Magnetic Forces and Fields

(This lab experiment has been adapted from Electric and Magnetic Interactions by Sherwood and Chabay.)

 Permanent Magnets

When a compass needle turns and points in a particular direction, we say that there is a "magnetic field" pointing in that direction, which forces the needle to line up with it. For the time being we’ll simply define magnetic field as "whatever it is that is detected by a compass." The twist of a compass needle is an indicator of the presence of a magnetic field. We define the direction of a magnetic field B at a particular location as the direction that the "north" end of a compass points to when placed at that location. In some cases, we can use the amount of deflection to find the magnitude of the field.

In this lab you will doing many small experiments of the "try this and see what happens" variety and only a few measurements. Begin by cleaning off a work area on your table. As much as possible there should not be any steel objects nearby that might interfere with the magnetic forces. Place your compass on the table and turn it so that the needle points to the "N". Now you know the direction of the Earth’s magnetic field at your location. Place one of your cylindrical magnets so that one end points toward the "W" on the compass and the compass deflects east. The compass needle now shows the vector sum of the Earth’s magnetic field and the field from the bar magnet. Assuming the magnet creates a magnetic field along its axis, draw a picture in your notebook showing the direction of Bearth, Bmagnet, and Bnet. For future reference, mark the end of the magnet pointing toward the compass with white tape. This is its North end.  Use the same procedure to determine the North end of your other magnet and mark it also.

Turn your magnet 180 degrees. How does the compass needle deflect, and what can you conclude about the direction of the magnetic field at the South end of your magnet?

Suspend your magnet from a thread. Which end points toward the north? What can you conclude about a compass needle?

Briefly describe the interaction your two bar magnets have with each other. Check to see what other objects your magnets will interact with. Be sure to try some metal objects and some non-metal objects. Make a charged tape as you did in the lab on electric forces and check to see if the charged tape interacts with the magnet. How can you tell if the interaction is due to the magnet or if it is just the interaction the tape would have with any uncharged object?

Place two magnets in various positions near your compass to demonstrate that the superposition principle holds for magnetic field; that is, the net field of the two magnets is the vector sum of the two fields. Record in your notebook at least one example that supports your conclusion.

The magnitude of magnetic fields is measured in a unit called the "tesla." The horizontal component of the earth’s magnetic field, which is the component that affects a horizontally held compass, is different at different latitudes, depending on the distance from the "magnetic poles" of the earth. Over much of the central US, the horizontal component of the Earth’s magnetic field is about 2 x 10-5 tesla. Next you will use this to measure the strength of your cylindrical magnet. As you did before, place your magnet on the west side of your compass, oriented in such a way as to make the compass deflect to the east. Place the magnet far enough away that the compass deflection away from north is 60 degrees. Measure the distance from the center of the compass to the center of the magnet. Referring to the diagram you made at the beginning of this lab, use vector addition and the magnitude of the earth’s field to calculate the magnetic field of the magnet at this distance from the magnet. Now move the magnet farther away by a factor of two; that is, place the magnet so that the distance from the center of the magnet to the center of the compass is twice what it was before. Record the new distance and angle. Calculate the magnetic field of the magnet at this new distance from the magnet. By what ratio did the magnetic field of the magnet decrease when the distance was doubled?

The magnetic field of the magnet gets smaller with distance, and it is plausible to guess that the magnetic field of the magnet might vary as l/dn, where d is measured to the center of the magnet, and n is initially unknown. According to your measurements, what is n? (You may wish to make measurements at some other distances to give further support to your analysis.) Extrapolating to a location near one end of your bar magnet, approximately how strong is the magnetic field in tesla near one end of your magnet? (Remember d is measured to the center of the magnet.)

Measure the magnetic field on the side of the magnet.  Use approximately the same distance from the magnet as you did for the first measurement of the field on the end.  How do the magnitudes of the field compare on the side and the end?  In what direction does the side field point? Note that you must always position the magnet and compass in such a way that the magnet’s magnetic field is perpendicular to the earth’s magnetic field.

 

Fields due to Currents

Assemble a two-battery circuit with a bulb in a socket. Place your magnetic compass on a flat surface under one of the wires. Keep the compass away from the steel-jacketed batteries. (For flexibility in placement, you may find it useful to make a  long wire by connecting two of your wires together.) With the bulb lit, do the following:

  1. Lift up the wire up above the compass.
  2. Orient the wire to be horizontal and lined up with the compass needle.
  3. Bring the aligned wire down onto the compass.
  4. Observe the effect of the electric current on the compass needle. Record the amount and direction of the deflection.

Now lift the wire up again and turn it so it is perpendicular to the compass needle. Bring it down again and observe the effect.

This time instead of lifting the wire lift the compass up above the wire. Align the wire in parallel with the needle again. Now lower the compass down to the wire. How is the effect different? Explain your three observations in terms of the magnetic field produced by a wire.

Disconnect the batteries, turn them around, and reconnect them so that they current moves in the opposite direction now. Repeat steps 1 - 4 above. Has reversing the current changed anything?

Repeat steps 1 - 4 with the batteries disconnected to make sure that the effect you are observing is associated with the current itself not some property of the wire. Record your observation.

In the the next part of the lab, you will connect about 2 meters of insulated wire to your battery. Since you are essentially shorting the battery, make the connection only long enough to note each measurement. Keep one end of the wire connected to the battery at all times so that you don’t inadvertently change the direction of current flow. Lay the wire down on the table so that a large section of the wire is straight along a north-south line and not close to the steel battery casing. Hold the compass above the wire at a height where the compass deflection is 20o when you turn the current on. Measure the height to the center of the compass. Now raise the compass to twice this height and note the compass deflection. Use vector addition again to compute the magnetic field at the two positions. Is your data consistent with the theoretical prediction that the magnetic field near a long straight wire is proportional to 1/x, where x is the perpendicular distance to the wire?

Take an insulated wire about 2 meters in length and wrap a coil of about N = 20 turns loosely around two fingers. Remove the coil from your fingers and tie with a twist-tie. Use clip leads to connect the coil to a battery. By appropriate placement and orientation of your current loop with respect to your compass, determine the location of the North pole of the magnetic field created by the coil. Mark the north side of the coil with white tape.

Hold the compass in front of your coil so that the needle is at about 45 degrees when current is present. Place a nail through the middle of the coil. What happens? Add another nail. Is there a change? Explain this change using the idea of magnetic domains.

To check to see whether the nails are permanently magnetized after being inserted into the coil, break the circuit connection. If the compass still deflects significantly without any current in the coil, the nails have become strongly magnetized. Are they? If not, check for weaker magnetization by bring one end of the nail near the compass, first one end then the other end. Is the nail slightly magnetized?

There is yet another way in which the behavior of a magnet and of a current-carrying coil of wire is very similar. Hang your coil over the side of the table and bring your bar magnet near the hanging coil, along the axis of the coil. Reverse the magnet. Repeat from the other side of the coil. Is this behavior similar to the interaction you have observed between two bar magnets?

Summarize what you have learned about magnetic fields.