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AQA GCSE Physics 3-3
Electromagnetism
GCSE Physics pages 254 to 265
April 10th 2010
AQA GCSE Specification
THE MOTOR EFFECT TRANSFORMERS
13.7 How can electricity be used to make things move? 13.9 How do transformers work?
Using skills, knowledge and understanding of how science works:
• to explain how the motor effect is used in simple devices. Using skills, knowledge and understanding of how science works:
• to determine which type of transformer should be used for a
Skills, knowledge and understanding of how science works set in the context of: particular application.
• When a conductor carrying an electric current is placed in a magnetic field, it may
experience a force.
• The size of the force can be increased by: Skills, knowledge and understanding of how science works set in the
– increasing the strength of the magnetic field context of:
– increasing the size of the current. • The basic structure of the transformer.
• The conductor will not experience a force if it is parallel to the magnetic field. • An alternating current in the primary coil produces a changing
• The direction of the force is reversed if either the direction of the current or the magnetic field in the iron core and hence in the secondary coil.
direction of the magnetic field is reversed.
This induces an alternating potential difference across the
ELECTRICAL GENERATORS
ends of the
13.8 How do generators work? secondary coil.
• The potential difference (p.d.) across the primary and secondary
Using skills, knowledge and understanding of how science works: coils of a transformer are related by the equation:
• to explain from a diagram how an a.c. generator works, including the purpose of p.d. across primary / p.d. across secondary = number of turns on
the slip rings and brushes.
primary / number of turns on secondary
Skills, knowledge and understanding of how science works set in the context of: • In a step-up transformer the potential difference across the
• If an electrical conductor .cuts. through magnetic field lines, an electrical potential secondary coil is greater than the potential difference across
difference is induced across the ends of the conductor. the primary coil.
• If a magnet is moved into a coil of wire, an electrical potential difference is induced • In a step-down transformer the potential difference across the
across the ends of the coil. secondary coil is less than the potential difference across the
• If the wire is part of a complete circuit, a current is induced in the wire. primary coil.
• If the direction of motion, or the polarity of the magnet, is reversed, the direction of
the induced potential difference and the induced current is reversed. • The uses of step-up and step-down transformers in the National
• The generator effect also occurs if the magnetic field is stationary and the coil is Grid.
moved.
• The size of the induced potential difference increases when:
– the speed of the movement increases
– the strength of the magnetic field increases
– the number of turns on the coil increases
– the area of the coil is greater.
The motor effect
When a conductor
carrying an electric + S -
current is placed in a
magnetic field,
it may experience a
-
+ -
+
force. N
This is called the - +
motor effect.
Motor effect - Fendt
The force increases if:
– the strength of the magnetic field is increased
– the current is increased
The direction of the force is reversed if either
the direction of the current or the direction of
the magnetic field is reversed.
The conductor will not experience a force if it
is parallel to the magnetic field.
Motor effect - Fendt
The left-hand motor rule
Note:
Magnetic field direction is from NORTH to SOUTH
Current direction is from PLUS to MINUS
Motor effect - Fendt
Insert the missing information
Q1. Force direction ? Q2 Current direction ?
N S S N
Q3 N and S poles ?
Q4 Force directions ?
N S N S
Note: means current out of the page
means current into the page
Motor effect - Fendt
The electric motor
Electric current flowing around the
coil of the electric motor produces
oppositely directed forces on each
side of the coil.
These forces cause the coil to
rotate.
Every half revolution the split ring
commutator causes the current in
the coil to reverse otherwise the
coil would stop in the vertical
position.
Electric motor - Fendt
rotation
axis
N S
contact brush
regain contact the
in contact with with
Brushes lose contact with the
the ring commutator.
splitsplit ring commutator.
flows through the
Current no longer flows
through the motor coil.
coil original
motor coil.but in the opposite
split-ring commutator direction.
Forces will a clockwise
The coilexertcontinue to rotate
+ Forces exert clockwise
turning effect a to its
clockwise dueon the coil
momentum. on the coil.
turning effect
Electric motor - Fendt
Model electric motor
Electric motor - Fendt
The loudspeaker
The sound signal consists of an
alternating current supplied by the
amplifier.
This current flows through the coil of
the loudspeaker.
Due to the motor effect, the magnetic
field around the coil causes the coil to
vibrate in step with the alternating
current.
The coil causes the diaphragm
(speaker cone) to vibrate in step with
the original sound signal.
The diaphragm causes air to vibrate
and so produces a sound wave.
Question
Choose appropriate words to fill in the gaps below:
The motor effect occurs when a _______ carrying wire is
current
placed inside a ________ field.
magnetic
The force exerted is __________ when the wire is at 90° to the
maximum
magnetic field __________ but is zero if the wire is ________ to
direction parallel
the field.
