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Electromagnetism
Investigations
Autumn 2015
PDST Physics Support
Electromagnetism Investigations
ELECTROMAGNETISM
Investigations
Table of Contents
Magnetic effect of an electric current*
2
Force on a current-carrying conductor in a magnetic field*
6
Faraday’s law of electromagnetic induction*
7
Lenz’s law
9
Induction motor – Arago’s disc
11
Mutual induction
12
Transformer
14
Self-induction (back emf)
15
Appendix 1: Force on a current-carrying conductor
in a magnetic field
18
Appendix 2: Electromagnetic induction
19
Appendix 3: Lenz’s law
21
Appendix 4: Eddy currents in a copper plate
22
Appendix 5: Electromagnetic induction (with dataloggers)
23
Appendix 6: Pending changes to SI Base Units
24
*Denotes it is suitable for Junior Cycle and TY students
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Electromagnetism Investigations
INVESTIGATING THE MAGNETIC EFFECT OF AN
ELECTRIC CURRENT (Oersted’s Experiments)
In 1820 Oersted established a clear connection between electricity and magnetism. He
discovered that an electric current in a wire produced a magnetic effect.
Apparatus
6 V power supply, a large (e.g. an orienteering compass), a long lead.
Procedure
1. Place the lead over the compass so that it is parallel to the compass needle as shown
in Arrangement A.
2. Turn on the switch and allow a current to flow for 1 or 2 seconds.
3. Observe and record the action of the compass needle.
Arrangement A
4. Repeat the experiment only this time place the lead perpendicular to the compass
needle as in Arrangement B.
5. Observe and record the action of the compass needle this time.
Arrangement B
Issues to be explored
Explain how the needle moves in one arrangement but not the
other?

Repeat Arrangement A, i.e. let wire run parallel to the compass needle, only this time
place the compass above the wire.
What is the effect on the compass needle?

Repeat Arrangement A, i.e. let wire run parallel to the compass needle, only this time
change the direction of the current back and forth a number of times.
What is the effect on the compass needle?

Repeat Arrangement B i.e. let the wire run perpendicular to the compass needle, only
this time change the direction of the current back and forth.
What is the effect on the compass needle?
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Teacher Notes
The compass needle moves when the current is parallel to the needle direction and then
remains perpendicular to the wire.
It may give an initial rotation when the current is perpendicular to the compass needle but will
then remain perpendicularly aligned. We conclude that a magnetic field is induced by current
and that the magnetic field direction is perpendicular to the direction of the current.
We observed that the direction of the current affects the direction of the compass needle and
conclude that the direction of induced the magnetic field is determined by the current
direction.
Insert a resistor in the circuit for student use.
Extension: Investigating the direction and shape of the induced magnetic field.
Let the wire run perpendicularly through a solid stage e.g. a CD.
Now move a single plotting compass around the platform and see can you determine the
direction and shape of the magnetic field.
Current
Magnetic field
Note the circular
magnetic field
RIGHT HAND GRIP RULE
We can use what is known as the right hand grip rule to determine the direction of the
magnetic field, if we arrange our right hand as in the diagram, so that the thumb points in the
direction of positive conventional current then the fingers point in the direction of the
magnetic field.
RIGHT HAND GRIP RULE
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Electromagnetism Investigations
Issues to be explored
What shape would the magnetic field take if instead of a straight
wire we had a single loop of coil?
We can apply the right hand rule to each side of the coil, the
current goes up one side of the coil and down the other and we
see that inside the coil the direction of the field is the same due to
each side.
What would the shape of the magnetic field be if we had a coil of an increased number of
turns?
We can apply the right hand grip rule to each turn and we see that the direction of the field is
the same all along the inside of the coil.
The magnetic field generated by a current carrying long
straight coil is the same as the magnetic field of a bar
magnet so a coil of wire with a current flowing through it
could be used as a magnet, we call such a magnet an
electromagnet.
The electromagnet has the added advantage that it can be
turned on and off at will.
Applications of the magnetic effect of an electric current
include:
Motors, doorbells, electromagnetic cranes (for lifting scrap iron), magnetic hotel door card
keys, etc.
