Magnetism and Motors Laboratory

Before coming to lab, be certain you have read the Magnetism and Generators/Motors section for this module. In addition, you should read the Exploratorium's A simple DC motor site.

1. Magnetic Field of a Bar Magnet

Using a small compass, you will trace a few magnetic field lines for a bar magnet. Place the magnet on a large sheet of paper. Trace around the magnet and mark "N" and "S" inside the outline as indicated by the magnet. (With this outline, it will be easy to reposition the magnet after it is moved.) Mark 4 dots on either side of the magnet spaced 1 cm apart. Mark two dots at the end of the magnet.

Move the bar magnet far from the paper and use the compass to find the direction of the earth's field. Orient the page such that magnet outline is along the earth's field. Replace the magnet and place the magnet next to one of the dots, such that one end of the compass points directly towards the dot. Make a dot at the opposite side of the compass. Move the compass to that dot that you just made and repeat the process. At first, this process will take you out away from the magnet. But eventually it will return you back to magnet at the opposite end. Do this for all the dots and you will have a nice "map" of the magnetic field lines. Do the process with some care, but it should not take you more than 30 minutes.

2. 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.

3. The DC Motor

You will be provided with the raw materials to build your own motor/generator. The Exploratorium's A simple DC motor site will provide you with one example.

It is not necessary to follow exactly the instructions provided above. Rather, try to develop a sense of how and why the design works. If you can devise an alternative design, all the better. Discuss any new designs with the instructor before beginning construction. Good luck, and may your coil spin freely!