Qualitative Magnetism Laboratory

Transcription

1 Qualitative Magnetism Laboratory 1 Object To learn about magnetism and the many facets of induction from eight dierent experimental stations where various aspects of magnetism will be shown. 2 Equipment Wires, light bulb, bar magnets, hollow Al tube with accessories, power supplies, horseshoe magnet with knife blade assembly, galvanometer, solenoid, St. Louis motor and accessories, jumping ring apparatus, banana plugs, etc. 3 Theory - ish In this lab you will be asked to explain the eects you see at each station. In each case there is some type of magnetic phenomenon going on. You will rst need to understand the set-up how things might be wired, what materials things are made of and then you will follow the directions to see each phenomenon. Your task for this lab is to properly carry out the instructions, observe the eect, and then to clearly and correctly explain (in words, and with equations and sketches) what the physics of each is. You will have 15 minutes at each of the eight (8) stations, afterwhich you will move to the next station. After two hours you will have one additional hour to nish your explanations. At the end of three hours, if you haven't nished sooner, I'll collect your papers. One nal note: although you will be with your partner at each station, and may discuss what is happening, each of you will turn in your own report, which should reect your own thoughts.

2 Field Mapping The circuit diagram for this station is shown below in gure 1. A Grey DC PS Power R Al Rod Figure 1: Schematic circuit diagram for eld mapping. Turn on the power supply. Current should now be owing around the circuit in a direction you should be able to determine. Allow no more than ve amperes (5 A) of current to ow. There should be at least one compass at the station. You should place it on the small table and move it toward the Aluminum bar. Observe what happens to the compass needle and explain in detail why it does what it does.

3 Knife blades and Horseshoe magnet In this station you should nd the very large horseshoe magnet which is mounted upon a wooden base. There are two Aluminum knife blades suspended above the pole gap in the magnet. You should pull the blades back together through some angle (about 45 works well) and release them together so that they will swing through the gap (they should not hit the pole pieces if they do, repeat more carefully). Observe what happens to each blade. You may want to do this a couple of times just to be sure. Explain in detail what happens (and why) to each of the blades.

4 Aluminum Tube with slugs Here there is a long Aluminum tube with two slugs which are of just the right diameter to t into the tube. Start with the blue slug and allow it to fall down the tube when the tube is vertical. Next, allow the red slug to fall down the same way. Again, you might want to do this a couple of times just to be sure. Explain in detail what happens to each and why.

5 St. Louis Motor Here is a D.C. electric motor (sometimes called a St. Louis motor) which should be pre-wired to work correctly. These are notoriously testy items, so you might want to ask the instructor or TA for some assistance if things do not work right away. When the circuit is completed, the motor should do what a motor does. Your job (after getting it to work) is to explain the working of the motor in words and diagrams so that we who grade can understand that you understand what is going on.

6 Galvanometer and Solenoid This is a simple circuit with a galvanometer and solenoid hooked up together. You should pick up the bar magnet and move it toward the solenoid's hole. Wiggle the magnet in the hole and watch the needle on the solenoid. You need to explain what is happening and why in detail, including the direction of the deection of the needle, the polarity of the magnet, and the sense of the windings on the soleniod.

7 Light bulb with Large solenoid The circuit diagram for this set-up is shown in gure 2 below. switch Grey DC PS Large Solenoid wooden base flashlight bulb Put the wrapped metal rods into the solenoid core Figure 2: Circuit for light bulb set-up. After turning on the power supply (it should be set to a proper voltage already) you will need to close the switch. The light bulb should light up very shortly after the switch is closed. Now watch the bulb closely as you open the switch. If you missed it, try closing and opening the switch again. Explain what you have seen, and explain why it happens.

8 Transformers Take a close look at the transformer. Note that you can unscrew the black handled knob and remove the top piece of the metal core. (You don't need to do this.) The plastic pieces on either side enclose coils of wire, called the primary (input) and secondary (output) coils. The primary is hooked up to an A.C. voltage source, and the secondary is hooked up to some type of load (circuit). On the plastic is a small letter n, which indicates the number of turns of wire for each coil. This is depicted schematically in gure 3. Primary Coil Secondary Coil Function Generator Oscilloscope Figure 3: Schematic of a transformer. Hook the function generator (set for a sine wave output) up to the coil with the higher number of turns (if you've taken the transformer apart, put it back together rst). Next, hook the oscilloscope channel #1 up to this input signal. Hook the other coil (secondary, lower turns) up to channel two of the oscilloscope. Make sure the volts/div knobs for both channels are set to the same value and that both allow you to see both waveforms completely. Compare the value of the output amplitude to that of the input amplitude as a ratio. How does this compare to the ratio of the number of turns of the secondary coil to the primary coil? Now switch the inputs and outputs so the primary has the lower number of turns and the secondary has the higher number of turns. Make the same measurements and ratio as above and compare the two. Explain why things work like this.

9 Jumping ring This set-up has the potential for personal injury, so please exercise caution. Here, there is a coil of wire which is plugged into an ordinary outlet. Before closing the switch, you are to place the short aluminum tube around the top of the apparatus. There are some metal rods protruding from the top of the main tube, and the aluminum tube needs to go around these and should t over the main tube as well. The switch is like an ordinary house switch, but you should only turn it on for a very short time, or else the device will overheat. Have your partner then quickly close the switch and then open it again i.e., ip the switch quickly on and then o. Watch what happens, and try it again. Explain what happens when you close the switch, as well as why. How does the aluminum tube feel after you've done things a couple of times? Explain.