Chapter 15 – Electromagnetic machines
Lesson
15.1
An
electromagnetic
world
(14 lessons including test)
Content
An introductory circus of electromagnetic machines as a qualitative introduction.
Student experiments to show the action of the transformer, magnetic flux from a
coil and induced emf = rate of change of flux linked.
Useful links:
http://www.magnet.fsu.edu/mediacenter/features/meetthemagnets/multishot.html
Click on Overview Video in the bottom right hand corner.
http://phet.colorado.edu/index.php
Find 'Generator' and open the transformer tab, then click on ac.
 Review GCSE work on electromagnetism.
 Know that electric currents are encircled by loops of magnetic flux.
 Draw lines of flux around current-carrying wires and coils of wire.
 Use the right hand grip rule to determine the direction of flux due to a
current-carrying coil.
 Know the meaning of the term flux linkage.
 Know that flux paths tend to be as short and straight as possible.
 Appreciate the vast range of devices making use of electromagnetic
effects.
Lesson 1: GCSE work on electromagnetic effects could be reviewed for
homework in advance of this lesson. Begin by investigating magnetic field
shapes by doing some of the experiments from Activity 40E. Students should
practice drawing field patterns. The question sets are useful as a way of linking
with prior GCSE work. Students should be clear about the concept of flux loops
and current loops being linked, with field direction determined by the right hand
grip rule (p138), and also the tendency of flux paths to be as short and straight
as possible (p138).
Students should then do Activity 10E to get an appreciation of the vast range of
appliances that make use of electromagnetic machines.
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A. M. James, Matthew Arnold School, Oxford
Activities
Homework
Activity
40E
‘Magnetic
field
shapes seen as
flux patterns’
Activity
60E
‘Factors affecting
magnetic flux n a
coil’
Activity
10E
‘Commercial
machines’ and see
also
DM 180S ‘Large
electromagnetic
machines’ and
Qs 10W ‘Magnetism
reminders’
Qs 20W ‘Magnetic flux’
Qs 30S ‘Magnetic
circuits’
Qs 40S ‘Sketching flux
patterns’
Reading 10T ‘People
and electromagnetism:
the discoverers’
1
Lesson
Content
Activities
DM
290S
catalogue
motors’
Homework
‘A
of


Investigate electromagnetic induction experimentally.
Explain observations in terms of induction of an e.m.f. when flux linkage
is changing, with the magnitude of the e.m.f. being proportional to the
rate at which flux linkage changes.
 Know that the e.m.f. induced opposes the change producing it.
Lesson 2: Students investigate electromagnetic induction by doing Activities
30E and 70E or alternatively equivalent GCSE experiments. Use the C cores
and insulated wire. Oscilloscopes can be used in place of microvoltmeters, and
in the absence of lots of signal generators use the a.c. output of power packs.
Key points to stress are: (1) that a continuous loop of iron linking a primary to a
secondary coil of wire will maximize the flux linkage; (2) an e.m.f. is induced only
when the flux linkage is changing, with size proportional to rate of change; and
(3) the e.m.f. opposes the change producing it. Challenge – who can make the
biggest voltage?
 Know the meaning of the terms flux, flux density, flux linkage.
 Recall and use the equation B = Φ/A in calculations involving flux and
flux density.
 Explain electromagnetic induction using the equation ε = -NdΦ/dt.
 Use the equation ε = -NdΦ/dt to sketch and interpret graphs showing
the variation of current, flux and induced e.m.f. with time.
 Use the equation ε = -NdΦ/dt to do calculations involving induced e.m.f.
Lesson 3: Faraday’s law. Define the terms flux, flux density and flux linkage,
noting that flux and flux density are related through B = Φ/A. Students are often
unsure of the difference between flux and flux density: a diagram showing the
flux lines in a transformer core which has a narrow section is a useful way to
illustrate the difference.
Introduce the equation for electromagnetic induction, ε = -NdΦ/dt, and discuss
the experimental observations of the last lesson in the context of this equation.
To reinforce, do the following demonstration: demonstration transformer coil
connected to the input of the storage oscilloscope, dropping a bar magnet
through the coil from a fixed height and observe the shape and height of the
trace obtained. Vary coil to vary N, and vary release height to vary dΦ/dt.
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A. M. James, Matthew Arnold School, Oxford
Activity
30E
‘Faraday’s law’
Activity
70E
‘Investigating
electromagnetic
induction’
Qs 60S ‘Changes in
flux linkage’
Induction
demonstration with
storage
scope,
transformer coils
and bar magnet.
Activity
90S
‘Building
up
a
model
of
electromagnetic
induction’
Activity
180S
‘Changing
flux
Qs 150S ‘Flux or flux
linkage?’
Qs 80S ‘Rates of
change’
Qs 90S ‘Bugging’
Qs 170S ‘Graphs of
changing
flux
and
e.m.f.’ very good for
exam practice
Reading 20T ‘Michael
Faraday’s vision’
2
Lesson
Content
Activity 180S is useful to illustrate the different ways in which flux linkage can be
changed.
Do some examples from exam questions to show how the equation is used.
Graphical questions where the flux-time graph must be sketched, given the emftime graph (or vice versa) are popular, as are questions where the peak emf
must be estimated by estimating dΦ/dt, most simply as change in flux / ¼ x
period.
 Explain the operation of a transformer in terms of Faraday’s law, the
equation ε = -NdΦ/dt, and the confinement of the flux loop to the
magnetic circuit defined by the iron core.
 Explain the origin of the transformer equation Vs/Vp = Ns/Np.
Lesson 4: Faraday’ law and transformers. Recap transformer work from GCSE.
Students should do Activity 100E to re-familiarise themselves with Vs/Vp = Ns/Np.
Discuss how this equation arises (p141) from the fact that the same flux links
both coils, so dΦ/dt is the same for both, assuming that primary and secondary
resistances are low. Reinforce understanding by having students do Activity
120S. Discuss the implications of stepping up voltage on the current, and
discuss use of step-up and step-down transformers in electrical energy
transmission. Demonstrate induction heating/welding using Activity 130D.
Activities
linkage’
Homework
FPP June 07 Q4,
Q12; Jan. 08 Q7,
10; Jan. 07 Q3;
June 05 Q3
Activity
100E
‘Model
transformers’ class
practical
Activity
110P
‘Building
up
a
transformer’
Activity
120S
‘Modelling
transformers’
Activity
130D
‘Demountable
transformer’
Qs
100S
‘Transformers’
Qs 110S ‘The circuit
breaker’
FPP
G495
Specimen Q8; Jan.
07 Q10; June 04
Q1





