Physics 222
Lab #6
Magnetic Fields
Objective: The objective of this laboratory is to investigate the source of magnetic fields and the forces that
magnetic fields exert on magnets and on moving charged particles.
Prediction:
1. There is a current through a straight wire, coming out of the paper.
a. On the right side of the wire, what is the direction of the magnetic field due to the current?
b. On the left side of the wire, what is the direction of the magnetic field due to the current?
A horseshoe magnet is now placed so that the north pole of the magnet is on the right side of the wire
and the south pole is on the left side of the wire. (A horseshoe magnet has a U shape with the poles at
the ends of the U.)
c. What is the direction of the magnetic force on the north pole of the magnet?
d. What is the direction of the magnetic force on the south pole of the magnet?
e. What is the direction of the force on the wire due to the magnet?
2. Draw the free body diagram for these two scenarios.
a. A horseshoe magnet of mass m sitting on the pan of a scale in the absence of a magnetic field.
b. The same horseshoe magnet sitting on the pan of a scale in the presence of a magnetic field that
exerts an upwards force on the magnet. (The magnetic force is less than the force of gravity.)
Part #1 : Force on a Beam of Electrons
I will show a demonstration of the force on a beam of electrons. Record the direction of the magnetic field, the
direction of the electron’s velocity, and the direction of the deflection of the electron beam.
Write a short summary of your observations. Make sure to state if your observations agree or disagree
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with the equation F  qv  B , where q is the charge, v is the velocity of the charged particle, B is the
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magnetic field, and F is the force on the charged particle from the magnetic field.
Part #2 : Magnetic Field due to a Loop of Wire
In this part of the lab you will use a Vernier Magnetic Field Sensor to measure the magnetic field. The sensor
utilizes the Hall Effect, which we will study in class. One end of the sensor has a white dot on it. The sensor
measures the component of the magnetic field perpendicular to this dot. (Or, to put it another way, it measures
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B  n̂ , where B is the magnetic field vector and n̂ is the unit vector normal to the surface of the dot.) A
positive reading indicates the field is pointing out of the sensor surface.
This section is to illustrate the principle that a current through a loop of wire generates a magnetic field. You
will investigate how the field at the center of the loop depends on the current through the wire and the number of
current loops.
As you begin, make sure the Magnetic Field Sensor is set to the 0.3 mT switch setting.
Set up the tangent galvanometer. Tape your magnetic field sensor to the tangent galvanometer table so that
the field sensor is perpendicular to the wires. The white dot on the sensor should point directly
perpendicular to the loop of wires and be located at the center of the loop. ( n̂ of the sensor should be
parallel to the area vector of the loop of wire.)
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You should set up the table so that there are only two or three loops of wire around the frame.
Zero the magnetic field sensor once you are finished making adjustments. Remember that the Earth has a
magnetic field around it. So any time you move the table, you must re-zero your sensor with the
current turned off.
Connect your power supply to the ends of the wire wrapped around the tangent galvanometer. This will
provide current to your wire loop. The direction of current through the wire does not matter (although
reversing the current direction will change the sign of your B field measured by your sensor).
WARNING: Be sure to leave the current turned on for as little time as possible. The wires will get very
hot otherwise! After each 5 seconds of data, turn off the current with the switch until you
are ready for the next run.
Investigation A: Changing the Current
1. Once you have set up the tangent galvanometer, zeroed your sensor, and connected your power supply,
you may begin making magnetic field measurements. Turn the power supply current up to about 0.5 A.
Record the magnetic field by taking data for 5 seconds, then turn off the power supply switch. Use the
“stat” button to get the average magnetic field for the run.
2. Continue by increasing your current in 0.5 A increments until you get to 5 A or the limit of the sensor.
For each current, measure the magnetic field. There is no need to re-zero your magnetic field sensor
unless you move the tangent galvanometer for some reason.
3. Take one last measurement with the current running the other direction through the wire.
Investigation B: Changing the Number of Loops
For this section, be sure to turn off the power supply with the switch. Do NOT turn the current back to zero
with the knob. As long as you do not change the wire, the resistance of the wire will remain constant and
the current through the wire should be unchanged when you turn the switch back on.
1. Make sure your magnetic field sensor is still aligned properly. Start with 1 loop of wire wrapped around
the tangent galvanometer. Turn the current to approximately 0.5 A. Run the sensor and record the
average magnetic field for about 5 seconds. Turn off the power supply.
2. Wrap a second loop of the wire around the galvanometer table. Keep the wire tight and wrap the wire in
the same direction as your first loop.