The force increases with _________ or current strength, the
field
force __________ in direction if either are reversed.
reverses
Applications include the electric motor and ___________.
loudspeaker
WORD SELECTION:
parallel reverses loudspeaker direction
current magnetic field maximum
The motor effect
Notes questions from pages 254 & 255
1. What is the motor effect?
2. Copy out the bullet points at the bottom of page 254 listing the factors that
affect the force on a current carrying wire inside a magnetic field.
3. Copy and answer question (a) on page 254.
4. Copy Figure 3 on page 255 and explain how a simple electric motor
works. Your account should include the purpose of the split-ring
commutator.
5. Copy and answer question (b) on page 255.
6. Copy Figure 4 on page 255 and explain how a moving coil loudspeaker
works.
7. Copy and answer question (c) on page 255.
8. Copy the ‘Key points’ table on page 255.
9. Answer the summary questions on page 255.
Motor effect - Fendt Electric motor - Fendt
The motor effect
ANSWERS
In text questions: Summary questions:
(a) No change, the actions 1. (a) Current, coil, force, coil.
cancel each other out. (b) Current, force, coil.
(b) The material must conduct 2. (a) The direction of the
electricity. current is reversed and so the
(c) A direct current will not force on the coil is in the
produce a changing opposite direction.
magnetic field. (b) (i) Faster because the coil
is lighter
(ii) Faster because the field is
much stronger due to the
presence of iron.
The generator effect
If an electrical conductor cuts.
through magnetic field lines, an
electrical potential difference is
induced across the ends of the
conductor.
If the wire is part of a complete
circuit, a current is induced in
the wire.
This is also called
electromagnetic induction.
Generator - Fendt
If a magnet is moved into a coil of
wire, an electrical potential
difference is induced across the
ends of the coil.
If the direction of motion, or the
polarity of the magnet, is reversed,
then the direction of the induced
potential difference and the induced
current are also reversed.
The generator effect also occurs if
the magnetic field is stationary and
the coil is moved.
Generator - Fendt
The size of the induced potential difference
increases when:
– the speed of the movement increases
– the strength of the magnetic field increases
– the number of turns on the coil increases
– the area of the coil is greater.
Generator - Fendt
Alternating Current Generators
Most electricity is produced using the ‘generator
effect’.
The simplest generators and the types used in
power stations produce alternating current (A.C.)
Generator - Fendt
Moving Coil A.C. Generator
Generator - Fendt
Generator - Fendt
This like an electric motor in reverse.
As the coil is rotated electromagnetic induction occurs.
An alternating voltage is induced in the coil.
An alternating current is drawn off through two slip rings.
The faster the coil is rotated:
- the greater is the amplitude of the voltage and current
- the higher is the frequency of the a.c.
Generator - Fendt
Bicycle generator
When the wheel turns the
magnet is made to rotate
next to the fixed coil of wire.
Electromagnetic induction
occurs and a alternating
potential difference is
induced in the coil.
This causes an alternating
current to flow to the light
bulb of the bicycle.
Generator - Fendt
Question 1
The graph opposite shows PD
the potential difference of a
generator varies in time.
Using the same set of axes
show how the potential
difference would vary if the
rotational speed of the
generator was doubled. time
The new potential difference will have
TWICE the amplitude AND frequency of
the original.
Question 2
Choose appropriate words to fill in the gaps below:
The _________ effect occurs when a conductor is moved
generator
relative to a ____________ field. This is also known as
magnetic
electromagnetic ___________.
induction
The greater the relative __________ of the conductor and
movement
magnetic field the _______ is the potential difference ________.
greater induced
complete
If the conductor is part of a ________ circuit an electric
current will flow.
___________ current is produced if the direction of movement
alternating
is continually _________.
reversed
WORD SELECTION:
magnetic complete alternating generator reversed
induction induced greater movement
Electromagnetic induction
Notes questions from pages 256 & 257
1. What is induced in a wire because of the dynamo effect?
2. Copy and answer question (a) on page 256.
3. Copy Figure 2 on page 256 and explain how a cycle dynamo
works.
4. Copy and answer questions (b) and (c) on page 256.
5. Explain how the alternating current generator on page 257 works.
Your explanation should include a copy of both parts of Figure 4.
6. Copy the ‘Key points’ table on page 257.
7. Answer the summary questions on page 257.
Generator - Fendt
Electromagnetic induction
ANSWERS
In text questions: Summary questions:
(a) (i) The current increases. 1. (a) The pointer would move to
(ii) The direction of the the right but not as far.
current reverses. (b) The pointer returns to zero.