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WORKSHEET ON THE MAGNETIC EFFECT OF AN ELECTRIC
CURRENT - Teacher copy
1. When the wire is placed parallel to the north south direction of the plotting compass
does the needle move? ……..yes, because the magnetic field due to the wire is
perpendicular to the wire and much stronger than the earth’s magnetic field.
2. When the wire is placed perpendicular to the north south direction of the plotting
compass does the needle move?.........no, because the compass is already pointing in
the direction of the magnetic field due to the current.
3. When the wire is placed parallel to the north south direction of the plotting compass
and the direction of the current changes does the needle move? ……..yes, the compass
needle constantly changes direction as the current changes direction, this shows that
the direction of the current determines the direction of the field.
4. When the wire is placed perpendicular to the north south direction of the plotting
compass and the direction of the current changes does the needle move?.........yes, the
compass needle flips back and forth but the direction is always perpendicular to the
wire.
5. Let the wire run perpendicularly through a solid stage e.g. a CD
Now move a single plotting compass around the platform and see can you determine
the shape of the magnetic field?........yes, the field is clearly circular in shape around
the wire.
WORKSHEET ON THE MAGNETIC EFFECT OF AN ELECTRIC
CURRENT - Student copy
1. When the wire is placed parallel to the compass needle, how does the needle move?
2. When the wire is placed underneath the compass and parallel to the compass needle
how does the needle move?
3. When the wire is placed perpendicular to the compass needle, how does the needle
move? .
4. When the wire is placed parallel to the compass needle and the direction of the
current changes, how does the needle move? …………………………………….
5. If the wire is placed perpendicular to the compass needle and the direction
of the current changes does the needle move?....................................
6. Let the wire run vertically through a solid stage e.g. a CD
Now move a single plotting compass around the platform and see can you
determine the shape of the magnetic field?.
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Electromagnetism Investigations
INVESTIGATING THE FORCE ON A CURRENT- CARRYING
CONDUCTOR IN A MAGNETIC FIELD
Apparatus
6 V power supply, 10  resistor (5 W), aluminium foil, U-shaped alnico magnet.
6V
10 
S
N
U-shaped magnet
Procedure
1.
2.
3.
4.
5.
6.
Aluminium foil
Set up the apparatus as shown with the foil at right angles to the magnetic field.
Close the switch, or complete the circuit, and observe the aluminium foil.
Reverse the direction of the current flowing in the foil.
Observe and record what happens.
Reverse the direction of the magnetic field.
Observe and record what happens to the foil when a current flows.
Teacher Notes
See appendix 1.
The 10  resistor should be rated at 5 watts. Overheating will occur if a 0.25 W or 0.5 W
resistor is used.
The foil moves when a current flows through it. Reversing the direction of the current or
the magnetic field reverses the direction of the movement of the foil.
Conclusion
When a current flows in the foil, it experiences a force.
This causes it to move. The direction of the force can be
found using the second right hand rule.
(See: https://en.wikipedia.org/wiki/Lorentz_force )
Applications
This is the principle of operation of the electric motor, the moving coil meter and the
moving coil loudspeaker.
Sonometer connected to wave generator with a Ushaped magnet under the sonometer wire
Simple motor https://www.youtube.com/watch?v=zOdboRYf1hM
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Electromagnetism Investigations
INVESTIGATING THE FARADAY’S LAW OF
ELECTROMAGNETIC INDUCTION
Apparatus
Bar magnet, coil with 800 turns, galvanometer.
Magnet
Coil of wire
Galvanometer
Procedure
1.
2.
3.
4.
Set up the apparatus as shown.
Push the magnet into the coil and note the deflection of the galvanometer.
Pull the magnet out of the coil and note the deflection.
Repeat, moving the magnet with different speeds, and observe what happens to the
deflection (direction and size) on the galvanometer:
 As you move the magnet into the coil (a) quickly, (b) slowly
 If you change the direction of motion of the magnet
 If you turn the magnet around (poles swapped)
 If you use a coil with more (less) turns
 If you use a more powerful magnet (you may tape two magnets together, with
like poles side by side)
Issues to be explored
What energy conversions take place in this experiment?