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Appreciate the correspondence between electric and magnetic circuits.
Know the meaning of the terms permeance and permeability.
Recall and use the equation Λ = μA/l in making magnetic circuit
calculations.
Know that current-turns drive flux in a magnetic circuit as voltage drives
current in an electric one.
Calculate the number of current-turns required to produce a given flux,
A. M. James, Matthew Arnold School, Oxford
3
Lesson
Content
using the equations Λ = μA/l, B = Φ/A and Φ = ΛNI.
 Know and explain the design features of a transformer that optimize its
performance.
Lesson 5: Transformer design. Begin by revising basic electric circuits in terms
of conductance = current/voltage, getting students to predict effects on current of
changing conductance and voltage. Introduce the magnetic circuit, discussing
the correspondence between electric and magnetic circuits, using p143 and a
comparison table (see below), and introducing the terms permeance and
permeability, and the equation Λ = μA/l. It is a useful exercise to get students to
consider a magnetic circuit modified by introducing a “pinch” (narrowing) of the
core, and explain the effect of this modification on the permeance, resulting flux
and flux density.
Activities
Homework
Qs 2-3 p146
Q2 p162
Qs
70S
‘Electromagnetism’
Reading
30T
‘Transformers:
designed
for
a
purpose’
FPP June 08 Q4,
Q9; June 06 Q12
Discuss the important features of transformer design (p144-145) and how to
calculate flux required from flux = current-turns x permeance. Note that high
permeance is desirable in the same way as high conductance in electric circuits.
Question 2-3 on p146 and question 2 on p162 give useful practice in using the
equations. Discuss losses in transformers due to eddy currents (p145) and how
they can be minimized by laminations. It is helpful to draw the path of an
induced current loop and a flux loop on a diagram of a transformer core to show
how the laminations work.
Drawing a labeled diagram or poster of transformer optimized design is a useful
exercise to conclude this section.
Electric circuit
Magnetic circuit
Current I
Flux Φ
Conductance G
Permeance Λ
Conductivity σ
Permeability μ
Emf V
Current turns NI
I = GV
Φ = ΛNI
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A. M. James, Matthew Arnold School, Oxford
4
Lesson
Content
G
A
L

Activities
Homework
Activity
20E
‘Introducing eddy
currents’
Activity
220D
‘Jumping ring’
Qs 50S ‘Magnet down
a tube’
Qs
120S
‘Eddy
currents and Lenz’s
law’
Qs student book p146
Qs 130X ‘Explaining
with induction’
Activity
170D
‘Rotating magnetic
fields’ good
Activity
180S
‘Changing
flux
Qs 140X ‘A bicycle
speedometer’
Qs
160X
‘The
geophone’
Qs 150S ‘Flux or flux
A
L