After you have added the second loop, you must re-zero your magnetic field sensor because you have
changed the orientation of your table. You MUST do this before turning the current back on!
After you have re-zeroed the sensor, turn the current back on (the current should go back to what you
had previously). Use the sensor to collect data for 2 loops of wire. Turn off the switch.
3. Continue this process for as many loops of wire as you can get by adding one or two loops of wire at a
time. Remember to re-zero the sensor (with the current OFF) any time you move the galvanometer.
4. Make graphs of the magnetic field strength vs. current, and magnetic field strength vs. number of loops.
Find the equations for these relationships. Also find the combined relationship (magnetic field strength
as a function of both current and number of loops.)
5. Describe the relationship between the magnetic field, and the current and number of loops.
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6. How does the direction of the magnetic field depend on the direction of the current through the loop?
Part #3 : Magnetic Force due to Current
In this part of the experiment you will use a Current Balance to measure the force that the magnetic field
generated by a current exerts on a permanent magnet. The permanent magnet is a horseshoe magnet. The
Current Balance has 6 different current “loops” that you can insert at the END of the current arm. Each “loop”
has a section of straight wire that will sit between the poles of the permanent magnet. The loops are labeled
“SF” and the following chart shows the length of straight wire for each current loop:
Current Loop
SF 40
SF 37
SF 39
SF 38
SF 41
SF 42
Length of Wire
1.2 cm
2.2 cm
3.2 cm
4.2 cm
6.4 cm
8.4 cm
Set up the Current Balance so that you can measure the force on the permanent magnet (horseshoe magnet)
exerted by the magnetic field generated by the straight wire. Place the permanent magnet on the balance
pan.
Place the SF 42 current loop into the END of the current arm. Set up the current arm so that the current loop
is inside the jaws of the permanent magnet but not touching the magnet. Connect the power supply to the
current arm but DO NOT TURN IT ON! Measure the mass of the permanent magnet in the absence of a
magnetic field.
Make sure that the power supply voltage and current knobs are turned all the way down to zero. Turn on the
power supply. Notice that changing the current through the current arm will cause the “effective mass” of
the magnet to change (reads lighter or heavier depending upon current direction). The balance does not
actually measure the mass of the magnet. What it measures is the normal force on the balance pan. What it
displays is the mass that has the weight equal to the measured normal force. (Your free body diagrams from
the prelab may help you understand this.)
Investigation A: Varying the Current
1. Using the SF 42 current loop, investigate the effect that varying the current has on the force exerted on
the magnet. You should vary the current by 0.5 A increments between 0.5 A and 4.5 A and measure the
“mass” with the balance. If there is a delay between measurements, turn off the current with the switch.
2. Make one measurement with the current going the opposite direction.
3. Make one measurement with the north and south poles of the magnet reversed.
4. From your data, calculate the magnetic force produced by the interaction between the current and
magnets. (Do not leave the lab until you understand how to do this calculation.) Remember to
include a sample calculation of this in the Calculations section of your lab report.
5. Produce a graph of magnetic force vs current for current loop SF 42. Describe the relationship between
the magnetic force and the current.
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6. How does the direction of the current and the direction of the magnetic field from the permanent magnet
determine the direction of the force on the magnet?
Investigation B: Varying the Magnetic Field
1. Notice that the permanent magnet is made of 6 small removable magnets. Again use the SF 42 current
loop and investigate how the force depends on the magnetic field strength. You should use a fixed
current of 3 A (be sure to turn off your power supply between measurements) and measure the “mass”
associated with decreasing numbers of magnets (6 magnets, 5 magnets, . . . 1 magnet.) You MUST be
careful and re-measure the actual mass of the remaining magnets at each step (with the current turned
off) since you must be able to calculate the mass difference between when the current is on and off. Be
sure to align the magnets properly according to the colored paint. Putting a magnet in backwards will
cause incorrect results.
2. From your data, calculate the magnetic force produced by the interaction between the current and
magnets.
3. Make a graph of magnetic force vs number of magnets for SF 42. Describe the relationship between the
magnetic force and the magnetic field strength.
Investigation C: Varying the Current Loop
1. You have 6 different current loops that can be swapped out. Use all six magnets and a current of 3 A.
Measure the magnet’s “mass” for each current loop.
Turn off the power supply while you are switching the current loop.
You should NOT change the dials. Just turn the power supply off and on with the switch.
2. From your data, calculate the magnetic force produced by the interaction between the current and
magnets.
3. Produce a graph of magnetic force vs. current length. Describe the relationship between the magnetic
force and the length of the wire carrying the current.
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