Faraday’s Law of Electromagnetic Induction implies that to generate an e.m.f. in a coil
the magnetic flux through the coil must be changing. How is that done in this
experiment?
How do you make the flux change more rapidly?
If you double the number of turns in the coil does the e.m.f. double?
State Faraday’s Law of electromagnetic induction
To explain accurately what happens in this experiment the concept of magnetic flux is
used. Define what is meant by magnetic flux.
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A current will flow in the coil if there is a complete circuit. When a current flows in
the coil it behaves like a bar magnet. If the N pole of the magnet is entering the coil,
explain why, using the Law of Conservation of energy, the end of the coil facing the
magnet must also be a N pole.
Teacher Notes
Observation
The faster the magnet is moved, the greater the deflection of the galvanometer.
Conclusion
E = IR. The coil has a fixed resistance. An increase in current indicates a corresponding
increase in emf. The faster the motion of the magnet the greater the current indicated by the
galvanometer, which implies a greater emf induced.
 This may also be shown by using a stronger magnet or taping two magnets together,
with like poles side by side. The resulting increase in flux ( produces a greater
deflection.
 See appendix 2.
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INVESTIGATING LENZ’S LAW
Apparatus
Copper pipe, plastic pipe, stopwatch, strong neodymium magnet, piece of unmagnetised
neodymium or iron, (same size as magnet).
Magnet
Magnet
Copper pipe
Plastic pipe
Procedure/Observations
1. Drop the neodymium magnet down through the length of copper pipe held vertically
and note the duration of fall.
2. Repeat the same procedure using the plastic tube.
3. Compare the time taken for the magnet to fall through both tubes?
4. In which tube was the overall velocity of the magnet least?
5. In which direction does the force of gravity act?
6. In which direction is the force, causing the magnet to slow down, acting?
Further Discussion:
1. You have already learned about the nature of magnetic fields: they have magnitude and
direction. You also know that there is a magnetic field produced whenever there is an
electric current in, let’s say, a copper wire or loop. Do you think that the moving magnet
has induced some kind of electric current in the copper?
2. If there is an electric current induced in the copper, it should create a magnetic field. This
newly created magnetic field exerts a force on the falling magnet. Does it oppose the
motion of the magnet? (Hint: the magnet slows down).
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Teacher Notes
Observation
The time taken for the magnet to fall down through the copper tube is much greater than the
time taken for the magnet in the plastic tube or the piece of neodymium in either tube.
Explanation
The moving magnet induces an electric current in the copper. This current creates a magnetic
field that exerts a force to oppose the motion of the magnet and hence slows it down.
The copper and plastic pipes used are available from
plumbing suppliers.
As an alternative to copper pipe use an aluminium rail
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Electromagnetism Investigations
INVESTIGATING THE INDUCTION MOTOR – ARAGO’S DISC
Apparatus
Aluminium or copper disc (centre punched), strong magnet, pivot.
Rotating
magnet
Spinning spiral disc &
neodymium magnet
Disc
Pivot
Procedure
1. Place the disc on the pivot.
2. Move the magnet quickly in a circular motion above the rim of
the disc.
Issues to be explored
1. What do you observe happening to the disc as the magnet
rotates above it?
2. In what direction does the disc rotate?
3. Now move the magnet quickly in a circular motion above the
rim of the disc, in the opposite direction to that in step 2.
4. What do you notice about the rotation of the disc this time?
5. Why does this happen?
Alternatively use a soft
drinks can balanced on
the tip of a pencil,
supported by blu-tack &
neodymium magnet
Teacher Notes
The moving magnet induces a current in the disc. This current creates a magnetic field that
exerts a force to oppose the motion of the magnet. The magnet exerts an equal and opposite
force and the disc rotates. The relative motion between the magnet and the disc is reduced.
For more details visit: https://vimeo.com/20847392
A copper or aluminium calorimeter balanced on a point could also be used for this
demonstration.
