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Lab 27: Faraday Fun Day

Goals: Improve teamwork and communication capacities; Gain experience with magnetic induction, Faraday’s Law, and Lenz’s Law. Groups & Lab Notebook: Work in groups of 3. Update Table of Contents. Take good notes.

Some activities are equipment limited. If you reach such an activity and the equipment is all out, move on to the next activity and then come back and finish when equipment is available.

 WARNING: The magnets that you will use today are quite strong and need to be treated with significant caution. Do not allow them to slam together (they will likely break). Do not allow them to pinch your body (they may bruise, cut, or crush). Do not allow them near computers (they will wipe out magnetic hard drives and cause motor fans to seize). The compasses, alligator clip leads, etc. used in this lab will all respond strongly to these magnets.

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A. Magnet Polarity

Equipment:

Cylinder bar magnet (1 per group)

Compass (1 per group)

Tape

In a previous lab, you used a compass to determine the N pole and S pole of a cylinder bar magnet, and marked the N pole end with a piece of tape. Either verify or determine the N pole end of your bar magnet and mark it with a piece of tape.

A reminder in case you need it: The windows in 2046 face out roughly towards the north. Whichever end of the compass needle is pointing roughly in that direction should be the North Magnetic Pole of the little bar magnet in the compass (this is different for different compasses).

Here’s a point of possible confusion: if that’s the North Magnetic Pole of your compass, isn’t it attracted to the South Magnetic Pole of the earth?

It is – the South Magnetic Pole of the earth is actually near the geographic North pole!

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B1. Moving a Conductor in a Magnetic Field

Equipment:Cylinder bar magnet with N pole end marked (from previous)3200-turn coilMicroammeterBanana plug leads (1 red and black, should come in a pair).

a) Connect the microammeter to the coil as follows: using the red lead, connect the side of the coil labeled 3200 to the left hand plug on the microammeter; using the black lead, connect the other side of the coil to the right hand plug on the ammeter. Make sure the leads are long enough that you can move the coil around but leave the ammeter stationary. If you need, get longer leads or attach several sets of leads together.

b) One partner should hold the cylinder magnet perfectly stationary with its axis vertical and N pole end up. Another partner should hold the coil with the 3200 labeled end pointed down, directly above the N pole end of the magnet, such that it can be lowered with the magnet axis and the coil axis aligned and the magnet enters the coil opening. The third partner should watch the ammeter and then direct the other partners to notice the important relevant features.

c) Holding the magnet perfectly stationary, slowly lower the coil towards the magnet. Note the deflection of the microammeter needle: both direction and magnitude. What happens when the coil stops moving?

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B2. Moving Conductor, Stationary magnet

d) Slowly raise the coil directly up away from the stationary magnet. Again note the direction and magnitude of needle deflection.

e) Repeat, but with coil moved faster, but towards and away.

f) Repeat, with S pole end up, coil moved towards and away, fast and slow. Repeat all previous, but this time with 3200-labeled side pointed up not down.

g) Summarize your findings. A table might be a nice way to do this.

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C. Stationary Conductor, Moving Magnet

Equipment: same as before

a) Repeat the various permutations as before, but this time place the coil so that it is stationary on the table and move the magnet up and down. Start with the 3200-labeled side up and the N pole down. Also make sure to have the magnet enter the coil (you may not have done that last time).

b) Again, summarize your findings in a table. Note the symmetries you observe.

c) We can understand our observations via the principle of relativity. However, there is a problem understanding the observations in this part using the classical magnetic force law. What is the problem? Hint: look at the first phrase of the title of this part, above.

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D. Moving Magnet, Moving Conductor

Equipment: Same as before

a) IMPORTANT: Discuss and make your prediction before testing it out. What do you think will happen if you move the coil and the magnet together?

b) Try it and see. Record your observations. If you were surprised, note why you were surprised. If you were not surprised, put yourself in someone else’s shoes and see if you can describe why a smart other observer might have been surprised.

Return Microammeter and banana plug leads. Keep rest of equipment.

