# PHY 124 Lab 4 - Magnetic Force and Induction

If you need the .pdf version of these instructions you can get them here

## Goals

In Part I you will observe the consequences of the force exerted on moving electrons by the magnetic field of a bar magnet. In Part II you will measure the voltage induced across a solenoid coil by the change of the magnetic flux through the coil. In Part III you will observe the voltage induced across a solenoid coil by an alternating current (AC) from a second solenoid coil. These two solenoid coils are arranged similar to a transformer. Since you have a lot to do in this lab, make sure you prepare well. First, view/review relevant material in the online notes for PHY 122, such as Ch18 Sh5-7, Sh26-28, Sh30-31; Ch19, Sh2-4, Sh25-29. Then view/review similar material in Chap. 24, “Magnetic Fields and Forces” and in Chap. 25, “EM Induction and EM Waves” in Knight, Jones, and Field, College Physics: A Strategic Approach, 2nd edition, the optional textbook for the course. (If you don't have a copy, you'll find one bolted to a table in the Help Room, A-131 physics building. A few copies are also on closed reserve in the Math/Physics Library, “C” level, physics building.) Then carefully watch the video, read this entire Lab Manual, and do the online Lab 4 pretest.

## Equipment

• 1 oscilloscope
• 1 bar magnet
• 1 small solenoid coil (see Fig. 24.23 on p. 788 of KJF2)
• 1 large solenoid coil (see Fig. 24.23 on p. 788 of KJF2)
• 1 DC power supply
• 1 galvanometer
• 1 AC voltage generator
• test leads (wires) with banana plugs

Figure 1 is just above here

## Part I - Magnetic Force

Your goal here is to determine the magnetic north (N) and south (S) poles of the bar magnet (see Ch18 Sh5 and Fig. 24.8 on p. 783 of KJF2) by moving a spot of light on the oscilloscope screen. You accomplish this indirectly by deflecting the electron beam inside the cathode-ray tube inside the oscilloscope. This beam is a nearly parallel, tightly focused (by a knob you can adjust) stream of negatively charged particles moving with the same kinetic energy in vacuo. The spot of light is created when the electrons hit the phosphor coating on the inside of the glass surface that makes up the oscilloscope screen. The right-hand rule (see FigureRHR here and also http://en.wikipedia.org/wiki/Right-hand_rule) for the magnetic force on a moving charge (see Ch18 Sh7 and Chap. 24.5, “Magnetic Fields Exert Forces on Moving Charges”, in KJF2, pp. 789-795; Fig. 24.25 on p. 790) is useful for interpreting your observation of the deflection because it allows you to see the relationship between the velocity of a charged particle and the magnetic force acting on that particle because of its motion through a magnetic field. In this experiment the velocity of the charged particle is along the direction the electrons are traveling in the oscilloscope (toward you when you are directly in front of the oscilloscope screen), and you control the direction of the magnetic field according to how you hold the bar magnet just in front of the screen. You will use the right hand rule to solve for the direction of that magnetic field, but BE CAREFUL: When using the right hand rule remember that the electron has a negative charge!

FigureRHR is just above here

### Procedure

Set the controls on the oscilloscope to:

• VERT MODE to CH2
• X-Y pushed in
• ← POS → to mid range
• The white button at the bottom center to ANALOG

The positions of the other switches (what you turn on and off), knobs (what you turn cw or ccw), and buttons (what you push or pull) should not matter unless they were maladjusted by the person(s) using the oscilloscope before you.

Turn the oscilloscope power on and adjust the vertical position knob (POS) for CH2 such that you see a stationary spot at approximately the center of the screen. Be careful to turn down the INTENSITY control (ccw lowers the intensity, whereas cw increases it) so that the spot is not too bright. It's too bright when you are seeing a halo around the spot; this is not good for the oscilloscope screen.

You will now use a bar magnet to deflect the electron beam.

First watch out: You may find bar magnets already labeled with N and S. You CANNOT rely on this being correct!

Second watch out: Do NOT attempt this deflection experiment outside this laboratory with an older-style television (one that uses a cathode-ray tube rather than an LCD flat screen) and a really powerful magnet that you or a friend may happen to have. We know of students who tried this in the past, and in so doing they permanently messed up the picture seen on the TV they used. The reason is that a really powerful magnet can permanently magnetize internal elements inside such older-style TVs, particularly color TVs. When this occurs, moving the really powerful magnet away from the screen does not remove the remanent field(s) of the now-magnetized internal elements that deflect the electron beam inside the TV in an uncontrollable way. That messes up the picture. Even color TVs with degaussing coils installed can be permanently messed up. Gauss is a unit of magnetic field (1 gauss = 10-4 tesla) so degaussing refers to removing or greatly decreasing the remanent field in magnetized materials; see http://en.wikipedia.org/wiki/Degaussing.