15.2
Generators and
motors
Know the meaning of the term eddy currents, and explain how they arise
in accordance with Lenz’s law.
 Explain how to minimize eddy currents in transformers.
Lesson 6: Lenz’s law and eddy currents. Having dealt with eddy currents briefly
in the context of transformer losses in the last lesson, now deal with Lenz’s law
more formally. Do Activity 20E, as demonstration or circus, explaining the results
in terms of an e.m.f., and hence currents, being induced that give rise to a
magnetic field that opposes motion. It is helpful to consider the sign of the poles
induced when the plate in Activity 20E is at different points in its swing. You can
also show how the eddy currents are much reduced by cutting slots in the plate:
it will be much less quickly damped, as eddy currents do not circulate so easily.
A magnet dropped down a long bit of copper pipe is particularly eye-catching.
Activity 220D on the jumping ring is another illustration of eddy currents in
action.
Class experiments to show design and operation of dynamo and motor.
Discussion focused on designs of real machines.

Explain the operation of a dynamo in terms of changing flux linkage,
Faraday’s law and the tendency of flux lines to shorten and straighten.
 Sketch and interpret graphs showing the variation with time of current,
flux and induced e.m.f. for dynamos and generators.
 Explain the operation of an a.c. generator (alternator), noting how it
differs from a simple dynamo.
 Know and explain the design features that maximize the power output
from a generator.
 Explain the operation of a three-phase generator.
Lesson 7-8: Demonstrate dynamo and Activity 170D to show the effect of a
rotating magnetic field on a nearby coil. When discussing, relate the output on
the oscilloscope to the movement of the magnet, noting when the rate of cutting
flux is a maximum. Students can now do Activity 180S to explore different ways
of changing the flux to induce an e.m.f. Introduce a.c. generator (alternator),
stressing similarities to transformer (p148), and note that it differs from the
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A. M. James, Matthew Arnold School, Oxford
5
Lesson
Content
simple dynamo in having a rotating d.c. electromagnet rather than a permanent
magnet. The images in Activity 190E, or a real demonstration alternator, will help
to make the design clear.
Discuss the design of large alternators (p149) and three-phase generators
(p149-150). With three-phase, make clear that the induced voltage in a
particular set of coils peaks when a pole of the rotating magnet sweeps past the
coil.
Students should be able to draw graphs of the variation of current, flux and
induced e.m.f. Use Qs 170S and past exam questions for practice.
Activities
linkage’
Activity
190E
‘Examining
real
dynamos
and
alternators’
Activity 200E ‘A
three
phase
generator’
Activity
210S
‘Building up an
alternator’
Homework
linkage?’
Qs 170S ‘Graphs of
changing
flux
and
e.m.f.’
Qs 180S ‘Alternating
current generators’
Qs from past papers
on graphs
FPP June 08 Q2;
June 06 Q8; June
04 Q9

Explain the operation of a rotating field motor using a permanent magnet
rotor.
 Explain the operation of an induction motor in terms of either eddy
currents/Lenz’s law, or in terms of attraction between opposite poles, or
in terms of flux lines tending to shorten and straighten.
 Explain the operation of a shaded pole motor.
Lesson 9: Induction motors. Discuss the rotating field motor described on p151,
noting that it has a very simple design, relying on the attraction between the
poles of the rotating field and those of a permanent magnet rotor. Activity 230S
very clearly illustrates how rotating flux may be generated from alternating fluxes
differing in phase. It may be helpful to draw a complete flux loop passing through
the rotor magnet poles and the stator to show how the tendency of flux lines to
shorten and straighten gives rise to rotation.
If not done so already, demonstrate the jumping ring to show that forces can act
on induced currents. This leads in to a discussion of induction motors, in which
the rotor is itself a coil in which a current is induced by the rotating flux of the
stator coils (p152). The action can be visualized in terms of eddy currents which
tend to reduce the change producing them, hence dragging the rotor around
(compare eddy currents pendulum). Alternatively, explain in terms of alignment
of opposite poles on rotor and stator, as in the case of the motor with the
permanent magnet. Demonstrate a shaded-pole induction motor as a very
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A. M. James, Matthew Arnold School, Oxford
Activity
240D
‘Shaded
pole
induction motor’ or
demonstrate
commercial
shaded pole motor
from
fish
tank
pump or biology
aerator pump
Activity
250D
‘Model three phase
induction motor’
Activity
230S
‘Making flux rotate’
Qs
200X
‘The
induction motor’: very
good
for
testing
understanding
of
induction motor effects
Qs 220X ‘A variable
speed linkage’
Qs 190X ‘Electronic
ignition’
Reading 60T ‘People
and electromagnetism:
the
inventors
and
engineers’
Qs student book p153
6
Lesson
Content
simple way of producing low-power motor operation.
Activities
Homework
FPP June 05 Q10
15.3 A question
of power
Operation of d.c. motor. F = ILB. Flux cutting. Emf = vLB.
Looking to the future.
 Know that a force acts on a current-carrying conductor in a magnetic
field.
 Use the left hand rule (conventional current) to predict the direction of
the force.
 Use the left hand rule to explain the operation of a simple d.c. motor,
noting how its operation differs from induction motors.
 Explain the direction of the force using diagrams showing the addition of
the fields due to the current and the external field (catapult principle).
Lesson 10-11: D.C. motors. Begin by demonstrating the force on a currentcarrying wire, as per activity 290E or 300E. Students should be clear about the
current, force and field direction being at right angles to each other (left hand
rule with conventional current). The equation F = IlB can be introduced, but is
derived later. Now students can try building a simple d.c. motor using the kits,
the challenge being to produce the fastest/most powerful motor. Discussion
should focus on explaining its operation in terms of the forces acting on the
current-carrying wires. To show how the force arises, use the diagrams on p154,
showing addition of flux lines from the current and the external field, recalling
that flux lines tend to straighten and shorten. Note also how the operation of a
d.c. motor differs from the induction motors considered previously, and the
drawbacks of continually making and breaking electrical contact.
Examples
from
Activity
290E
‘Forces
on
currents’ AND/OR
Activity
300E
‘Force
on
a
current-carrying
wire’
Motors kits OR
Activity 270E ‘A
simple motor’
Activity
340E
‘Motors that make
our
world
go
round’
Qs 230S ‘Sketching
flux
patterns,
predicting forces’
Qs 240S ‘Forces and
currents’
Qs 260S ‘EMF in an
airliner’
FPP
G495
Specimen Q5; Jan.
08 Q4; June 05 Q7



