University of Puget Sound Introductory Physics Laboratory
11. The magnetic current balance and the electromagnetic motor
Name:____________________
Date:___________________
Objective
1. To investigate the nature of magnetic forces on current carrying wires.
2. To observe the operation of an electromagnetic motor.
Equipment
Solenoid, compass, bar magnets, current balance, power supplies, ammeters,
LCR meter, electronic balance and a dissectible electromagnetic motor.
Introduction
You have learned in class that magnetic forces are only felt by moving
charges (the Lorentz force law). But everybody knows that magnets exert forces
on other magnets, even when they are sitting still and uncharged. Today you
will observe that wires that carry current (moving charge) both create a magnetic
force field and are repelled or attracted to other magnets. Thus it is plausible to
explain magnetic materials as objects which sustain internal currents.
Today you'll use a device called a solenoid (a coil of wire) that creates a
moderately large magnetic field when you pass a current through it. It's easy to
see that a solenoid acts just like a magnet: bring a compass or another magnet
near it. The solenoid exerts a force on the compass or magnet, and therefore it
also feels a force due to the presence of the other magnet (Newton's third law,
right?). But if you turn off the flow of current, there's no force. It seems like we
have two kinds of objects that act similarly, magnets and current-carrying wires.
On a microscopic scale, it's possible to understand magnets as collections of
atoms with internal currents. So physicists boil it down to a single phenomena:
moving charge exerts a force on other moving charges. Today you will examine
this interaction between moving charges using a force balance. As an example of
the technological utility of this interaction you will also "dissect" an
electromagnetic motor, with the aim of understanding how it works in terms of
magnetic forces.
The solenoid
Pick up the solenoid and examine it. When the solenoid carries current, why
don’t the wires short together? Why do you think this partcular geometry is
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useful (compared to other shapes, like a straight wire, or wire wrapped like a ball
of twine)? Explain.
The magnetic field inside a solenoid is proportional to the current flowing
through the solenoid windings:
B
L
I
NA
where A is the cross sectional area of the solenoid, N is the total number of turns,
and L is called the inductance of the solenoid. Inductance is measured in Henries.
If you express the cross sectional area in square meters and current in Amps,
then the magnetic field comes out in Teslas (all SI units). You have a device
called an LCR meter capable of measuring inductance, capacitance and
resistance. Measure the inductance of your solenoid as well as its cross sectional
area. Estimate the number of turns N, and determine the proportionality
constant L/NA between the current and the magnetic field.
You can drive a current through the solenoid using one of the DC power
supplies at your table. Make sure you are using the DC and not the AC output!
You will be pumping a relatively large current through the solenoid. You can
measure the current using a DMM, but you need to be careful to use the 10-Amp
unfused current input port. Wire the solenoid, power supply and ammeter in
series using the bananna-jack cables. To connect to the solenoid you will need to
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use some alligator clips. Before you turn on the juice, check that the ammeter is in
current mode on a high scale and that the volume knob on the power supply is
turned all the way down. It also can't hurt to have your lab instructor check your
circuit before you start pumping current.
Slowly increase the power-supply voltage while monitoring the current on the
ammeter. If everything is hooked up right, the current should vary in proportion
to the supply voltage. Bring the current up to 4 Amps. What B field would you
predict inside the solenoid? Compare this value of B with the vertical
component of Earth’s magnetic field, which is about 2 x 10-5 T. Do you know the
direction of the magnetic field inside the solenoid? Can you change it?
Warning: Prolonged use of high current can cause parts of the solenoid to melt and short
out. If the solenoid gets warm, turn off the current for several minutes and let it cool
before resuming.
The current balance
You now have a controlled way to make a uniform magnetic field. By setting up
a current balance we can test this idea of magnetic forces on moving charges.
We want a way (1) to insert an object carrying a current into the magnetic field of
the solenoid and (2) to balance the magnetic force on the object with a
gravitational force. Examine the little circuit board at your table and the set of
copper clips. See how it will work? Puzzle over it. Hook the clips onto the end
of the solenoid so that they create a fulcrum for the axle on the circuit board.
Balance the rectangular circuit board on the clips carefully, making it as close to
horizontal as possible. Be sure that the part of the board that carries the current
is inside the solenoid! The copper clips also serve as connectors to pump the
current through the circuit board. Use alligator clips to connect the support
hooks to another power supply with another ammeter in series also set to 10-A
maximum reading. Fiddle with the arrangement until it seems solid and
reasonably balanced.
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Measurements
Before taking data let's first check out how it works. Pump 4 Amps through the
solenoid and then slowly increase the current through the balance. If it doesn't
work you'll have to re-examine your circuit and/or get help. Keep increasing the
balance current until you see the balance tip. Turn off the balance power supply,
reverse the leads on the balance, and pump current through the balance in the
other direction. Do the same for the current through the solenoid (always turn
off the current before disconnecting the leads).
In class you have learned about the right-hand rule. What is the direction of the
force on each of the three segments of the copper wire in the magnetic field? Are
you using your right hand? How does the force on the segment that matters
change when you reverse the direction of the current? When you reverse the
direction of the solenoid current? Does this agree with you experimental
observations?
OK let's make a quantitative measurement. Here's a reasonable hypothesis: the
magnetic force on the balance is directly proportional to both the current in the
solenoid and the current in the balance.
m  C Ib Is
where m is the mass hung from the balance, Ib is the balance current, Is is the
solenoid current, and C is a constant. To set up the balance to check this, arrange
the direction of the current in the solenoid and current balance so that the outer
end of the balance will deflect upwards when both currents are on. For a fixed
value of current in the solenoid, turn up the balance current until you get a
noticeable deflection. Now hang a small wire on the outer end of the balance
until it returns to its original, horizontal position. You can always adjust the
current a bit to achieve balance. Measure the mass of the small wire needed to
achieve balance using the electronic scale. Record the mass and the values of the
two currents in the table below. Perform this measurement for four values of the
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balance current for each of three values of the solenoid current, keeping the
solenoid current under 5 A.
Mass (grams)
Solenoid current (A)
Balance current (A)
Make a graph of the counterbalance mass m versus the product of the currents Ib
and Is. Does your experimental data support the hypothesis? In the space
below, state clearly how you think the graph should look and why, and discuss
the hypothesis in light of your data.
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The electromagmetic motor
Electromagnetic motors are everywhere around you, making your life easier, and
it's neat to see how they work. Check out the design of our demonstration DC
motor. Follow the path of the current from the power supply around a full loop.
The trick is in the "commutator," which causes the current direction to reverse
each half cycle. Where does the force that drive the motor come from? Turn the
power supply on, and give the moveable coil a kick to get the motor going. You
may need to adjust the position of the commutator brushes to get it to spin at all.
This is analogous to adjuting the timing in an internal combustion engine. Once
you've got it timed so that it is running smoothly, reduce the power supply
voltage to slow the motor down to a near stop. You can grab the moveable coil
without shocking yourself (grab the painted part, but don't touch the
commutator or any bare wires). Manually move the coils past each other. Is the
force attractive? Repulsive? Both? During what part of the cycle?
To turn in: Write up a brief report about your measurements, including graphs
to show your results. If your data looks reasonably linear, report a best-fit value
for the constant C. From what you know (and what is in your textbook) you
should be able to calculate C in terms of quantities you have measured today.
Derive a formula for the proportionality constant C, and compare the prediction
of your formula with your experimental value. Comment on the support of your
data for the hypothesis, including estimates for possible experimental
uncertainties and their influence on your derived value for C.
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