1. With the bar magnet oriented vertically, bring it toward the screen of the oscilloscope as shown in Fig. 2a. Make sure you note which end of the magnet is on the top; mark that end with a small dot. Whenever you must label this bar magnet in your worksheet, include that mark in your sketch. Since you need to find the north and south poles of the magnet, you must know and record how it was oriented when you observed the deflection and applied the right hand rule.

Figure 2 is just above here

Always keeping the same orientation, repeat bringing the magnet to the screen several times. You should notice that as you bring the magnet closer the electron beam moves in some direction from its original position. Record in sketch a) on Part I of your worksheet your observation of this deflection .

2. Turn the magnet by 180 degrees, i.e., flip it end-for-end so that it is in the other vertical orientation as shown in Fig. 2b. Repeat step 1 and record your new observations in sketch b) on Part I of your worksheet.

3. Now turn the magnet into the horizontal orientation shown in Fig. 2c. Once more, repeat the instructions from step 1. Make sure you note where the marked end of the magnet is, on the left or on the right. Record your observations on Part I of your worksheet in sketch c).

For the 3 orientations in steps 1 through 3, apply the right hand rule to determine which way the magnetic field of the bar magnet points. Once you know the direction of the field, determine the north pole of the magnet and mark it. Record the poles on sketches a), b), and c) on your worksheet for Part I. These sketches should show all of the following: (i) the direction the electron beam is deflected, (ii) the direction of the magnetic field, (iii) the two poles of the magnet, and (iv) an indication of which end of the magnet you put a dot on.

You have now a bar magnet with the north and south poles determined and marked. In any sketches for Part II always include both the label N for the north pole of your bar magnet and where you put the original mark on the bar magnet. This will provide you with a check to make sure that your observations from Part 1 were correct.

## Part II - Induction

### A - Induction using a Bar Magnet

Your goal now is to verify Faraday's Law and Lenz's Law (see KJF2, Ch. 25.3, 25.4) of magnetic induction. For this part you will be making only qualitative observations of the current induced in a coil of wire by a changing magnetic flux (see Ch19 Sh2, ff. and KJF2, Figs. 25.10, ff). The magnetic flux is due to the magnetic field produced by the bar magnet whose north pole you have already determined in Part I. By moving that magnet in and out of a coil, you create a time-varying magnetic flux through it, and this induces the current in the coil.

Before you make any observations, set up a simple circuit to measure when a current is flowing through the coil. Connect the large coil to the terminals of a galvanometer (a current meter) as shown below. IMPORTANT: You must inspect your coil to find out how its wire is wound, that is, whether the upper or lower picture in the sketch represents how your particular coil is wound. The direction of the winding is important because for the rest of this lab you will use the right-hand-rule for induced current. To do this correctly you need to know which way the current travels around the coil. Part II of your worksheet shows two options for the large coil. You need to pick the one that corresponds to how your coil is wound and then complete the incomplete sketch on the worksheet by adding how your connections were made.

Figure 3 is just above here

The galvanometer indicates whether a current flows through it by the deflection of the needle on its scale. Deflection to the right means the current is going into (not out of) the red or positive terminal of the galvanometer. This is important to know because to use the right-hand-rule when you observe a deflection, you must know the direction of current flow. You will use the right-hand-rule for the magnetic field made by a current in a wire loop (Fig. 25.14 on p. 784 of KJF2) or solenoid coil (Figs. 24.15 and 24.16 on p. 785 of KJF2)and apply Lenz's Law to figure out in which sense the induced current flows through your coil (see KJF2, Ch. 25.3).

Inserting [or withdrawing] the bar magnet into [away from] the coil changes the magnetic flux through the coil. The result is an induced voltage (see Ch19 Sh2 and Figs. 25.12-25.14 on pp. 823-8244 of KJF2) across the coil that causes an induced current to flow through it. This current generates a magnetic field in the coil itself, the direction of which is given by the right-hand-rule for the magnetic field made by the induced current.

Note: Here the dominant contribution to the magnetic flux inside the coil is made by the magnetic field near and inside the bar magnet, indicated by the high density of field lines at either end of the bar. There are important differences between the magnetic field inside and outside a permanent magnet, and confusion and mistakes arise when trying to think about isolated north and south poles. (Though such “magnetic monopoles” have never been observed, there are theoretical reasons to believe they could exist. However, isolated electric monopoles, viz., positive and negative charges, do exist.) It makes more physical sense to describe magnetism in a permanent magnet as the result of small (atomic sized) current loops distributed throughout its magnetic material. The dipole field outside the bar magnet does go away from what we call north, loops around through space and then returns to the magnet at what we call south. It'ss useful, though still a bit perplexing, to compare a bar magnet to a solenoid electromagnet. Compare the “solenoid part” of the figure on p. 782 of KJF2 to Fig. 24.8 on p. 783 for a bar magnet. Note that the field lines outside the solenoid look like those outside the bar magnet, which invites you to think of the left-hand end of the solenoid coil as its “south” (S) end and the right-hand end as its “north” (N) end. Outside both the solenoid coil and the bar magnet, field lines loop around from the N pole to the S pole. However, note that inside the solenoid coil the field lines go from S to N. They must do this because the field lines are continuous because they could end only on isolated S or isolated N poles (the magnetic monopoles mentioned above). Contrast this with the case of electrostatics, where electric field lines begin on isolated + charges and end on isolated - charges, the electric monopoles. For now it's better not to think too deeply about the direction of the magnetic field inside a permanent magnet such as a bar magnet.

