Magnetic Fields and Magnetic Forces (5/7/2015)

Field Directions Around a Bar Magnet

Using a bar magnet and compass, we were asked to draw the direction at which the compass pointed at various areas around the magnet, as shown below.

A physical representation of a magnetic field around a bar magnet.

We then converted our arrows into magnetic field lines, and drew three enclosed surfaces in order to observe and record the number of field lines entering and exiting each surface. Surface one enclosed the negative end, surface 2 enclosed the positive end, and surface 3 enclosed both ends. The results are show above. What we learn is that the magnetic flux is related to the number of poles enclosed. A net flux requires a net number of poles.

Cutting a Magnet in Half

Shown above is a magnetized paper clip, which was then cut in half. What ended up happening is we now had two magnets. The reason for this is that were are many domains within the magnet, not just one north and south pole. If we cut a magnet into pieces, the domains still exist inside, but are just spread between two pieces. In order to completely shut down the magnetic field, the domain would have to be cut in half.

Magnetic Forces on Moving Charges

As seen above, when a magnet is moved toward the powered on oscilloscope, it affects the beam of electrons. Shown below are drawings of the forces involved in various situations similar to the one above, with V being the direction of motion of charge (in this case, pointing out at us), B being the direction of the magnetic field, and F being the direction of the force.

Magnetic Force on a Current Carrying Wire

What we see in the photos above, is that when a current is pass through the wire, the affecting forces either pull down the wire (as shown in the first two pictures above), or pull the wire upwards (as seen in the last two photos above).

Drawn below are diagrams explaining what has happened. When the current flows from left to right (while the magnetic field is constantly pointing at us), the force is downward and when the current flows from right to left, the force is upward. 

Magnetic Forces on a Current Loop

For this activity, we were asked to draw a detailed picture of a loop with current flowing through it, and calculate various aspects of it, such as its torque, force on each side, and net force. The work can be seen below. After the drawing, we were asked what would happen if the loop was placed within a magnetic field. There were several options, but we (correctly) chose option C. We stated that the loop would turn 90 degrees and stay there horizontally.

This was due to the fact that the forces in this situation produce a torque which rotates the loop, but once the loop turns past 90 degrees, the torque changes direction, which causes the loop to go back in the other direction. This continuous back and forth results in the constantly horizontal loop shown below.

Summary:

In summation, we delved deeper into understanding magnetic forces and fields, and looked into several situations in which they play a significant factor. One of the biggest takeaways is how a line of charge reacts when confronted with a magnetic field. This results in a variation of the Right-Hand Rule, which allows us to predict the direction of the force on the object if we know the direction of charged particles, and of the magnetic field. We have also now tied torque to magnetism, which present to us the basics of how some engines function. The last activity would not have produced a decent engine, for example, because the loop did not keep spinning and stayed stationary.