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EDEXCEL IGCSE PHYSICS 6-2 Electric Motors and Electromagnetic Induction

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EDEXCEL IGCSE PHYSICS 6-2 Electric Motors and Electromagnetic Induction Edexcel IGCSE Physics pages 187 to 196 THIS POWERPOINT IS NOT DUE FOR COMPLETION UNTIL JULY 2012 – PowerPoint PPT presentation

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Title: EDEXCEL IGCSE PHYSICS 6-2 Electric Motors and Electromagnetic Induction

1
EDEXCEL IGCSE PHYSICS 6-2 Electric Motors and
Electromagnetic Induction
• Edexcel IGCSE Physics pages 187 to 196

THIS POWERPOINT IS NOT DUE FOR COMPLETION UNTIL
JULY 2012
Content applying to Triple Science only is shown
in red type on the next slide and is indicated on
subsequent slides by TRIPLE ONLY
July 14th 2011
2
Edexcel IGCSE Specification
• Section 6 Magnetism and electromagnetism
• c) Electromagnetism
• appreciate 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
• recall 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
• recall that 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
• recall 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 also
recall 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
• recall 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
• recall 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
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
4
• 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
5
The left-hand motor rule
Note Magnetic field direction is from NORTH to
SOUTH Current direction is from PLUS to MINUS
Motor effect - Fendt
6
Insert the missing information
Q1. Force direction ?
Q2 Current direction ?
Q3 N and S poles ?
Q4 Force directions ?
Motor effect - Fendt
7
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
8
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
9
Model electric motor
Electric motor - Fendt
10
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.

11
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
12
The generator effect
• 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 also called electromagnetic induction.

Generator - Fendt
13
• 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.
• The generator effect also occurs if the magnetic
field is stationary and the coil is moved.

Generator - Fendt
14
• 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
15
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
16
Moving Coil A.C. Generator
Generator - Fendt
17
Generator - Fendt
18
• 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
19
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
20
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.
21
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
22
The transformer
• A transformer is a device that is used to change
one alternating voltage level to another.

Transformer - eChalk
23
Structure of a transformer
• A transformer consists of at least two coils of
wire wrapped around a laminated iron core.

Transformer - eChalk
24
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
25
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.

26
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
27
Step-up transformers
• 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
28
Step-down transformers
• 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
29
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
30
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
31
Complete
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
32
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.

33
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.
34
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.
35
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.

36
Question 2
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
37
Electromagnetism Simulations
• Motor effect - Fendt
• Electric motor - Fendt
• Faraday Electromagnetic Lab PhET Play with a
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

38
Electric Motors and Electromagnetic Induction
Notes questions from pages 187 to 196
1. Answer the questions on pages 195 and 196.
2. Verify that you can do all of the items listed in
the end of chapter checklist on page 195.

39
Online Simulations