This demonstration is from the Applied Electricity section of the syllabus
Applications
Induction motors are used in speedometers, tachometers and some electric clocks. They are
also used as large motors in factories as they do not have brushes, commutators, etc. to wear
out.
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Electromagnetism Investigations
INVESTIGATING MUTUAL INDUCTION
Apparatus
Coils of wire – 400 and 800 turns, galvanometer, soft iron core, 6 V battery.
6V
Procedure
1.
2.
3.
4.
5.
6.
Set up the apparatus as shown with the two coils side by side.
Connect one coil to the 6 V supply.
Close the switch – a deflection is seen on the galvanometer.
Open the switch – a larger deflection is observed.
Repeat moving the coils closer together and note the galvanometer deflection.
Repeat inserting a soft iron core into the coils. Move the coils closer together and note
the galvanometer deflection.
Teacher Notes
Each time the circuit is completed or broken, a deflection is observed on the galvanometer.
The deflection at the break is greater than at the make.
Conclusion
At the make and break of the circuit there is a change in the magnetic flux linking the coils
and so an emf is induced in the secondary coil. The break is faster than the make and so the
rate of change of flux is greater at the break. This creates a greater emf and so a larger current
is produced at the break of the circuit.
If both coils are mounted on a shared iron core a much greater deflection is obtained. This is
dΦ
because the magnetic flux Φ has been increased and, from Faraday’s law, E 
dt
6V
Iron
core
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INVESTIGATING MUTUAL INDUCTION CONTINUED
Apparatus
6 V a.c. power supply, coils of wire – 400 turns and 800 turns, soft iron core, two a.c.
voltmeters.
800
400
turns
turns
Vin
V
V
Vout
Procedure
1. Set up the apparatus as shown above.
2. Switch on the 6 V a.c. supply.
Issues to be explored
Record the readings obtained from each voltmeter?
What happens to the output reading Vout when the coils are
moved closer together?
Insert the U-shaped iron core and record Vout.
Complete the full core and record Vout.
What can you conclude?
Teacher Notes
The a.c. produces a constantly changing magnetic field. Hence an emf is continuously
induced in the other coil. The iron core increases the magnetic flux .
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INVESTIGATING THE TRANSFORMER
Iron core
Primary
coil Np
V
Vin
Secondary
coil Ns
V
Vout
Procedure
1.
2.
3.
4.
Set up the apparatus.
Read the voltages on both coils.
Read the number of turns on both coils, Np and Ns.
Switch the coils and repeat.
Results
Vin / V
Vout / V
Vin
Vout
Np
Ns
Teacher Notes
It is found that
Vin N P

Vout N s
Applications
Transporting energy/power, Mobile phone chargers, Televisions, Computers, Power stations.
For more visit: https://en.wikibooks.org/wiki/Basic_Electrical_Generation_and_Distribution
or visit: http://c21.phas.ubc.ca/article/transformers
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INVESTIGATING SELF-INDUCTION (BACK EMF)
(a) Apparatus
6 V a.c. power supply, coil of wire with 1200 turns, soft iron core, 6 V filament lamp.
Iron core
6V
Procedure
1.
2.
3.
4.
Connect the bulb, coil and a.c. supply in series.
Switch on the power supply and observe the lamp.
Insert the iron core into the coil and observe the lamp.
Explain your observations.
Teacher Notes
The a.c. produces a changing magnetic field in the coil. This induces an emf and hence a
current that opposes the applied current. The iron core increases the magnetic flux and hence
the induced opposing current is increased. The resultant current in the circuit is reduced and
the bulb becomes dimmer.
This is the principle on which large dimmer switches for stage lights in theatres operate.
If this circuit is set up using a d.c. power supply, no dimming occurs with the core in the coil
as there is no changing magnetic field.
This effect is best demonstrated by putting the coil on the completed transformer core. This
gives a much greater change in magnetic flux and so a larger opposing current.
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(b) Apparatus
1.5 V cell /Suitable d.c. power supply, electric motor/toy electric car, ammeter.
Motor
M
A
1.5 V
Procedure
1.
