EC-7b  Induction - Dropping Magnet

OBJECTIVE:

To show that a moving magnet induces an emf within a coil of wire.

INTRODUCTION:

After discovering the phenomenon of induction using two coils wrapped around an iron ring, Faraday was able to show that plunging a magnet into a coil of wire also generated a momentary induced current. Faraday did not use the concept of Changing Flux instead he thought of the `lines of force', and concluded that when these lines move across a wire an emf is induced. Can you see the correspondence of this concept with the concept of flux? Think about this.

Figure 3: A moving magnet - lines move across a wire

EXPERIMENT:

You will be repeating Faraday's experiment by observing the emf induced in a coil by a fast moving magnet, and you will check that the emf you observe obeys the `right hand rule'. If you have the time you can see the effect of changing the speed and the polarity of the magnet.

EQUIPMENT:

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PC and PASCO interface.

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A voltage sensor is connected to the coil and to port A of the interface. Be sure the red wire is connected to the top of the coil. In this way a positive voltage corresponds to a current in the direction of the arrow on the top of the coil.

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A stand holding a plastic tube and a coil.

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A long bar magnet, this magnet has a small cut at one end, that is the North pole.

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A pair of red and green bar magnets

PROCEDURE I: (15 min)

1.

Click on the Launch EC-7B icon below (web version) to initiate the PASCO software window.

2.

Set the distance $d_1 \sim 15 cm$ between the top of the plastic tube and the top of the coil. Record this in your lab notebook. Be sure that the red terminal of the voltage sensor is plugged on the top plug of the coil.

3. The long bar magnet has a narrow cut at one end, this is the North pole of the magnet. 4. Hold the long bar magnet at the top of the tube; the end with the cut should be down, and just one centimeter or so inside the tube. 5. DCLK on RECORD and drop the magnet immediately after. 6. to see the whole graph, and measure the heights $h_1$ and $h_2$ of the two peaks using the cursor cross-hairs (5). 7. Slide the tube upwards, remove the rubber stopper and the magnet, replace the rubber stopper at the bottom of the plastic tube, and slide the tube down again. 8. Print the graph on printer.

Figure 4: The setup

QUESTIONS: (10 min)

Q1

The graph you obtained has a fall, a rise, a very short flat part, then a rise and finally a fall. Explain what is happening at these various times.

Q2

Explain why the two voltage peaks you measured in 1.6 are not the same size. Can you give an approximately quantitative justification for the ratio $h_1/h_2$?

Q3

Examine the direction of the winding in the coil; verify that the directions of the peaks is what you would expect using the right hand rule (curled fingers = current;    thumb = magnetic field).

PROCEDURE II: (10 min)

Repeat procedure I with the magnet inverted (the South Pole at the bottom).

QUESTION:

Q1

Explain in what way(s) and why the new graph is different.

PROCEDURE III: (20 min)

1.

Move the coil further down on the tube making the new distance $d_2$ at least about two and a half times larger than $d_1$. Record this new distance $d_2$.

2.

Repeat procedure I. Measure and record the heights $h^\prime _1$ and $h^\prime _2$ of the new peaks.

QUESTIONS:

Q1

Explain the reason for the difference between this graph, and the one obtained in procedure I.

Q2

Can you give an approximate quantitative explanation for the ratio $h^\prime _1 / h_1$? HINT: remember Physics 103 formula for free fall $v=\sqrt{2gx}$.