Experiment 18: Effect of Magnetic Fields on Currents
Good work and very good answers.
However note the following, which is very important:
The force exerted by a magnetic field on a current segment is perpendicular to the field, and perpendicular to the segment. Your observations that the current appeared to be attracted to the magnet are natural enough, but the apparent attraction results from secondary effects (related to the deflection of current across the width of the wire and to variations in magnitude and direction of the magnetic field as you move across the face of the magnet). Don't repeat the experiment, but I do recommend that you set up the aluminum segments above the magnet once more and see if you can observe its back-and-forth motion perpendicular to the field. This will help you make more sense of the textbook explanations.
A straight bare copper wire is balanced on a knife edge, with a thin wire hanging from one end and immersed in water to provide stable equilibrium. Current is passed through a segment of wire at the other end, with part of the segment positioned between the poles of a ceramic magnet. The deflection of the wire with current is observed and from the diameter of the wire hanging in water the force of the magnetic field on the current is determined.
A light wire segment is suspended from the edge of a table by aluminum strips and the nature of the forces on the segment resulting from magnetic fields in various directions relative to the segment are observed.
The effects of magnetic fields in various directions on a similarly suspended loop of lightweight wire are investigated.
In this experiment you will investigate the effect of a magnetic field on the current in a short segment of wire, then you will investigate the effect of a magnetic field on a loop of wire.
The 'wire loop' in this experiment is a strip of aluminum foil attached around three sides of a cardboard square, and around part of the fourth side.
To start, suspend the loop from the edge of a table.
Position the magnet next to the aluminum strips, as indicated on the video clip.
Attach the leads of the generator as shown, and turn the crank fast. It should require some force to crank the generator; otherwise the aluminum strip is probably not making good contact with the leads.
What happens to positions of the aluminum strips?
The aluminum strips move towards the magnet.
What happens if the direction of the crank is reversed?
The magnet repels the strips if the crank is reversed.
What happens if the direction of the magnet is reversed?
Magnet still has same effect on wire and aluminum strips except the direction the generator must be turned to repel/attract the wire is opposite what it was before.
Position the magnet, as indicated on the video clip, so that it is in a vertical plane parallel to the wire strip, and close to the strip.
Give the handle of the generator a fast turn and see if the position of the wire segment changes.
The wire segment moves to the magnet. It is a stronger reaction than that of the aluminum strips.
Turn the magnet upside down and repeat, carefully noting any difference in what happens.
The wire segment it repelled rather than attracted as long as generator is cranked in the same way.
Turn the crank of the generator in the opposite direction and carefully note what happens.
With the magnet upside down and the crank turning in the opposite direction the wire is again attracted to the magnet.
Now position the ceramic magnet beneath the loop, making sure that the bottom of the loop is horizontal and that the magnet is lying on a horizontal surface as close as possible to the bottom.
Measure the length of the aluminum strips, from the tabletop to the wire segment.
The aluminum strips are 52.6mm long.
Slowly crank the generator and see if the wire segment changes position.
The wire segment moves sideways wither one direction or the other depending on which direction the generator is being cranked.
Now crank the generator, slowly at first, then more and more quickly, and see what happens to the position of the wire segment.
The segment moves farther from the middle of the magnet the faster the generator is cranked.
Add the wooden dowel weight to the system as shown the video clip, and place a thin ruler over the magnet in order to measure the position of the wire strip.
The system will probably tend to oscillate about some equilibrium position. Try to determine the central point of its oscillations; this central point is the equilibrium position.
Using the BEEPS program to set your 'beat', turn the crank of the generator at 1 revolution / sec and see how far the equilibrium position of the wire strip is displaced.
1 rev/sec : 0.4 mm displacement
2 rev/sec: 0.8 mm displacement
3 rev/sec: 1.5 mm displacement
Answer the following questions:
The field of the magnet is perpendicular to the plane of the magnet, as if it is coming through the hole in the middle of the magnet and out the ends. The current in the aluminum strips goes down one strip, through the wire segment, then up the other strip.
How does the apparent direction of the force exerted on the aluminum strips depend on the direction of the current and the direction of the magnetic field?
The direction of the force on the aluminum strips is dependant on which direction the crank is being turned (which end is + or -) and which side of the magnet is showing. If the magnets positive side is up then the aluminum strip through which the negative current is being passed will be attracted to it. If the magnets negative side is up then the aluminum strip through which the positive current is being passed will be attracted to it.