Connect the switched d.c. power supply, ammeter, and motor
in series.
2. Turn on the digital multimeter and set to dc A.
[Remember: Use the 'COM' and the 'A' ports].
**Any d.c milli-ammeter will suffice **
3. Switch on the power supply (to 1.5 V approx.)
4. What current is displayed on the digital multimeter (or your
milli-ammeter)?
5. Is the voltage displayed on your power supply? If so, make a note of its value.
* If the power supply does not display voltage or to check its accuracy – connect a
voltmeter in parallel with the power supply. *
6. Is the motor turning freely?
7. Place a finger on the rotating wheel to slow the rotations.
8. When the wheels slow down, do you notice any change on the digital ammeter
reading?
9. Why do you think this may have occurred?
10. Did the voltage value change?
11. Did you expect the voltage to change?
12. What conclusion can you draw from your investigation?
Teacher Notes
Explanation
When the coil of the motor is rotating in the magnetic field, a current is induced that opposes
the applied current. If the motor slows down, the rate of magnetic flux change is reduced.
This means that the induced e.m.f. is smaller. Therefore the induced opposing current is
reduced and hence the resultant current increases.
This is why many large motors have starter resistors incorporated. It also explains why
motors burn out if they cannot turn while the current is flowing. There is no opposing
induced e.m.f. so a larger current flows.
A scaled-electric car motor works well. Applying friction to the rotating wheels slows down
the motor and a noticeable increase in the resultant current is observed. This model of motor
can stimulate the interest of a pupil. It connects their past childhood with physics in a fun
way.
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(c) Apparatus
6 V battery, 90 V neon lamp/phase tester, coil with 1200 turns, soft iron core.
Iron core
90 V Neon lamp
6V
Procedure
1.
2.
3.
4.
5.
6.
7.
Connect the switch, coil and 6 V battery in series.
Connect the neon lamp/phase tester in parallel with coil.
Switch on the power supply
Close the switch and observe the neon lamp.
Open the switch and observe the neon lamp.
Record your observations.
Explain your observations.
Teacher Notes
Explanation
As the circuit is switched on or off, there is a changing magnetic field in the coil. This causes
an emf to be induced. With the large number of turns and the iron core, this emf is greater
than 90 V and so the lamp lights. The magnetic field is only changing when the circuit is
being switched on or off.
The flash is brighter on the break because the magnetic field takes longer to build up than to
collapse.
** The standard laboratory Neon lamp may also be replaced with a
common 220-240Vac Snap In Neon Red (pictured opposite). Connection
is via two 1/4" (6.35 mm) push on blade terminals separated by a plastic
insulator. Overall length is 33 mm and lens diameter is 12 mm.
Purchased in Maplin Stores - cost €2.
The only noticeable 'flicker of light' when using this lamp occurred when
the circuit was switched off, reinforcing that the magnetic field collapses
quicker than it builds and hence with this quicker change of magnetic field - a larger e.m.f. is
induced.
For more visit: https://www.youtube.com/watch?v=pKKsco9EgBQ
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Electromagnetism Investigations
Appendix 1
INVESTIGATING THE FORCE ON A CURRENT-CARRYING
CONDUCTOR IN A MAGNETIC FIELD
Apparatus
Signal generator, 10  (5 W) resistor, ammeter, U-shaped magnet, aluminium foil.
A
Signal
generator
U-shaped magnet
10 
Aluminium foil
Procedure
1. Set the signal generator at the square wave option.
2. The aluminium foil is connected in series with an ammeter and a high wattage 10 
protective resistor to the low impedance output of the signal generator as shown.
3. Ensure that the current does not exceed 0.4 A (or lower if indicated on the generator).
4. Set the frequency at 2 Hz and observe the foil.
5. Gradually increase the frequency. Observe the foil and listen.
6. Remove the magnet and observe.
Teacher Notes
Observation
The foil moves up and down. At frequencies >100 Hz, sound can be heard from the foil.
