Current Balance
Description of the experiment:
This equipment is designed to measure the force acting upon an electric conductor,
placed in a magnetic field. When an electric conductor, with the length L, is within a
magnetic field (B), and a current (I) is running through the conductor, it will be acted
upon by a force, called the Lorentz Force, which can be calculated by the following
formula (also known as Laplace’s law):
F = B . I . L.sin θ
F:
B:
I:
L:
θ:
Force on the wire due to the magnetic field (newton)
Magnetic field strength (tesla)
Current (ampere)
Length of wire (meter)
Angle (degree) between the B-field and the direction of the current
The equipment consists of a holder with 6 replaceable magnets, a holder for wire
frames, and 6 wire frames with conductors of different length.
With this equipment the different parameters in the formula above can be varied one
by one. The force is measured in a roundabout way. The magnet holder is placed on
a balance, which one then tares, and then one uses the Newton’s Third Law of Motion.
In this situation it means that the force acting upon the conductor from the magnetic
field, is equal in magnitude, but opposite in direction to the force acting upon the
magnetic field from the current in the conductor. If the force acting on the conductor
is directed upwards, then the force acting on the magnetic holder will be directed
downwards (upwards), which results in an apparent increase (decrease) of the weight
of the magnet holder. This force is directly measured by the sensitive balance used in
the experiment.
Procedure:
Part 1: Force versus Current
1. Mount the Main Unit on a retort stand as shown in Fig 1.
Fig 1
2. Select the Current Loop (SF42), and plug it into the ends of the arms of the
Main Unit, with the conductive foil facing down.
3. Connect the power supply as shown in Fig 2.
Fig 2
4. Place the Magnet Assembly 1 on the electronics balance. Insert the Current
Loop in between the magnets without touching any of the sides.
5. With no current flowing, press the “TARE” button on the balance to set the
reading to 0.0000 grams, so only the force caused by the current will be
measured. Turn the current on, if the reading is negative, reverse the leads
connected to the arms of the Main Unit.
6. Now set the current on to 0.5 A, and record the reading on the balance.
7. Increase the current in steps of 0.5 A to a maximum of 5.0 A, each time
recording the new “Force” value.
8. Reduce the current to 0.0 amp. Swing the arm of Main Unit up, to raise the
Current Loop out of the Magnetic field gap. Pull the Current Loop gently from
the arms of the Main Unit. Replace it with Current Loop SF40. Carefully,
lower the arm to reposition the Current Loop in the magnetic field. Repeat
steps 4-7.
9. Plot the graphs of Force (vertical axis) versus Current (horizontal axis).
10. Deduce the polarity of the magnets, ie, state which coloured pole is the north
pole. Indicate clearly the magnet sample number.
11. Using the value of the length of the conduction listed in the table below,
deduce the strength of the magnets used in your experiment.
Part 2: Force versus Length of Wire
1. Repeat steps 1 to 5 in part 1.
2. Now set the current on and adjust it to 2.0 A and record the value on the
balance.
3. Reduce the current to 0.0 amp, swing the arm of Main Unit up, to raise the
present Current Loop out of the Magnetic field gap. Pull the Current Loop
gently from the arms of the Main Unit. Replace it with a new Current Loops.
Carefully, lower the arm to reposition the Current Loop in the magnetic field.
4. Repeat steps 1-3 for the rest of Current Loops.
5. The lengths of the six Current Loops are given as:
Current Loop
SF40
SF37
SF39
SF38
SF41
SF42
Length
1.2cm
2.2cm
3.2cm
4.2cm
6.4cm
8.4cm
6. Plot a graph of Force (vertical axis) versus Length (horizontal axis).
Part 3 : Force versus Magnetic Field
1. Set current to 2.0 A, using the longest current loop.
2. Measure the force for different numbers of magnets (including no
magnet), with the balance carefully zeroed.
3. Repeat the experiment with the shortest current loop.
4. Plot force vs. number of magnets for both sets of results.
5. Explain whether any non-linearity is to be expected and compare with
your observations.
Part 4: Force versus Angle
1. Mount the Main Unit on a retort stand as shown in Fig 3.
Fig 3
2.
3.
4.
5.
Plug the Goniometer Assembly into the ends of the arms of the Main Unit.
Connect the power supply as shown in Fig 2.
Place the Magnet Assembly 2 on the electronics balance.
Set the dial on the Goniometer Assembly to 00.
6. Align the Magnet Assembly 2 so that the magnetic field is parallel with the
horizontal (bottom most) wires of the coil of the Goniometer Assembly.
7. There are two ways which you may try to align the Goniometer Assembly
with the magnetic field.
a. Visually align the assembly to the field by using the edge of the
magnet and goniometer assemblies as guides; or
b. Apply some current to the loop and adjust the magnet assembly or the
goniometer assembly until the reading on the balance is zero.
State how you align the goniometer assembly and provide a possible
explanation for any alignment using the two methods above may be different.
8. With no current flowing, press the “TARE” button on the balance to bring the
reading to 0.0000 grams.
9. Now set the current supply to 1.0 A. Record the reading on the balance.
Warning: The current passing through the Goniometer Assembly
(SF8608) should never exceed 2.0 amps.
10. Rotate the dial clockwise in 50 increments to 900, each time recording the new
“Force” value. Then rotate the dial counter-clockwise in –50 increments to –
900, at each angle, repeat the “Force” measurement.
11. Plot a graph of force (vertical axis) versus θ (horizontal axis).
12. Account for any offset or asymmetry in your graph.