Electromagnetism Investigations - PDST · electromagnet. The electromagnet has the added advantage that it can be turned on and off at will. Applications of the magnetic effect of

Electromagnetism

Investigations

Autumn 2015

PDST Physics Support Electromagnetism Investigations

Page 1

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 18

in a magnetic field

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


Page 2

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.

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.

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.


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.


Arrangement A

Arrangement B


Page 3

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.

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.

Current

Magnetic field

Note the circular

magnetic field

RIGHT HAND GRIP RULE


Page 4

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.


Page 5

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?.


Page 6

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.

Procedure 1. Set up the apparatus as shown with the foil at right angles to the magnetic field.

2. Close the switch, or complete the circuit, and observe the aluminium foil.

3. Reverse the direction of the current flowing in the foil.

4. Observe and record what happens.

5. Reverse the direction of the magnetic field.

6. 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

S N

10

U-shaped magnet

Aluminium foil

6 V

https://en.wikipedia.org/wiki/Lorentz_force

https://www.youtube.com/watch?v=zOdboRYf1hM


Page 7

INVESTIGATING THE FARADAY’S LAW OF ELECTROMAGNETIC INDUCTION

Apparatus Bar magnet, coil with 800 turns, galvanometer.

Procedure 1. Set up the apparatus as shown.

2. Push the magnet into the coil and note the deflection of the galvanometer.

3. Pull the magnet out of the coil and note the deflection.

4. 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.

Galvanometer

Magnet

Coil of wire


Page 8

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.


Page 9

INVESTIGATING LENZ’S LAW

Apparatus Copper pipe, plastic pipe, stopwatch, strong neodymium magnet, piece of unmagnetised

neodymium or iron, (same size as magnet).

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).

Magnet

Copper pipe Plastic pipe

Magnet


Page 10

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


Page 11

Alternatively use a soft

drinks can balanced on

the tip of a pencil,

supported by blu-tack &

neodymium magnet

INVESTIGATING THE INDUCTION MOTOR – ARAGO’S DISC

Apparatus Aluminium or copper disc (centre punched), strong magnet, 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?

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.

Disc

Rotating

magnet

Pivot

Spinning spiral disc &

neodymium magnet

https://vimeo.com/20847392


Page 12

INVESTIGATING MUTUAL INDUCTION Apparatus Coils of wire – 400 and 800 turns, galvanometer, soft iron core, 6 V battery.

Procedure

1. Set up the apparatus as shown with the two coils side by side.

2. Connect one coil to the 6 V supply.

3. Close the switch – a deflection is seen on the galvanometer.

4. Open the switch – a larger deflection is observed.

5. Repeat moving the coils closer together and note the galvanometer deflection.

6. 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

because the magnetic flux Φ has been increased and, from Faraday’s law, dt

dΦE

6 V

Iron

core

6 V


Page 13

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.

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 .

Vin V

800

turns

V Vout

400

turns


Page 14

INVESTIGATING THE TRANSFORMER

Procedure

1. Set up the apparatus.

2. Read the voltages on both coils.

3. Read the number of turns on both coils, Np and Ns.

4. Switch the coils and repeat.

Results

Teacher Notes

It is found that s

P

out

in

N

N

V

V

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

Vin / V Vout / V

out

in

V

V

s

p

N

N

Iron core

Primary

coil Np

Secondary

coil Ns

Vin V V Vout

https://en.wikibooks.org/wiki/Basic_Electrical_Generation_and_Distribution

http://c21.phas.ubc.ca/article/transformers


Page 15

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.

Procedure 1. Connect the bulb, coil and a.c. supply in series.

2. Switch on the power supply and observe the lamp.

3. Insert the iron core into the coil and observe the lamp.

4. 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.

6 V

Iron core


Page 16

(b) Apparatus 1.5 V cell /Suitable d.c. power supply, electric motor/toy electric car, ammeter.

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.

1.5 V

Motor

M

A


Page 17

(c) Apparatus

6 V battery, 90 V neon lamp/phase tester, coil with 1200 turns, soft iron core.

Procedure

1. Connect the switch, coil and 6 V battery in series.

2. Connect the neon lamp/phase tester in parallel with coil.

3. Switch on the power supply

4. Close the switch and observe the neon lamp.

5. Open the switch and observe the neon lamp.

6. Record 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

90 V Neon lamp

Iron core

6 V

https://www.youtube.com/watch?v=pKKsco9EgBQ


Page 18

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.

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.


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.

10

U-shaped magnet

Aluminium foil

Signal

generator

A

10

U-shaped magnet

Aluminium foil

Signal

generator Radio

A


Page 19

Appendix 2

INVESTIGATING ELECTROMAGNETIC INDUCTION Apparatus Coil of wire (10 000 turns), red LED, green LED, magnet.

.

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.


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

Magnet

Coil

of

wir

e


Page 20

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.


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


Page 21

Appendix 3

INVESTIGATING LENZ’S LAW Apparatus Aluminium ring, magnet, thread, retort stand.

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.



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.

Thread

Aluminium ring

Thread

Magnet


Page 22

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.


Page 23

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:


(a) What is the height of the peak in milli-Amps?

(b) What is the depth of the trough in milli-Amps?

(c) Which is greater and why?

(d) 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.

(e) What is the duration (in milli-seconds) of the peak?

(f) What is the duration (in milli-seconds) of the trough?

(g) Which is longer and why?

(h) Calculate the area enclosed by the peak and that enclosed by the trough.

(i) Which is bigger and why?

(j) 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.


Page 24

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.

https://en.wikipedia.org/wiki/CIPM

https://en.wikipedia.org/wiki/SI_base_units

https://en.wikipedia.org/wiki/CIPM

https://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measures

https://en.wikipedia.org/wiki/General_Conference_on_Weights_and_Measures

https://en.wikipedia.org/wiki/Hyperfine_structure

https://en.wikipedia.org/wiki/Ground_state

https://en.wikipedia.org/wiki/Ground_state

https://en.wikipedia.org/wiki/Caesium-133

https://en.wikipedia.org/wiki/Hz

https://en.wikipedia.org/wiki/Electric_current

https://en.wikipedia.org/wiki/Elementary_charge

https://en.wikipedia.org/wiki/Coulomb

Documents

Electromagnetism Investigations - PDST · electromagnet. The electromagnet has the added advantage that it can be turned on and off at will. Applications of the magnetic effect of