How does the apparent direction of the force exerted on the wire strip depend on the direction of the current and the direction of the magnetic field?
If the current flowing through the wire is positive it will be attracted to a negative magnetic field, but if the current flowing through the wire is negative then it will be repelled by the negative magnetic field.
Assume that the mass of the loop is 6 grams. The system consisting of the aluminum strips and the loop is pretty nearly a simple pendulum whose length is that of the aluminum strips, and its mass is that of the loop.
Using the length and mass of the pendulum, for small displacements from equilibrium you will recall that we can determine the force necessary for the displacement from the fact that the ratio of the displacement from equilibrium to the length of the pendulum is the same as the ratio of the force to the weight of the pendulum.
Use this principle to determine the force exerted by the magnetic field for the cranking rates 1 rev / sec, 2 rev / sec and 3 rev / sec.
1 rev/sec: ( .004/.526) = (F/.06) => F=4*10^-4
2 rev/sec: (.008/.526)=(F/.06) => F=9*10^-4
3 rev/sec: (.015/.526)=(F/.06) => F=1.7*10^-3
Save your force vs. cranking rate information-- you will use it again in experiment 20.
Now suspend the wire coil from the aluminum strips, as indicated on the videoclip. Be sure that the straw in which the central dowel is inserted is vertical and held securely.
You will position the magnet in three orientations:
Orientation 1: Magnet positioned so that its field passes through the plane of the loop in a direction perpendicular to the loop.
Orientation 2: Magnet positioned so that its field passes through the plane of the loop horizontally and parallel to the loop.
Orientation 3: Magnet positioned so that its field passes through the plane of the loop vertically and parallel to the loop.
Before you conduct the experiment, make the following predictions:
When current passes through the loop in Orientation 1, in what direction will the force on each of the four segments of the loop act?
Make the same prediction for the other to orientations of the magnetic field.
Since there is no current through the loop there will be no force on the loop no matter what orientation it is in.
For each orientation of the magnetic field, what do you expect will happen to the loop when a current is run through the loop?
1: The aluminum segment on the positive current side will be attracted to the magnetic field, as will the wire on that side so the loop will spin in a counterclockwise direction if the positive side is on the left.
2: The positive strip will be attracted to the magnet, so the force will be in a counterclockwise direction. The wire will be repelled by the field so it will move in a sideways motion either right or left. The negative strip will be repelled by the field so the force will be in the same counterclockwise motion.
3: The positive strip will be attracted to the fields so will stay in the field. The wire will be attracted to the field at one end and repelled at the other, and the negative strip will be repelled from the field. Thus the loop will end up with one side in the field and the other at an angle away from it, the angle depending on how strong the forces are.
Now conduct the experiment by positioning the magnet in each of the three orientations, as indicated on the video clip, and carefully note what happens when current is passed through the coil.
Finally, make a crude meter out of the loop.
Attach the horizontal 'pointer dowel' to the loop, as indicated on the video clip, and position a light pendulum as shown.
Position a ruler in order to measure the horizontal position of the pendulum.
Position the magnet so that the loop will tend to rotate in such a way that the pointer turns in the horizontal plane.
Using cranking rates of 1, 2, and 3 revolutions/second, send a current through the loop and for each rate determine how far the pointer displaces the pendulum bob.
1 rev/sec: 0 mm
2 rev/sec: 2 mm
3 rev/sec: 5 mm
Answer the following:
Explain in terms of the forces on the individual segments of the loop why the magnet must be oriented as it is in order to produce a rotation of the loop.
The magnet must be oriented so that the field affects one side of the loop more than the other to produce a rotational effect. If the field is directly parallel or perpendicular to the loop then the forces cancel each other out and there is no rotation.
Explain how the position of the pointer indicates the current through the loop.
The farther the pointer is from the equilibrium position the higher the current because there is more force pushing the loop around the rotational axis.
If you were to use the principle illustrated by this experiment to create a meter to measure current, how would you design the meter?
I would create a meter that had a cylinder in the middle of a circle with measurements of current around the outside so that if the 0 was oriented at the equilibrium point of the loop and the loop restricted by the cylinder to a rotational axis the user could easily measure the amount of current by reading off the number where the pointer on the loop ende