AQA GCSE Physics 3-3 Electromagnetism

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AQA GCSE Physics 3-3Electromagnetism

• GCSE Physics pages 254 to 265

April 10th 2010
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AQA GCSE Specification

• THE MOTOR EFFECT
• 13.7 How can electricity be used to make things
  move?
• Using skills, knowledge and understanding of how
  science works
• to explain how the motor effect is used in
  simple devices.
• Skills, knowledge and understanding of how
  science works set in the context of
• 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
• increasing the strength of the magnetic field
• increasing the size of the current.
• The conductor will not experience a force if
  it is parallel to the magnetic field.
• The direction of the force is reversed if
  either the direction of the current or the
  direction of the magnetic field is reversed.
• ELECTRICAL GENERATORS
• 13.8 How do generators work?
• Using skills, knowledge and understanding of how
  science works
• to explain from a diagram how an a.c.
  generator works, including the purpose of the
  slip rings and brushes.

• TRANSFORMERS
• 13.9 How do transformers work?
• Using skills, knowledge and understanding of how
  science works
• to determine which type of transformer should
  be used for a particular application.
• Skills, knowledge and understanding of how
  science works set in the context of
• The basic structure of the transformer.
• An alternating current in the primary coil
  produces a changing magnetic field in the iron
  core and hence in the secondary coil. This
  induces an alternating potential difference
  across the ends of the
• secondary coil.
• The potential difference (p.d.) across the
  primary and secondary coils of a transformer are
  related by the equation
• p.d. across primary / p.d. across secondary
  number of turns on primary / number of turns on
  secondary
• In a step-up transformer the potential
  difference across the secondary coil is greater
  than the potential difference across the primary
  coil.
• In a step-down transformer the potential
  difference across the secondary coil is less than
  the potential difference across the primary coil.
• The uses of step-up and step-down transformers
  in the National Grid.

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The motor effect

• When a conductor carrying an electric current is
  placed in a magnetic field,
• it may experience a force.
• This is called the motor effect.

Motor effect - Fendt
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• 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
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The left-hand motor rule
Note Magnetic field direction is from NORTH to
SOUTH Current direction is from PLUS to MINUS
Motor effect - Fendt
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Insert the missing information
Q1. Force direction ?
Q2 Current direction ?
Q3 N and S poles ?
Q4 Force directions ?
Motor effect - Fendt
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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
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rotation axis
contact brush
Brushes lose contact with the split ring
commutator. Current no longer flows through the
motor coil. The coil will continue to rotate
clockwise due to its momentum.
Brushes in contact with the split ring
commutator. Current flows through the motor
coil. Forces exert a clockwise turning effect on
the coil
Brushes regain contact with the split ring
commutator. Current flows through the motor coil
but in the opposite direction. Forces exert a
clockwise turning effect on the coil.
Brushes lose contact with the split ring
commutator. Current no longer flows through the
motor coil. The coil will continue to rotate
clockwise due to its momentum.
Brushes regain contact with the split ring
commutator. Current flows through the motor coil
but in the original direction. Forces exert a
clockwise turning effect on the coil.
split-ring commutator
Electric motor - Fendt
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Model electric motor
Electric motor - Fendt
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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.

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Question
Choose appropriate words to fill in the gaps
below The motor effect occurs when a _______
carrying wire is placed inside a ________ field.
The force exerted is __________ when the wire is
at 90 to the magnetic field __________ but is
zero if the wire is ________ to the field. The
force increases with _________ or current
strength, the force __________ in direction if
either are reversed. Applications include the
electric motor and ___________.
current
magnetic
maximum
direction
parallel
field
reverses
loudspeaker
WORD SELECTION
parallel
direction
loudspeaker
reverses
magnetic
maximum
current
field
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The motor effectNotes 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.

Electric motor - Fendt
Motor effect - Fendt
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The motor effect ANSWERS

• In text questions
• No change, the actions cancel each other out.
• The material must conduct electricity.
• A direct current will not produce a changing
  magnetic field.

• Summary questions
• (a) Current, coil, force, coil.
• (b) Current, force, coil.
• 2. (a) The direction of the current is reversed
  and so the force on the coil is in the opposite
  direction.
• (b) (i) Faster because the coil is lighter
• (ii) Faster because the field is much stronger
  due to the presence of iron.

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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
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• 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
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• 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
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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
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Moving Coil A.C. Generator
Generator - Fendt
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Generator - Fendt
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• 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
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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
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Question 1

• The graph opposite shows 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.