Insert the bar magnet slowly into the coil, left to right, north pole first, and observe the deflection of the galvanometer needle. Note whether the deflection stays at maximum or falls back to zero when you stop moving the magnet. Then, withdraw the magnet slowly from the coil, observing the direction and magnitude of the needle deflection. Repeat this insertion/withdrawal procedure a few times.

Now repeat this procedure with a significantly faster motion of the bar magnet.

Now apply Lenz's law to see if the direction of motion of the galvanometer needle in each case agrees with what you expect from your understanding of magnetic induction.

The form below will guide you through a set of questions from which you can deduce the direction the needle should have moved when you inserted and removed the bar magnet. When you click submit the computer will tell you whether your answers are correct based upon the connections you entered and the direction of the magnetization in the bar magnet. If some of the answers are marked as incorrect there are a few possible causes you should consider.

1. The wiring that you entered might be incorrect
2. You might have labeled the poles of the bar magnet incorrectly in Part I
3. You might have applied Lenz's law incorrectly
4. You might have applied the right hand rule for induction incorrectly

Hopefully by looking at which answers are incorrect you can work out which error you might have made and correct it until you get the right answer. (Of course, if you get it all right on the first go, you can move right on to the next part!)

Figure 4 is just above here

Which wire end was attached to terminal e? (a,b,c,d)

Which wire end was attached to terminal f? (a,b,c,d)

When inserting the bar magnet into the coil is the change in the magnetic field directed to the left or to the right?

When removing the bar magnet into the coil is the change in the magnetic field directed to the left or to the right?

Using Lenz's law, is the magnetic field induced in the coil to the left or right when inserting the bar magnet into the coil?

Using Lenz's law, is the magnetic field induced in the coil to the left or right when removing the bar magnet from the coil?

According to right hand rule does the current in the central part of the coil go as in diagram 1 or in diagram 2 when the magnet is inserted into the coil?

According to right hand rule does the current in the central part of the coil go as in diagram 1 or in diagram 2 when the magnet is removed from the coil

Which way, left or right, did the galvanometer needle move when you inserted the magnet?

Which way, left or right, did the galvanometer needle move when you removed the magnet?

Figure 5 is just above here

### B - Induction using an electromagnet

Make an electromagnet by passing a DC current through the smaller of the two solenoid coils. As before, use the right-hand-rule to find the magnetic field lines outside this small, tightly wound solenoid coil; notice that they are similar to the field lines outside the bar magnet. This part of the lab will be the same as Part IIA except that you replace the bar magnet with the electromagnet, as shown below.

Figure 6 is just below here

The large solenoid coil and the galvanometer will still be connected the same as in Part IIA. There is no need to change that part of the setup.

Connect the small coil to the DC power supply as shown in Fig. 6. As you did for the large coil, inspect your small coil carefully to get the correct winding for it. On Part IIB of the worksheet, choose the incomplete sketch of the coil with the correct winding as in your setup and draw dashed lines for the wire connections to the coil. Draw the directions of current flow on the wires of the small coil, taking into account how they are connected to the DC power supply. (Which end of the small coil is connected to the positive terminal of the power supply? to the negative terminal?) Use the right-hand-rule for a current loop (Ch18 Sh25-27 and Fig. 24.14 on p. 784 of KJF2) to determine the north pole of the electromagnet and label it in the sketch. Remember that outside a magnet the magnetic field lines go from the N pole to the S pole.

Place the small coil inside the large coil with the DC power off. Turn the knobs on the DC power supply to maximum (clockwise). Turn the power on and observe the needle carefully as it comes on. Note! The needle deflection is very small. If you can hardly see any needle deflection, insert the bar labeled steel into the small coil, which enhances the strength of the electromagnet. (Do you know why?) After the power has reached a maximum value, check whether the needle stays at maximum deflection or goes back to zero. Turn the power off and observe the needle carefully as it goes off. Do this a couple of times.

Now we will apply Lenz's law to see if the direction of motion agrees with what we expect from our understanding of magnetic induction. As before the form below will guide you through a set of questions from which you can deduce the direction the needle should have moved when you turned on and when you turned off the power supply. Insert your answers in the online form below.

