and calculations, and
summarize how well the results support equation (1).
Part III: Force vs. Magnetic Field
Use loop SF42 and a current of 2 amps. With
no magnets in the holder and all the magnets lying in
Current Loop Length
SF37
2.2 cm
SF38
4.2 cm
SF39
3.2 cm
SF40
1.2 cm
SF41
6.4 cm
SF42
8.4 cm
the scale pan, measure the mass reading. Repeat
with 2, 4, and 6 magnets in the holder; make a table of your results in your report.
(Q3) You know the drill, discuss how well equation (1) was supported by
your collected data. If your data for this part was noticeably worse than for earlier
parts, speculate on why that might be.
(Q4) With the same loop and current, use just two magnets in the holder
with their polarity reversed from each other. Discuss and explain the results.
Part IV: Force vs. Angle
Do not exceed a current of 2A during this part. Replace the current loop with
the rotating loop with angular scale; replace the magnet holder with the permanent
magnet. Start with the coil in the magnet, the loop set at zero degrees, and the loop
and magnet as “perpendicular” to each other as you can manage (the bottom wires
should be perpendicular to the white wall).
Collect mass data with the rotating loop set at angles of -15°, 0°, 15°, 30°,
45°, 60°, 75°, and 90°. You should measure the mass with I = 0 and subtract that
value from all the other numbers. Make a graph of mass versus angle for the data
that you collected, draw a best-fit curve for your data.
(Q5) Explain how your data fits (or doesn’t fit) with the prediction of equation
(1). [Extra credit available for highly detailed/mathematical answers.]