08/03/2016 1:47 AM
Know the meaning of the term back e.m.f.
Explain the origin of the force opposing motion in a generator, and the
back e.m.f. in a motor, in terms of Lenz’s law.
Understand how to derive the relationship F = IlB from a consideration
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7
Lesson
Content
of mechanical and electrical power in a generator.
 Know that an unloaded motor produces a back e.m.f. equal and
opposite to the applied e.m.f.
 Explain why, for a motor to do useful work, it must rotate more slowly to
generate a smaller back e.m.f.
Lesson 12: Go through the derivation of F = IlB as outlined on p156-157. It is
important to stress that in a generator, the force F that opposes the externallyproduced motion arises from the induced currents. Activity 310P helps to clarify
the important point that in a generator, the current that flows feels a force due to
the ‘motor effect’ in opposition to the force producing the motion, trying to slow
down rotation (Lenz’s law). Discuss the corresponding situation for a motor,
where the back e.m.f. increases with the speed of the motor until it equals the
applied e.m.f. It is important that students realise that in this situation, the motor
is doing no useful work as no current is being drawn. To do useful work, the
motor must slow down. Activity 320P illustrates this concept.
 Know the salient design features of the universal a.c./d.c. commutator
motor.
 Know that the main design challenge for a motor is to achieve a balance
between desired speed of rotation and torque.
 Compare and contrast the various types of motor design encountered in
this chapter, noting which ones achieve high conductance and
permeance.
 Explain how linear motors and electromagnetic braking work.
Lesson 13: Complete this section with a discussion of what makes a good
motor design. You could set some of the work in advance for homework.
Compare the universal a.c./d.c. commutator motor (DM 280O) with the simple
motors made earlier from kits. See also GCSE physics resources for comparison
of kit motors with commercial ones, and also Reading 80T and DM 290S.
Discuss also future developments, as per p159. Reading 110T is good on
electromagnetic braking. Review all motor types, using printed diagrams to
make posters.
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A. M. James, Matthew Arnold School, Oxford
Activities
Homework
Activity
320P
‘Torque from a
motor’ good
Qs 230S ‘Sketching
flux
patterns,
predicting forces’
Qs 240S ‘Forces and
currents’
Reading 80T
wide variety
motors’
DM
290S
catalogue
motors’
Reading 80T ‘A wide
variety of motors’
DM 290S ‘A catalogue
of motors’
Qs 250S ‘Thinking
about the design of the
d.c. motor’
Qs
270C
‘The
Birmingham maglev’
Qs 280X ‘ICT driven
by precision motors’
Reading 90T ‘Linear
‘A
of
‘A
of
8
Lesson
Content
Chapter 15 test.
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A. M. James, Matthew Arnold School, Oxford
Activities
Homework
motors:
from
Laithwaite to levitating
trains
and
rocket
launchers’
Reading 110T ‘The
Eurostar train’
Qs student book p160
Qs student book p162
9