(iii) No current is produced. (c) The pointer would move
(b) The wires leading to the coil rapidly to the left.
would get twisted up. No 2. (a) Spin the coil faster, use
brushes are needed. more loops of coil, use
(c) (i) There is no current. stronger magnets.
(ii) A p.d. is produced in the (b) The peak voltage would be
opposite direction. lower and the period would be
longer.
The transformer
A transformer is a
device that is used to
change one alternating
voltage level to another.
circuit symbol
Transformer - eChalk
Structure of a transformer
A transformer consists of at least two coils of wire
wrapped around a laminated iron core.
PRIMARY COIL SECONDARY COIL
of Np turns of Ns turns
PRIMARY SECONDARY
VOLTAGE Vp VOLTAGE Vs
laminated iron core
Transformer - eChalk
How a transformer works
When an alternating voltage, Vp is applied to the
primary coil of Np turns it causes an alternating to
flow in this coil.
This current causes a changing magnetic field in
the laminated iron core which cuts across the
secondary coil of Ns turns.
Electromagnetic induction occurs in this coil which
produces an alternating voltage, Vs.
Transformer - eChalk
Question
Why can a transformer not change the level of the
voltage output of a battery?
– A battery produces a steady (DC) voltage.
– This voltage would cause a constant direct current in
the primary coil of a transformer.
– This current would produce an unchanging magnetic
field in the iron core.
– This unchanging magnetic field would NOT cause
electromagnetic induction in the secondary coil.
– There would therefore be no secondary voltage.
Transformers
Notes questions from pages 258 & 259
1. Copy Figure 1 on page 258 and (a) explain what a
transformer is, (b) what a transformer does and (c) how a
transformer works.
2. Copy and answer questions (a), (b) and (c) on page 258.
3. Copy the circuit symbol for a transformer on page 259 and
explain why the electric current supplied to a transformer
must be alternating in order for the transformer to function.
4. Copy and answer question (d) on page 259.
5. Copy the ‘Key points’ table on page 259.
6. Answer the summary questions on page 259.
Transformer - eChalk
Transformers
ANSWERS
In text questions: Summary questions:
(a) The magnetic field in the core 1. Current, primary, magnetic field,
would be much weaker secondary, p.d., secondary.
because the core is not a 2. (a) Direct current in the primary
magnetic material. coil would not produce an
(b) The lamp would be brighter. alternating magnetic field, so no
(c) The lamp would not light up p.d. would be induced in the
with direct current in the secondary coil.
primary coil. (b) The current would short-circuit
(d) Iron is easier to magnetise and across the wires instead of
demagnetise as the alternating passing through them. This would
current increases and cause the coil to overheat if it did
decreases each half cycle. not cause the fuse to blow.
(c) Iron is a magnetic material, so
it makes the magnetic field much
stronger. It is easily magnetised
and demagnetised when the
current alternates.
The transformer equation
The voltages or potential differences across the
primary and secondary coils of a transformer are
related by the equation:
primary voltage = primary turns
secondary voltage secondary turns
Vp = Np
Vs Ns
Transformer - eChalk
Step-up transformers
In a step-up transformer the
potential difference across the
secondary coil is greater than the
potential difference across the
primary coil.
The secondary turns must be
greater than the primary turns.
Use: To increase the voltage
output from a power station from
25 kV (25 000 V) to up to 400 kV.
Transformer - eChalk
Step-down transformers
In a step-down transformer the potential
difference across the secondary coil is
smaller than the potential difference
across the primary coil.
The secondary turns must be smaller
than the primary turns.
Use: To decrease the voltage output
from the mains supply from 230V to 18V
to power and recharge a lap-top
computer.
Transformer - eChalk
Question 1
Calculate the secondary voltage of a transformer that has a
primary coil of 1200 turns and a secondary of 150 turns if
the primary is supplied with 230V.
primary voltage = primary turns
secondary voltage secondary turns
230 / Vs = 1200 / 150
230 / Vs = 8
230 = 8 x Vs
230 / 8 = Vs
Secondary voltage = 28.8 V
Transformer - eChalk
Question 2
Calculate the number of turns required for the primary coil
of a transformer if secondary has 400 turns and the primary
voltage is stepped up from 12V to a secondary voltage of
48V.
primary voltage = primary turns
secondary voltage secondary turns
12 / 48 = Np / 400
0.25 = Np / 400
0.25 x 400 = Np
Primary has 100 turns
Transformer - eChalk
Answers
Complete:
PRIMARY SECONDARY
Voltage Turns Voltage Turns
230 V 1000 11.5 V 50
230 V 500 46 V
46 V 100
230 V 200 920 V 800
9V 120 72 V 960
Transformer - eChalk
Transformers and the National Grid
The National Grid is the system of cables used to
deliver electrical power from power stations to
consumers.