Figure 7 is just below here

Which wire end of the small coil was attached to terminal e of the DC power supply? (a,b,c,d)

Which wire end of the small coil was attached to terminal f of the DC power supply? (a,b,c,d)

Which wire end of the large coil was attached to terminal e of the galvanometer? (a,b,c,d)

Which wire end of the large coil was attached to terminal f of the galvanometer? (a,b,c,d)

When the DC power is on does the current go round the coil like in diagram 3 or diagram 4?

When the DC power is turned on is the change in the magnetic field produced by the small coil directed to the left or to the right?

When the DC power is turned off is the change in the magnetic field produced by the small coil directed to the left or to the right?

Using Lenz's law, is the magnetic field induced in the large coil to the left or right when the DC power is turned on?

Using Lenz's law, is the magnetic field induced in the large coil to the left or right when the DC power is turned off?

According to right hand rule does the current in the central part of the large coil go like diagram 1 or 2 when the DC power is turned on?

According to right hand rule does the current in the central part of the large coil go like diagram 1 or 2 when the DC power is turned off?

Which way did the galvanometer needle move when you turned on the DC power?

Which way did the galvanometer needle move when you turned off the DC power?

## Part III: Induction using alternating current (AC) power

Your goal here is similar to that for PART II B, but now you energize your electromagnet (small solenoidal coil with $N_1$ windings of wire) with an AC voltage called $V_1$ below. This drives an AC current that creates a time-varying magnetic flux that induces an AC voltage, called $V_2$ below, across the large solenoidal coil (with $N_2$ windings of wire), all of it working like a transformer; see Ch19 Sh24-25 and KJF2, Ch. 26.2, pp. 855-858.

First, wire the large coil to Channel 2 of the oscilloscope in a way similar to how you previously wired it to the galvanometer. This will allow you to measure/observe the voltage $V_2$ induced across the large coil. If you inserted the steel rod for the previous part, before you do the measurements in Part III.

Next, wire the small coil to the AC power generator in a way similar to how you previously wired it to the DC power generator. Now you want to connect the small coil in parallel with Channel 1 of the oscilloscope. Connect the high voltage (red) terminal of the oscilloscope to the junction where the high voltage (red) terminal of the power supply meets the red wire leading to the small coil. Repeat this with the low voltage (black) terminals of the oscilloscope and power supply. This will allow you to measure/observe the voltage $V_1$ across this electromagnet. When you have done all this your set up should look like Fig. 8.

Figure 8 is just below here

Set the AC generator to produce a sine wave (push the button on the top right in) with frequency 1000 Hz (push the MULT 100 button in and set the Frequency to 10). Use MAIN OUT HI and turn the AMPLITUDE to maximum (all the way clockwise) if you are using the coils that have metal ends; if you are using the coils with the black plastic ends, set the AMPLITUDE about halfway toward maximum. Set the oscilloscope to

• VERT MODE to DUAL
• All push buttons (except the power button) out
• VOLTS/DIV of CH1 to ~ 50mV
• VOLTS/DIV of CH2 to ~ 0.2 V
• Both CH1 and CH2 inputs to AC
• TIME/DIV to ~ 0.2 ms
• COUPLING to AC
• SOURCE to CH1

Turn the TRIG LEVEL until you see a stationary picture of two sinewaves on the oscilloscope screen. Record the VOLTS/DIV settings of CH1 and CH2 and all your wire connections in the incomplete sketch shown on Part III of the worksheet; again show which case corresponds to actual windings of your small and large coils. Draw the observed voltage signals in the grid provided on your worksheet. Make sure you note which voltage signal is the induced one, $V_2$, from the large coil and which voltage signal $V_1$, is from the AC generator or across the small coil (you can switch the signals off simply by setting the input of that channel to GND temporarily). Label the signals on your graph and indicate the voltage and times scales of the boxes. Measure the peak-to-peak amplitude (from the minimum to the maximum) of your voltage signals with the oscilloscope (see Lab # 2) and record their values on the graph on your worksheet.

BE CAREFUL! When observing the signals from Channel 1 and Channel 2 remember that the signal for each channel has its own VOLT/DIV setting. Take this into account when labeling your sketch and when computing the amplitude of each wave. (Their TIME/DIV settings are the same.)

From the ratios of the voltages you can calculate the ratio of the number of turns in the two coils; see Ch19 Sh24-25 or KJF2, Ch. 26.2 and manipulate Eq. (26.15) on p. 856 to get $\Large \frac{N_{2}}{N_{1}} = \frac{V_{2}}{V_{1}}$.

Are the frequencies of the two sinewaves the same? Discuss with your TA whether they are or aren't and whether you observation is consistent with your expectation.