PHY 242 Laboratory for 10/8/98

Magnetic Field of a Line Current

You will use compasses to determine the direction and relative strength of the magnetic field (B) created by a current (I) flowing in a straight wire. The task is made more difficult by the ever-present magnetic field of the earth. It will be necessary to make observations for several different orientations of the wire and infer the direction of the current's magnetic field by trying to "subtract" the effect of the earth's field. All observations will be qualitative, so be sure to record those observations accurately and completely.

The setup consists of a loop containing a 12 V deep cycle battery, a current-limiting rheostat resistor, two sections of straight wire, and a deadman switch. The two straight sections of wire are arranged horizontally and vertically. All other sections of the circuit are kept far enough away as to mitigate their effect on measurements.

Procedure:

1. First, use the bar magnet (whose poles are clearly marked) to establish which end of the compasses represent N and S poles. With no current, record the direction of the earth's field as indicated by the compasses.
2. Place compasses very near the wire segments (within about one inch) and press the deadman switch. Record your observations.
3. Move the compasses a few inches away from the wire segments repeat the above. Repeat again at a distance of several inches.
4. Reverse the battery leads and repeat the second step.

In your report, describe the strength and direction of the magnetic field based upon your observations. If there is a difference between the behavior of the compasses on different sides of the wire, be sure to explain these differences.

(NOTE: In addition to writing, make a clear sketch of your observations for all sections of this lab.)

Magnetic Induction

Current can be induced to flow in a wire by an external magnetic field. We will explore how that occurs using coils of wire. The device provided for the first two procedures consist of two coaxial coils. The primary coil is wound on a solid cylinder and can slide into the secondary coil, which is wound on a hollow cylinder. The last procedure will use two identical coils that can be placed side by side.

1. Current Induced by a Moving Permanent Magnet.

You will detect the flow of current in the secondary (hollow) coil using a galvonometer connected to the terminals of the coil. You will need to establish the direction of the needle deflection of the galvonometer by quickly tapping the terminals of a weak battery to the terminals of the galvonometer. Connect the coil to the galvonometer and carefully inspect the wire to determine the direction the wire is wrapped around the cylinder. Your instructor will show you how to sketch the coil to minimize confusion.

1. Move the North pole of the magnet rapidly towards one end of the coil and observe the galvonometer deflection. Carefully record the results and indicate the direction of magnet movement and current flow in a sketch. Now move the magnet quickly away from the coil.
2. Repeat with South pole of the magnet.
3. Repeat the previous two steps with a slower movement of the magnet.

• Is the flow of current consistent with Lenz's law?
• What effect does the speed of magnet have on the current?
• Note the effect of the magnet's speed, and consider how current, charge and time are related. What quantity would you surmise might depend only upon the net change in magnetic flux? (net change, not the time rate of change!)

2. Current Induced by a Moving Coil.

Connect the primary coil in series with the power supply, rheostat and deadman switch. (The rheostat has already been adjusted to provide a reasonable current in the primary.) Note the direction that the current will flow and determine which pole (N or S) is present at the end of the primary. Confirm your determination with a compass.

1. Close the switch and repeat step 1 in the previous section using the coil as you did the permanent magnet.
2. Reverse the leads to the power supply and repeat.

Is there any discernible subjective difference between using the permanent magnet and using the primary coil?

3. Transformer Action.

It is possible to induce a current without any actual movement of the coils. By orienting two coils such that the magnetic flux of one coil passes through the loops of the other a current can be induced. Place the two free standing coils back-to-back. Connect the terminals of one coil to the galvonometer and the other to the power supply circuit.

1. Press and hold the deadman switch for a few seconds and record your observations.
2. Release the switch and observe what happens. Be sure to explain your results in terms of Lenz's Law.
3. Place the each of the metal cylinders through both coils and repeat step 1, observing only the magnitude of the induced current. (The cylinders are labeled. There should be one each made of copper, brass, soft iron, stainless steel, aluminum and zinc.) From your results determine which of these metals are ferromagnetic and which are paramagnetic.
4. Place the coils at right angles and repeat steps 1 and 2. Describe and explain the results.