Transcript IGCSE-62-Electric Motors & Electromagnetic Induction Presenation

EDEXCEL IGCSE PHYSICS 6-2
Electric Motors and
Electromagnetic Induction
Edexcel IGCSE Physics pages 187 to 196
Content applying to Triple Science only is shown in
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POWERPOINT
IS onNOT
red
type on the next slide and is indicated
subsequent slides by ‘TRIPLE ONLY’
DUE FOR COMPLETION
UNTIL
July 14 2011
JULY 2012
th
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.
Red type: Triple Science Only
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
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
+
S
+-
-
-+
N
+
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 ?
N
Q2 Current direction ?
S
S
N
Q3 N and S poles ?
Q4 Force directions ?
N
S
Note:
N
means current out of the page
means current into the page
Motor effect - Fendt
S
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
Brushes regain
in contact
lose
contact
contact
with
with
with
thethe
splitsplit
the
ringring
commutator.
commutator.
split-ring commutator
+
Electric motor - Fendt
Current flows
no longer
through
flowsthe
through
motor
coil.
coil
thebut
motor
in the
coil.
opposite
original
direction.
Forces
The
coilexert
will continue
a clockwise
to rotate
turning effect
clockwise
Forces
exert
dueaon
to
clockwise
the
its coil
momentum.
turning
effect on the coil.
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:
current carrying wire is
The motor effect occurs when a _______
magnetic field.
placed inside a ________
maximum when the wire is at 90° to the
The force exerted is __________
direction
parallel to
magnetic field __________
but is zero if the wire is ________
the field.
field
The force increases with _________
or current strength, the
reverses
force __________
in direction if either are reversed.
loudspeaker
Applications include the electric motor and ___________.
WORD SELECTION:
parallel reverses loudspeaker direction
field
current magnetic
maximum
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
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
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
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
voltage 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 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.
V
time
The new voltage will have TWICE the
amplitude AND frequency of the
original.
Question 2
Choose appropriate words to fill in the gaps below:
The _________
generator effect occurs when a conductor is moved
magnetic
relative to a ____________
field. This is also known as
induction
electromagnetic ___________.
movement of the conductor and
The greater the relative __________
greater is the voltage ________.
induced
magnetic field the _______
complete circuit an electric
If the conductor is part of a ________
current will flow.
alternating
___________
current is produced if the direction of movement
reversed
is continually _________.
WORD SELECTION:
generator
magnetic complete alternating
induction
induced
greater
reversed
movement
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
of Np turns
SECONDARY COIL
of Ns turns
PRIMARY
VOLTAGE Vp
SECONDARY
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.
The transformer equation
The voltages or potential differences across the
primary and secondary coils of a transformer are
related by the equation:
primary voltage
secondary voltage
Vp
Vs
Transformer - eChalk
=
=
primary turns
secondary turns
Np
Ns
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
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
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
=
secondary voltage
primary turns
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
=
secondary voltage
12 / 48 = Np / 400
0.25 = Np / 400
0.25 x 400 = Np
Primary has 100 turns
Transformer - eChalk
primary turns
secondary turns
Answers
Complete:
PRIMARY
SECONDARY
Voltage
Turns
Voltage
Turns
230 V
1000
11.5 V
50
230 V
500
46 VV
46
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 stepdown 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 ___________
voltage
alternating
level to another. They do not work with ____________current.
direct
increase the voltage because their
Step-up transformers _________
secondary
___________
coil has more turns than the primary.
25 kV
National
Transformers are used in the __________
Grid. The _______
400 kV
output of a power station is increased to up to _______.
A high
energy
resistance
voltage reduces the ________
lost to heat due to the _________
of the power lines.
WORD SELECTION:
energy
direct
increase
National
400 kV
secondary
resistance
alternating 25 kV
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
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.
Online Simulations