Physics 2212, Lab 10: Magnetic Force
Edward Thomas, Summer 1999
revised by Eric Murray, Fall 2006
Question these experiments will enable you to answer: Is the
magnetic force on a current-carrying wire linearly proportional to its length? Is the
magnetic force on a current-carrying wire linearly proportional to the current?
Features: A current balance
technique will be used to measure
the magnetic force on a current-carrying wire. A permanent magnet is placed on a balance and a
wire segment positioned within its magnetic field. The magnetic force on the wire is the difference
between the weight of the permanent magnet and its apparent weight when current passes through the wire.
Preliminaries: Careful work on this experiment can not only yield good
results, but protects the equipment from damage. In particular
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Never drop the magnets; it reduces their strength.
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Currents through the straight wire samples must never exceed 5 A!
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The wires must be positioned so that they do not touch the magnets; there must be air
space all around them. If they touch then not only will the force measurements will be affected,
but there is a danger of damaging the wire by shorting it out.
You have a wire support assembly held on a conventional stand, a beam balance, and a box
of wire samples. Take the wire sample labelled SF 42 and plug it into the end of the support
assembly. The magnet (found in the box of straight wire samples) is placed on the balance
pan with the open channel upwards. The straight wires sample is placed into the throat of
this magnet assembly (between the red and white bars).
The electrical connection is to the DC supply with one of the multimeters placed in series to
measure current. Plug the red lead to the multimeter into the 10 A socket. Turn the
meter control to the 10 A setting. As a precaution turn the voltage and current
controls on the supply fully counter-clockwise so that they are set to give zero output.
Current is important in this experiment, and the power supply should be operated in a constant
current mode. Current should be measured with the multimeter, as it is more accurate and more
easily readable than the current meter on the power supply.
Experiment 1: Find the mass of your magnet by moving the riders on
the various horizontal beams of the balance until the index at the beam end (far right)
is level with the fixed mark on the balance. Be very careful about the positioning of these
various riders. To get accurate (meaningful) readings the rider must be positioned in one of the
grooves, which also means the number corresponding to that weight is visible through the aperture
on the rider. Only the last rider (0.1 to 0.9 grams) slides along its bar continuously.
The mass of the magnet assembly is about 160 grams. Record the mass of the magnet. You can
measure to 1/1000th of a gram by carefully estimating the position of the
rider between the 0.01 gram markings.
At this point make sure the wire sample is not touching the magnet (if it is then the
mass measurement you just made is wrong), is positioned centrally in the magnet throat and
is as low in the throat as possible without hitting the bottom.
Turn on the power supply. Turn up the voltage a little, then turn up the current to 4 amps.
The balance should move due to the force between the wire and the magnet. Try re-balancing
the balance by moving the riders. It takes some skill and care to get this quite right. The
change in the apparent mass
of the magnet (from the mass when there was no
current) is determined from the change in the mass
indicated by the balance.
The change in mass change multiplied by the acceleration due to gravity is the force
between magnet and wire.
The magnet has a white side and a red side. From the direction of the force on the wire,
determine whether the red is the North end or the South end of the magnets.
Experiment 2: For the straight wire sample (such as SF 42), examine how
the force between the magnet and wire depends on current. Measure the force for several
currents between zero and 4 A in approximately half-amp steps. Plot the force as a
function of current. Does the graph show that force is proportional to current? Fit a straight
line to the data. Report the slope with its uncertainty, and calculate the magnetic field
magnitude with its uncertainty.
Experiment 3: For a particular current of approximately two to
three amps, examine how the force between the magnet and wire depends on the wire length.
Measure the force for at least five lengths. Plot the force as a function of wire length.
Does the graph show that force is proportional to wire length? Fit a straight line to the
data. Report the slope with its uncertainty, and calculate the magnetic field magnitude
with its uncertainty.
Summary: Review your worksheet. Think about the goals of these
experiments, your results, and the expectations from theory while writing your discussion.
The electrical connection is to the DC supply with one of the multimeters placed in series to
measure current. Plug the red lead to the multimeter into the 10 A socket. Turn the
meter control to the 10 A setting. As a precaution turn the voltage and current
controls on the supply fully counter-clockwise so that they are set to give zero output.
Current is important in this experiment, and the power supply should be operated in a constant
current mode. Current should be measured with the multimeter, as it is more accurate and more
easily readable than the current meter on the power supply.