The higher the voltage used, the greater is the
efficiency of energy transmission.
Lower voltages result in higher electric currents
and greater energy loss to heat due to the
resistance of the cables.
At power stations the output voltage of the generators is
stepped up by transformers from 25kV to 132kV.
The voltage may be further increased to up to 400 kV for
transmission over long distance pylon lines.
The voltage is reduced in stages by step-down
transformers to different levels for different types of
consumer.
The lowest level is 230V for domestic use. The final step-
down transformer will be at sub station within a few
hundred metres of each group of houses.
Question 1
Why is electrical energy transmitted over the
National Grid in the form of alternating current?
– To maximise efficiency high voltages must be used.
– Voltage therefore needs to be changed in level.
– Transformers are needed to change voltage levels.
– Transformers only work with alternating current.
Question 2
Choose appropriate words to fill in the gaps below:
Transformers are used to change one ___________ potential
alternating
difference level to another. They do not work with
____________current.
direct
Step-up transformers _________ the voltage because their
increase
___________ coil has more turns than the primary.
secondary
Transformers are used in the __________ Grid. The _______
National 25 kV
output of a power station is increased to up to _______. A high
400 kV
energy
voltage reduces the ________ lost to heat due to the _________
resistance
of the power lines.
WORD SELECTION:
energy direct National secondary resistance
increase 400 kV alternating 25 kV
Transformers and the National Grid
Notes questions from pages 260 & 261
1. (a) Why are transformers used in the National grid? (b) What is the
advantage of using high voltages?
2. Copy the transformer equation on page 260.
3. Copy a version of the worked example on page 260 but in your version
change the number of turns on the secondary coil from 60 to 30.
4. What is the purpose of (a) step-up and (b) step-down transformers?
5. Explain how the number of turns on the coils of a transformer determine
whether a transformer is step-up or step-down.
6. State how the currents and voltages associated with the primary and
secondary coils are related to each other with a 100% efficient
transformer.
7. Copy and answer questions (a) and (b) on page 261.
8. Copy the ‘Key points’ table on page 261.
9. Answer the summary questions on page 261.
Transformer - eChalk
Transformers and the National Grid
ANSWERS
In text questions: Summary questions:
(a) 60 turns 1. (a) (i) Secondary,
(b) (i) 6A (ii) 0.26A primary.
(b) Up, down.
2. (a) 2000 turns
(b) (i) 3A (ii) 0.15A
More power to you
Notes questions from pages 262 & 263
1. Answer questions 1 and 2 on page 263.
Electromagnetism Simulations
Motor effect - Fendt Faraday's Law - PhET - Light a
Electric motor - Fendt light bulb by waving a magnet.
Faraday Electromagnetic Lab – This demonstration of Faraday's
PhET Play with a bar magnet and Law shows you how to reduce
coils to learn about Faraday's law. your power bill at the expense of
Move a bar magnet near one or your grocery bill.
two coils to make a light bulb Generator - Fendt
glow. View the magnetic field Transformer - load can be
lines. A meter shows the direction changed but not turns ration -
and magnitude of the current. netfirms
View the magnetic field lines or Transformer - eChalk
use a meter to show the direction
and magnitude of the current. You
can also play with
electromagnets, generators and
transformers!
More power to you
ANSWERS
1. (a) They would not need heavy iron magnets.
(b) There would be no power wasted in the wires, as
the wires would have no resistance.
2. (a) Ionising radiation, carcinogenic (cancer-causing)
substances.
(b) People are at risk due to other causes. There is an
extra risk to those exposed to these magnetic fields.
(c) A hypothesis is put forward as an ‘unproven’ theory
to be tested by scientific experiments. If lots of
experiments are carried out and they all support the
hypothesis, it gains scientific credibility and is accepted
as a theory. But at any stage, it could be overthrown
by any conflicting scientific evidence.
How Science Works
ANSWERS
a) The voltmeter was not d) 0.01V
sensitive enough. It would e) Not at the greater heights.
also not give a read-out of f) Improve the sensitivity of the
the voltage, so it would be oscilloscope. Repeat his
impossible to get an results.
accurate result even if it was
g) By checking it against other
sensitive enough.
data/other similar research/get
b) Height on the X-axis, voltage someone else to repeat his
on the Y-axis. Axes fully work or calculate theoretical
labelled and plots correctly relationships.
plotted.
h) For example: Measuring the
c) In part. The voltage speed of an object through a
increased as height tube.
increased, but it was not
directly proportional.
