PHYSICS 122 Lab EXPERIMENT NO. 4
MAGNETIC FORCE AND INDUCTION
The purpose of this laboratory is in Part I to observe the magnetic force on moving
electrons due to the magnetic field from a bar magnet. The purpose of Part II is to observe the
induction of a voltage in a coil by the change of the magnetic flux through the coil. In Part III the
voltage induced by an alternating current (AC) is observed. Two coils are arranged similar to a
transformer.
Equipment:
1 oscilloscope
1 bar magnet
1 small coil
1 large coil
1 DC power supply
1 galvanometer
1 AC voltage generator
AC Voltage Generator
Galvanometer
Small Coil
Oscilloscope
DC Power Supply
Large Coil
Wires
Bar Magnet and a similarly looking steel bar
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PART I: MAGNETIC FORCE
In this part your goal is to determine the magnetic north and south poles of the bar magnet by
using its magnetic field to deflect moving electrons (the beam which “paints” the graph on the
oscilloscope screen). You use the right-hand-rule for the magnetic force on a moving charge and
observe the deflection of the electron beam. When using the right-hand-rule remember that the
force equation is F = q v x B and you use the right-hand-rule to determine the direction of the
vector v x B and that does not include the sign of the charge. Since we are dealing with electrons
here, q is negative and will therefore need to reverse the direction of the vector that you found
using the right hand rule. For example if the beam deflected left and we know that the electrons
are moving out of the screen and so that would say that the magnetic field is in the upward
direction, but because of the electron’s negative charge, the magnetic field is actually moving
down.
On a magnet, the magnetic field points out of the north pole and into the south pole as
shown below.
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
(no other button should matter)
Turn the oscilloscope power on and adjust the vertical position button (POS) for CH2 such that
you see a stationary spot on the screen at the center. This spot is caused by electrons hitting the
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fluorescent screen which lights up. Be careful to turn down the INTENSITY control so that the
spot is not too bright (i.e., no "halo").
a) Bring the bar magnet oriented vertically from the front close to the screen of the
oscilloscope as shown in the sketch a) below. Make sure you note which of the ends of the
magnet is on the top. Mark it with a pen (place a dot or…). Whenever you label this bar magnet
in your lab report include that mark in your sketch. Watch out! You may find bar magnets
already labeled with “N” and “S”. You CANNOT rely on this being correct! Since you have to
find the north pole and south pole of the magnet, you need to know how it was oriented when
you observed the deflection and applied the right hand rule. Repeat a couple of times. Observe
the deflection of the spot and record your observations in your lab book.
b) Turn the magnet by 1800 , repeat step a) and record your observations in your lab book.
c) Now turn the magnet in the horizontal position and repeat the above. Make sure you note
where the marked end of the magnet is, on the left or on the right. Record your observations in
your lab book.
Analysis:
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For the 3 orientations in steps a) to c) apply the right hand rule to determine the north pole
of the magnet.
Q1: Are your determinations of which end of the magnet is the north pole consistent for the
3 orientations?
You have now a bar magnet with the north and south poles determined. However, in Part
II always include your label “N” for the north pole of your bar magnet plus the comment where
your original mark (the dot or… you marked on the bar above) on the bar magnet is. This allows
you, during the write up of your report, to revise your determination of the north pole of the bar
magnet in case you did it wrong while in a hurry in class.
PART II
A. INDUCTION USING A BAR MAGNET:
Your goal is to verify Faraday’s Law and Lenz’ Law of magnetic induction qualitatively
only.
In this part of the experiment the voltage induced in a coil of wire by a changing
magnetic flux will be observed. The magnetic flux is due to the magnetic field made by the bar
magnet for which you have determined the north pole in Part I. You connect the large coil to the
terminals of a galvanometer (a current meter) as shown below. You have to inspect your coil and
find out how the wire is wound so that you can use the right-hand-rule to determine the direction
of the induced current.
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Galvanometer
Bar Magnet
Large Coil
The galvanometer indicates whether a current flows through it by deflecting the needle
on the scale. The deflection is to the right when the current goes into, and not out of, the red or
positive terminal of the galvanometer. This is important to know when you observe a deflection
and confirm it by using the right-hand-rule for the magnetic field made by a current in a wire
loop or coil, and by applying Lenz’ Law to figure out in which sense the induced current flows
through your coil.
Procedure:
You insert the bar magnet into the coil and thus change the magnetic flux through the
coil. The result is an induced voltage across the coil which causes an induced current to flow
through the coil. This current generates an induced magnetic field the direction of which is given
by the right-hand-rule for the magnetic field made by the induced current in the coil.
Insert the bar magnet slowly into the coil and observe the needle deflection. Note whether
the needle deflection stays at maximum or falls back to zero when the magnet motion ceases.
Then withdraw the magnet slowly from the coil and observe the needle deflection. Note the
direction of the needle deflection. Do it a couple of times.
Repeat this procedure with a fast motion of the bar magnet.
Record your observations into your lab book and sketch the process labeling the north
pole of the bar magnet (determined in Part I) and the direction that you move the magnet relative
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to the coil, and also the direction of the induced magnetic field from the coil and the direction of
the induced current along the coil with arrows on the top and bottom of the coil. Draw this for
both moving the magnet into and out of the coil.
Analysis:
Q2: Does the induced current stay at its maximum when the motion of the bar magnet
ceases or does it return to zero? Explain.
Q3: How does the magnitude of the induced current (the needle deflection) depend on the
speed of the magnet motion when inserting (withdrawing)? Explain.
B. INDUCTION USING AN ELECTRO-MAGNET:
You make an “electro-magnet” by passing a DC current through the smaller coil of the
induction set. The small coil with a current through it is a magnet with a north and a south pole
like the bar magnet. You simply replace the bar magnet by the electro – magnet as shown below.
Galvanometer
DC Power Supply
Large Coil
Small Coil
Procedure:
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Connect the small coil to the DC power supply as shown. Turn the knobs on the DC power
supply to maximum (clockwise). Turn the power on, insert the small coil into the large one and
observe the needle. 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
electro magnet. Check whether the needle stays at maximum deflection or goes back down to
zero. Turn the power off and observe the needle. Do this a couple of times.
Record your observations in your lab book.
Analysis:
Q4: Does the induced current stay at its maximum after the switch has been thrown or does
it return to zero? Explain.
Q5: Which end of the small coil was the north-pole? Figure this out by comparing the
direction of the deflection of the needle on the galvanometer.
Write-Up
Your lab report should include:
1) Answers to questions 1-5
2) Sketches of putting the magnet into and out of the large coil
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