EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 6-2 Electric Motors and Electromagnetic Induction

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EDEXCEL IGCSE / CERTIFICATE IN PHYSICS
6-2Electric Motors and Electromagnetic Induction

• Edexcel IGCSE Physics pages 187 to 196

Content applying to Triple Science only is shown
in red type on the next slide and is indicated on
subsequent slides by TRIPLE ONLY
June 17th 2012
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Edexcel Specification

• Section 6 Magnetism and electromagnetism
• c) Electromagnetism
• undertand that there is a force on a charged
  particle when it moves in a magnetic field as
  long as its motion is not parallel to the field
• undertand that a force is exerted on a
  current-carrying wire in a magnetic field, and
  how this effect is applied in simple d.c.
  electric motors and loudspeakers
• use the left hand rule to predict the direction
  of the resulting force when a wire carries a
  current perpendicular to a magnetic field
• describe how the force on a current-carrying
  conductor in a magnetic field increases with the
  strength of the field and with the current.

• d) Electromagnetic induction
• understand that a voltage is induced in a
  conductor or a coil when it moves through a
  magnetic field or when a magnetic field changes
  through it and describe the factors which affect
  the size of the induced voltage
• describe the generation of electricity by the
  rotation of a magnet within a coil of wire and of
  a coil of wire within a magnetic field also
  describe the factors which affect the size of the
  induced voltage
• describe the structure of a transformer, and
  understand that a transformer changes the size of
  an alternating voltage by having different
  numbers of turns on the input and output sides
• explain the use of step-up and step-down
  transformers in the large-scale generation and
  transmission of electrical energy
• know and use the relationship
• input (primary) voltage / output (secondary)
  voltage primary turns / secondary turns
• Vp / Vs np / ns
• recall and use the relationship input power
  output power
• VP IP Vs Is for 100 efficiency

Red type Triple Science Only
3
Charge deflection by a magnetic field
TRIPLE ONLY
Electric charges are deflected by magnetic fields
provided they are not travelling parallel to the
field lines. Positive and negative charges are
deflected in opposite directions.
4
The motor effect

• When a current carrying conductor carrying an
  electric current is placed in a magnetic field,
  it will experience a force provided that the
  conductor is not placed parallel to the field
  lines.
• 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.

Motor effect - Fendt
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Flemings 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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Electromagnetic induction

• If an electrical conductor cuts through magnetic
  field lines, a voltage 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 called electromagnetic induction and is
  sometimes called the generator effect.

Generator - Fendt
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• If a magnet is moved into a coil of wire, a
  voltage 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 voltage and the induced current are
  also reversed.
• Electromagnetic induction also occurs if the
  magnetic field is stationary and the coil is
  moved.

Generator - Fendt
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• The size of the induced voltage 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
19

• 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 voltage 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 how the voltage of a
  generator varies in time. Using the same set of
  axes show how the voltage would vary if the
  rotational speed of the generator was doubled.

The new voltage 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 voltage ________. 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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The transformer
TRIPLE ONLY

• 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
TRIPLE ONLY

• 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
TRIPLE ONLY

• 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
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Question
TRIPLE ONLY

• 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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The transformer equation
TRIPLE ONLY

• 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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Question 1
TRIPLE ONLY

• 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.
• Vp Np
• Vs Ns
• 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
TRIPLE ONLY

• 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.
• Vp Np
• Vs Ns
• 12 / 48 Np / 400
• 0.25 Np / 400
• 0.25 x 400 Np
• Primary has 100 turns

Transformer - eChalk
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Complete
TRIPLE ONLY
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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Transformer power transfer equation
TRIPLE ONLY

• If a transformer is 100 efficient then the power
  input to the primary coil is equalled by the
  power output from the secondary coil.
• as power current x voltage
• then
• Ip x Vp Is x Vs

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Question 1
TRIPLE ONLY

• Calculate the primary current if when a
  transformer is supplied with 230V the secondary
  provides 4A at a voltage of 13V. Assume that the
  transformer is 100 efficient.
• Ip x Vp Is x Vs
• Ip x 230V 4A x 13V
• Ip 52 / 230
• Primary current 0.226 A

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Question 2
TRIPLE ONLY

• Calculate the secondary current from a
  transformer supplying a secondary voltage of 6V
  if the primary is supplied with a current of
  0.20A at 230V. Assume that the transformer is
  100 efficient.
• Ip x Vp Is x Vs
• 0.2A x 230V Is x 6V
• Is 46 / 6
• Secondary current 7.67 A

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Complete
TRIPLE ONLY
Answers
PRIMARY PRIMARY PRIMARY SECONDARY SECONDARY SECONDARY
Np Vp Ip Ns Vp Is
600 200V 0.4 A 30 10V 8 A
100 12V 8 A 4000 480V 0.2 A
300 72V 0.4 A 50 12V 2.4 A
50 25V 10 A 250 125V 2 A
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Step-up transformers
TRIPLE ONLY

• In a step-up transformer the voltage across the
  secondary coil is greater than the voltage 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
TRIPLE ONLY

• In a step-down transformer the voltage across the
  secondary coil is smaller than the voltage 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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Transformers and the National Grid
TRIPLE ONLY

• 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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TRIPLE ONLY
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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TRIPLE ONLY
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
TRIPLE ONLY

• 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
TRIPLE ONLY
Choose appropriate words to fill in the gaps
below Transformers are used to change one
___________ voltage 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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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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Electric Motors and Electromagnetic Induction
Notes questions from pages 187 to 196
TRIPLE ONLY

1. (a) What is the motor effect? (b) What factors
   determine the size of the force exerted on a
   conductor in a magnetic field? (c) With the aid
   of a diagram show how Flemings left-hand rule
   can be used to find the direction of the force on
   a conductor.
2. Copy figures 22.5 and 22.6 and explain how a
   moving coil loudspeaker and electric motor work.
3. (a) Draw diagrams and explain what is meant by
   electromagnetic induction? (b) What factors
   determine the size of the voltage produced?
4. Copy figure 22.12 and use it to explain how a
   simple generator works.
5. Copy figure 22.16 and use it to explain how a
   transformer works.
6. Copy the two transformer equations on pages 193
   and 194 and find the secondary current and
   voltage for a 100 efficient transformer that has
   a primary coil of 800 turns supplied with 2A at
   40V if the secondary coil has 100 turns.
7. Explain what is meant by step-up and step-down
   transformers and how they are used in the UKs
   National Grid system.
8. Answer the questions on pages 195 and 196.
9. Verify that you can do all of the items listed in
   the end of chapter checklist on page 195.

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Electric Motors and Electromagnetic Induction
Notes questions from pages 187 to 196
DOUBLE SCIENCE ONLY

1. (a) What is the motor effect? (b) What factors
   determine the size of the force exerted on a
   conductor in a magnetic field? (c) With the aid
   of a diagram show how Flemings left-hand rule
   can be used to find the direction of the force on
   a conductor.
2. Copy figures 22.5 and 22.6 and explain how a
   moving coil loudspeaker and electric motor work.
3. (a) Draw diagrams and explain what is meant by
   electromagnetic induction? (b) What factors
   determine the size of the voltage produced?
4. Copy figure 22.12 and use it to explain how a
   simple generator works.
5. Answer questions 1, 2 and 3 on pages 195 and 196.