The new potential difference will have TWICE the
amplitude AND frequency of the original.
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Question 2
Choose appropriate words to fill in the gaps
below The _________ effect occurs when a
conductor is moved relative to a ____________
field. This is also known as electromagnetic
___________. The greater the relative __________
of the conductor and magnetic field the _______
is the potential difference ________. If the
conductor is part of a ________ circuit an
electric current will flow. ___________ current
is produced if the direction of movement is
continually _________.
generator
magnetic
induction
movement
greater
induced
complete
alternating
reversed
WORD SELECTION
generator
alternating
reversed
magnetic
complete
induction
movement
greater
induced
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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
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Electromagnetic induction ANSWERS

• In text questions
• (a) (i) The current increases.
• (ii) The direction of the current reverses.
• (iii) No current is produced.
• The wires leading to the coil would get twisted
  up. No brushes are needed.
• (i) There is no current.
• (ii) A p.d. is produced in the opposite
  direction.

• Summary questions
• 1. (a) The pointer would move to the right but
  not as far.
• (b) The pointer returns to zero.
• (c) The pointer would move rapidly to the left.
• 2. (a) Spin the coil faster, use more loops of
  coil, use stronger magnets.
• (b) The peak voltage would be lower and the
  period would be longer.

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The transformer

• A transformer is a device that is used to change
  one alternating voltage level to another.

Transformer - eChalk
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Structure of a transformer

• A transformer consists of at least two coils of
  wire wrapped around a laminated iron core.

Transformer - eChalk
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How a transformer works

• When an alternating voltage, Vp is applied to the
  primary coil of Np turns it causes an alternating
  current 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
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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.

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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
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Transformers ANSWERS

• In text questions
• The magnetic field in the core would be much
  weaker because the core is not a magnetic
  material.
• The lamp would be brighter.
• The lamp would not light up with direct current
  in the primary coil.
• Iron is easier to magnetise and demagnetise as
  the alternating current increases and decreases
  each half cycle.

• Summary questions
• 1. Current, primary, magnetic field, secondary,
  p.d., secondary.
• 2. (a) Direct current in the primary coil would
  not produce an alternating magnetic field, so no
  p.d. would be induced in the secondary coil.
• (b) The current would short-circuit across the
  wires instead of passing through them. This would
  cause the coil to overheat if it did 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.

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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
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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
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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
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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
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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
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Complete
Answers
PRIMARY PRIMARY SECONDARY SECONDARY
Voltage Turns Voltage Turns
230 V 1000 11.5 V 50
230 V 500 46 V 100
230 V 200 920 V 800
9 V 120 72 V 960
50
46 V
200
9 V
Transformer - eChalk
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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.

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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.
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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.
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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.

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Question 2
Choose appropriate words to fill in the gaps
below Transformers are used to change one
___________ potential difference level to
another. They do not work with ____________current
. Step-up transformers _________ the voltage
because their ___________ coil has more turns
than the primary. Transformers are used in the
__________ Grid. The _______ output of a power
station is increased to up to _______. A high
voltage reduces the ________ lost to heat due to
the _________ of the power lines.
alternating
direct
increase
secondary
25 kV
National
400 kV
energy
resistance
WORD SELECTION
energy
secondary
direct
National
resistance
alternating
400 kV
increase
25 kV
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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
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Transformers and the National Grid ANSWERS

• In text questions
• 60 turns
• (i) 6A (ii) 0.26A

• Summary questions
• 1. (a) (i) Secondary, primary.
• (b) Up, down.
• 2. (a) 2000 turns
• (b) (i) 3A (ii) 0.15A

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More power to you Notes questions from pages 262
263

• Answer questions 1 and 2 on page 263.

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Electromagnetism Simulations

• Motor effect - Fendt
• Electric motor - Fendt
• Faraday Electromagnetic Lab PhET Play with a
  bar magnet and coils to learn about Faraday's
  law. Move a bar magnet near one or two coils to
  make a light bulb glow. View the magnetic field
  lines. A meter shows the direction and magnitude
  of the current. View the magnetic field lines or
  use a meter to show the direction and magnitude
  of the current. You can also play with
  electromagnets, generators and transformers!

• Faraday's Law - PhET - Light a light bulb by
  waving a magnet. This demonstration of Faraday's
  Law shows you how to reduce your power bill at
  the expense of your grocery bill.
• Generator - Fendt
• Transformer - load can be changed but not turns
  ration - netfirms
• Transformer - eChalk

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More power to you ANSWERS

• (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.

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How Science Works ANSWERS

• 0.01V
• Not at the greater heights.
• Improve the sensitivity of the oscilloscope.
  Repeat his results.
• By checking it against other data/other similar
  research/get someone else to repeat his work or
  calculate theoretical relationships.
• For example Measuring the speed of an object
  through a tube.

1. The voltmeter was not sensitive enough. It would
   also not give a read-out of the voltage, so it
   would be impossible to get an accurate result
   even if it was sensitive enough.
2. Height on the X-axis, voltage on the Y-axis. Axes
   fully labelled and plots correctly plotted.
3. In part. The voltage increased as height
   increased, but it was not directly proportional.