Explanation
When a current from the signal generator flows through the foil, it experiences a force. Since
the current is a.c. the direction of the force changes with the direction of the current and so the
foil moves up and down. At high frequencies the vibrating foil produces sound (as in the
moving-coil loudspeaker). If the magnet is removed, the foil does not experience a force and
so motion and sound disappear.
A small radio/walkman with an earphone can also be used. Set the signal generator at the
amplifier option
. The output from the earphone is fed into the amplifier of the signal
generator. The foil is connected to the low impedance output of the signal generator as
shown. When the radio is turned on, the sound can be heard from the vibrating foil.
A
Radio
Signal
generator
U-shaped magnet
10 
Aluminium foil
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Appendix 2
INVESTIGATING ELECTROMAGNETIC INDUCTION
Apparatus
Coil of wire (10 000 turns), red LED, green LED, magnet.
.
Magnet
Coil
of
wir
e
Procedure
1. Connect the LEDs to a coil of wire as shown.
2. Push the magnet into the coil and observe the LEDs.
3. Withdraw the magnet from the coil and observe the LEDs.
Issues to be explored
Why are both LEDs not bright at the same time?
Use Faraday’s Law to explain what happens
Use Faraday’s Law to explain why neither LED lights if the magnet is moving slowly
Would the number of turns in the coil make any difference?
Use Faraday’s Law to explain why a powerful neodymium magnet works best in this
experiment
Give two reasons why LEDs are more suitable in this experiment than small filament
torchlight bulbs
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Teacher Notes
Observation
As the magnet is moved into the coil one of the LEDs lights and as it is being withdrawn the
other LED lights.
Explanation
As the magnet moves in or out of the coil, the magnetic flux linking the coil changes. An emf
is induced in the coil and current flows in the circuit. This current lights the LED.
The alternate lighting of the red and green LEDs arises because of Lenz’s law. The induced
current opposes the change causing it. The current flows in the opposite direction when the
motion of the magnet is reversed. Since the LEDs must be forward biased to conduct, only
one can light at any one time.
Conclusion
A changing magnetic flux in a coil induces an emf.
The direction of the current depends on the direction of the motion of the magnet.
Alternatively insert a neodymium magnet into a plastic cylindrical container surrounded by a
coil of about 500 turns connected to an LED. Shake the container and observe the LED.
Issues to be explored
What happens to the LED as the magnet moves up and down the tube?
Use Faraday’s Law to explain what happens
Use Faraday’s Law to explain why the LED does not light if the magnet is moving
slowly
Would the number of turns in the coil make any difference?
Use Faraday’s Law to explain why a powerful neodymium magnet works best in this
experiment
Explain why an LED is more suitable in this experiment than a small filament
torchlight bulb
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Appendix 3
INVESTIGATING LENZ’S LAW
Apparatus
Aluminium ring, magnet, thread, retort stand.
Thread
Thread
Aluminium ring
Magnet
Procedure
1. Suspend the ring from the retort stand, using two pieces of
thread for stability.
2. Move one end of the bar magnet towards and into the ring.
3. Observe and record what happens to the ring.
4. Hold the magnet in the ring and quickly pull it away.
5. Observe and record what happens to the ring.
6. Explain your observations.
Teacher Notes
Observation
When the magnet moves, the ring responds by moving in the same direction.
Explanation
The moving magnet induces a current in the ring. This current creates a magnetic field that
exerts a force to oppose the motion of the magnet. The magnet exerts an equal and opposite
force on the ring and so the ring moves as observed.
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Appendix 4
INVESTIGATING EDDY CURRENTS IN A COPPER PLATE
Apparatus
Use a neodymium magnet as the bob of a pendulum.
Procedure
Suspend two such pendulums from a metre stick clamped in a horizontal position.
Ensure that both pendulums can swing freely with a clearance of 3 or 4 mm above a table.
Place a copper plate under one pendulum. Cover it and the rest of the table with a sheet of
card before any student sees the apparatus.
Set the pendulums swinging at the same instant with the same initial amplitude.
Issues to be explored
Is there a difference between the pendulums in terms of:
(a) the duration of the oscillations and,
(b) the number of oscillations?
Why?