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E: Motors turn Electrical Energy into Mechanical Energy

via Magnetic Fields and Magnetic Force Law

Equipment:

1 AAA battery

Genecon generator (blue plastic case with white plastic handle and leads out front, looks a bit like a hand drill but no drilly end)

a) You’ve previously built some very simple motors. A motor uses a magnetic field to turn electrical energy into mechanical motion (mechanical energy). A potential difference drives a current in a conductor. The conductor is in a magnetic field. The magnetic force causes the mobile charge carries to deflect in the conductor, which then results in macroscopic motion of the conductor. If needed, review some parts of Lab 24 where you saw a conductor move in a magnetic field.

b) One partner holds the battery. Second partner holds the genecon by the grip (white handle should be free from obstruction. Third partner touches one lead to positive end of battery and other lead to negative end of battery. What do you observe?

c) Repeat, but reverse battery. What do you observe?

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F: Generators turn Mechanical Energy into Electrical Energy

via Magnetic Fields and Faraday’s Law

Equipment: same as part E, but also get a light bulb and socket.

a) With Genecon not attached to anything, each partner should turn the white handle to get a relative sense of the effort involved. Turn approximately 2 – 3 times per second.

b) Attach Genecon leads to either end of socket. One partner should turn the Genecon handle as before. Don’t say anything to your partners just yet! Each partner should take a turn. What do you notice?

c) One partner should turn the handle just a little bit faster (don’t turn too fast). What do you notice about the brightness?

d) Another partner should turn the handle at a steady rate. A second partner should disconnect the light bulb (just one lead is sufficient to break the circuit) and then in just a few seconds, reconnect. Do several times. Do with handle-turning partner having eyes shut. What does the handle-turning partner notice? Switch roles so all can experience this.

e) Ask your instructor to see a larger version of a Motor/Generator so you can more easily see the parts.

Return the Genecon, battery, light bulb, and socket.

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G: The World’s Simplest Generator?

Equipment:

Cylinder bar magnet (should still have from before)

3200-turn coil (should still have from before)

Banana plug to squeeze clip type leads

Red Light-Emitting Diode (LED)

Gray plastic tube.

a) Connect the coil to the LED. Hold the coil axis vertical, and put the tube through the coil so that everything is oriented vertically. Your goal is to be able to drop the magnet so it falls through the tube and thus falls vertically through the coil.

b) Make sure that one partner can catch magnet when it emerges from bottom of tube (don’t let it crash into table top, floor, etc.). Release the magnet and watch the LED. What do you notice?

Return: LED and plastic tube.

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H: The Magnet and the Pendulum

Equipment (all available in back corner where the ring stands are):Variable Gap MagnetRing Stand, Support Rod and connector (if needed)“Comb” Pendulum & “Plate” PendulumCylindrical Bar Magnet & Compass (should still have from before)

a) Use your bar magnet and your compass to verify that the plate pendulum and the comb pendulum are not by themselves magnetic.

b) Next, hook up the comb pendulum as shown in the figure. Make sure that the comb pendulum can swing freely and that it passes between the poles of the magnet assembly. Check with Krishna or Diane to make sure you’ve got the rightset-up.

c) Lift up the pendulum and release it from rest. Repeat several times. Do you observe this pendulum to swing back and forth pretty freely?

d) Replace the comb pendulum with the plate pendulum. Repeat the previous step.What do you observe? Specifically compare and contrast with the other pendulum motion.

e) Ask Diane or Krishna to show you the “spatula” pendulum. What do you think will happen here? Discuss and predict before testing.

Return everything to it original location.

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I: Motional EMF simulation

Using the Mozilla Firefox browser (ask if you’re not sure where to find it), work through the ActivPhysics simulation and activities at http://media.pearsoncmg.com/bc/aw_young_physics_11/pt2a/Media/Magnetism/1310MotionalEMF/Main.html

J: AC Generator

Work through the ActivPhysics simulation and activities athttp://media.pearsoncmg.com/bc/aw_young_physics_11/pt2a/Media/Magnetism/1309EMInduction/Main.html

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