PH 1121 Lab 7: Magnetic Induction

Overview

We conclude our experimental study of magnetism by having you make measurements that involve electromagnetic induction. We can't overemphasize the significance of electromagnetic induction. Given changes in the magnetic flux with time, you have already heard a lot about how to determine the polarity of induced EMFs in lecture. This exercise will give you practice in predicting and observing the polarity of induced EMFs.

Setup

• Unhook the magnet field probe from the LabPro Interface and replace it with the Differential Voltage probe. Connect the red and black leads of the voltage probe to the coil at your lab station, as described by your Lab Instructor. Place the blue foam circular pad on the bench, flat side up, and place the coil on that top flat surface with the central hole of the coil facing up. Open the Logger Pro File for Part 1.
• Click the "Collect" button to start the voltage recording, and then slide the bar magnet at your station rapidly into the central hole of the wire coil. You should see a voltage spike appear on the recording. Now pull out the magnet rapidly. You should see another voltage spike, but this one with opposite polarity. (Why opposite?)
• Experiment with the speed with which you slide the magnet into and out of the coil. In general, the faster you slide the magnet in or out, the bigger the induced EMF will be. As you slow down, however, you will reach a point where you will see NO effect from the changing magnet field—it's changing at just too slow a rate. Note that whenever the magnetic field is not changing (for example, if the magnet is still), there is no measurable EMF.
• Now turn over the magnet end-for-end and repeat the previous two bullet-points, noting that the polarities of the voltage spikes are opposite from before. (Why?)
• Now pick up the coil, holding it in one hand with its hole horizontal. Pick up the magnet in your other hand, and move the magnet back and forth in the hole of the coil, MAKING SURE that that you can always see the color of the portion of the magnet inserted into the coil hole as you move the magnet smoothly back and forth (in other words, the magnet should never be inserted beyond its midpoint into the coil—otherwise, the polarity of the other half of the magnet will start to have an effect). This is one way to generate an alternating voltage (with which you could generate an alternating current) – by changing the magnet field periodically with time. An even more convenient way (used by your local electric power company) of doing this same thing is to rotate a coil in a constant magnetic field (as discussed in lecture!), making the angular orientation between a coil and magnetic field vary in a regular periodic manner. That's how an electric generator works!
• Now change the back-and-forth frequency from low to high (or vice versa), and note what happens to the amplitude of the resulting induced EMF. (Observe carefully because there will be a Worksheet question about the results of this variation of inducing an EMF.) To see the detailed shape of your induced EMF waveform, you might want to expand the time axis of your recording by clicking on and changing one or both of the extreme t-axis values.
• When you think that you pretty well understand the physics implicit in this part, you might increase the amplitude of magnet insertion into the coil so that both polarities of the magnet are having an effect inside the coil. If you can think of other ways of inducing and EMF in the coil with the magnet, short of causing some experimental component to crash to the floor, you are welcome to try those ways now.
• Now something new—a new learning opportunity!!! Put the coil back on the flat side of the blue foam pad, coil-hole vertical, and open the Logger Pro File for Part 2 (which only records for a 5-second time interval).

Procedure

• Click on "Collect," and before your 5 seconds are up, insert the magnet into and withdraw it from the hole in the coil, JUST ONCE, for one up-down pair of peaks in the induced EMF recording. Resize the graph as necessary to see a well-spread-out graph of that pair of peaks. It turns out that the areas under the two peaks should be the same, independent of the speed with which the magnet entered or exited the hole. To have Vernier calculate the areas, place the cursor to the left of the first peak (where the voltage begins deviating from zero), click and drag the cursor to the point where the graph of that first peak returns to zero, release, and then click on the "Area" button located between the "Stat" and "R=" buttons up on the tool bar. A box will open up giving you the area under that first peak. Then repeat those steps for the second peak. The area of that second peak should be opposite in sign but similar, if not identical, in magnitude to the area of the first peak. Record these areas for later use.
• Repeat this experiment, first inserting the magnet quickly, and then withdrawing it more slowly. Obtain and record the areas under each peak.
• As a final test, one partner should grab the coil in one hand, turn the blue foam pad over, making sure that it is centered on the black rubber sheet at your station, both placed well back from the edge of the bench (so that nothing ends up bouncing to the floor!), and hold the magnet vertically suspended with the other hand so that it can be dropped through the hole in the coil to land GENTLY in the slot of the blue foam piece. NOW THIS NEXT IS REALLY IMPORTANT! The coil should be held high enough above the blue piece so that the magnet does NOT bounce back up anywhere close to the coil, AND the magnet should be held initially at least an inch above the coil so that it starts out having no effect on the coil. When one partner has the coil and magnet all lined up, with the magnet ready to drop, the other partner (or the Lab Instructor, in the absence of a partner) should click "Collect." When the recording starts, the magnet should be dropped, and if it drops cleanly through the hole to the foam piece underneath (and NOT bouncing to the floor!!!), that recording should provide good results. (If the drop wasn't clean, simply try again.)
• In the manner described above, determine the area under both the positive and negative peak in your recording. Record these numbers for later use on your Lab Report, AND be sure to observe the relative durations of the two peaks (specifically, are they the same or does one peak have a larger duration than the other?).

Logger Pro Files

• Logger Pro File for Part 1
• Logger Pro File for Part 2

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