Teacher Notes:
Eddy currents are induced in the copper plate which by Lenz’s law opposes the oscillation of
the magnet that passes over it. Hence this magnet comes to rest much sooner than the one that
doesn’t have a copper plate under it.
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Appendix 5
Electromagnetic Induction (with dataloggers)
Apparatus: 100 milli-Amp “current sensor”, datalogger, computer, coil, magnet
Connect a 100 milli-Amp “current sensor” to a datalogger
and to a coil.
Connect the datalogger to a laptop computer.
Launch the graphing software and set it to record with a
high sampling rate for a second after the trigger value is
reached.
Save the graph.
A single pulse of alternating current is generated when a
magnet falls once through a coil as shown.
A soft landing for the magnet needs to be provided.
Repeated hard blows to a magnet will reduce its strength.
A typical outcome is shown opposite.
Current is on vertical axis, time on horizontal.
Use the various analysis tools of the software to answer the
following questions:
Issues to be explored
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
What is the height of the peak in milli-Amps?
What is the depth of the trough in milli-Amps?
Which is greater and why?
What does the graph tell you about
1. The magnitude of the e.m.f. (voltage) as the magnet enters and leaves the coil
2. The direction of the e.m.f. as magnet enters and leaves the coil
Use the Laws of Electromagnetic Induction to explain 1. and 2.
What is the duration (in milli-seconds) of the peak?
What is the duration (in milli-seconds) of the trough?
Which is longer and why?
Calculate the area enclosed by the peak and that enclosed by the trough.
Which is bigger and why?
What physical quantity is represented by the area of the peak?
Extension activities
1. If the coil is replaced with one which has a different number of turns of wire, what is the
effect, if any?
2. If the magnet is replaced with a stronger magnet, what is the effect, if any?
Teacher Notes:
Answers: (c ) depth of trough is greater, because magnet is accelerating and so greater rate
of change of magnetic flux. (f) duration of peak is greater, because magnet accelerating and
so the trough is completed quicker. (1.) bigger pulse of current. (2.) bigger pulse of current.
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PDST Physics Support
Electromagnetism Investigations
Appendix 6
Pending changes to SI Base Units
A subcommittee of the International Committee for Weights and Measures (CIPM) has
proposed revised formal definitions of the SI base units, which are being examined by the
CIPM and which will likely be adopted at the 26th General Conference on Weights and
Measures in the autumn of 2018. Below are some of the most common.
The second
Current definition:
The second is the duration of 9 192 631 770 periods of the radiation corresponding to the
transition between the two hyperfine levels of the ground state of the caesium-133 atom.
Proposed definition:
The second, s, is the unit of time; its magnitude is set by fixing the numerical value of the
ground state hyperfine splitting frequency of the caesium-133 atom, at rest and at a
temperature of 0 K, to be equal to exactly 9 192 631 770 when it is expressed in the unit s−1,
which is equal to Hz.
The metre
Current definition:
The metre is the length of the path travelled by light in vacuum during a time interval of
1/299792458 of a second.
Proposed definition:
The metre, m, is the unit of length; its magnitude is set by fixing the numerical value of the
speed of light in vacuum to be equal to exactly 299 792 458 when it is expressed in the unit
m·s−1.
The kilogram
Current definition:
The kilogram is the unit of mass; it is equal to the mass of the international
prototype of the kilogram.
Proposed definition:
The kilogram, kg, is the unit of mass; its magnitude is set by fixing the
numerical value of the Planck constant to be equal to exactly 6.62606X×10−34
when it is expressed in the unit s−1·m2·kg, which is equal to J·s.
Two such spheres have been constructed, at a cost of $3.2 million each
The ampere
Current definition:
The ampere is that constant current which, if maintained in two straight parallel conductors of
infinite length, of negligible circular cross-section, and placed 1 m apart in vacuum, would
produce between these conductors a force equal to 2×10−7 newton per metre of length.
Proposed definition:
The ampere, A, is the unit of electric current; its magnitude is set by fixing the numerical
value of the elementary charge to be equal to exactly 1.60217X×10−19 when it is expressed in
the unit A·s, which is equal to C.
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