Download OVERVIEW: Electromagnetism

AQA GCSE Physics 3-3
Electromagnetism
GCSE Physics pages 254 to 265
April 10th 2010
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.
Skills, knowledge and understanding of how science works set in the context of:
• If an electrical conductor .cuts. through magnetic field lines, an electrical potential
difference is induced across the ends of the conductor.
• If a magnet is moved into a coil of wire, an electrical potential difference is induced
across the ends of the coil.
• If the wire is part of a complete circuit, a current is induced in the wire.
• 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 generator effect also occurs if the magnetic field is stationary and the coil is
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.
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.
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 motor effect
Notes questions from pages 254 & 255
1.
2.
3.
4.
5.
6.
7.
8.
9.
What is the motor effect?
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.
Copy and answer question (a) on page 254.
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.
Copy and answer question (b) on page 255.
Copy Figure 4 on page 255 and explain how a moving coil loudspeaker
works.
Copy and answer question (c) on page 255.
Copy the ‘Key points’ table on page 255.
Answer the summary questions on page 255.
Motor effect - Fendt
Electric motor - Fendt
The motor effect
ANSWERS
In text questions:
(a) No change, the actions
cancel each other out.
(b) The material must conduct
electricity.
(c) A direct current will not
produce a changing
magnetic field.
Summary questions:
1. (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.
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
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.
PD
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 _________
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 potential difference ________.
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
Electromagnetic induction
Notes questions from pages 256 & 257
1.
2.
3.
4.
5.
6.
7.
What is induced in a wire because of the dynamo effect?
Copy and answer question (a) on page 256.
Copy Figure 2 on page 256 and explain how a cycle dynamo
works.
Copy and answer questions (b) and (c) on page 256.
Explain how the alternating current generator on page 257 works.
Your explanation should include a copy of both parts of Figure 4.
Copy the ‘Key points’ table on page 257.
Answer the summary questions on page 257.
Generator - Fendt
Electromagnetic induction
ANSWERS
In text questions:
(a) (i) The current increases.
(ii) The direction of the
current reverses.
(iii) No current is produced.
(b) The wires leading to the coil
would get twisted up. No
brushes are needed.
(c) (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.
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.
Transformers
Notes questions from pages 258 & 259
1.
2.
3.
4.
5.
6.
Copy Figure 1 on page 258 and (a) explain what a
transformer is, (b) what a transformer does and (c) how a
transformer works.
Copy and answer questions (a), (b) and (c) on page 258.
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.
Copy and answer question (d) on page 259.
Copy the ‘Key points’ table on page 259.
Answer the summary questions on page 259.
Transformer - eChalk
Transformers
ANSWERS
In text questions:
(a) The magnetic field in the core
would be much weaker
because the core is not a
magnetic material.
(b) The lamp would be brighter.
(c)
The lamp would not light up
with direct current in the
primary coil.
(d) 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.
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
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
=
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 ___________
potential
alternating
difference level to another. They do not work with
direct
____________current.
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 lost to heat due to the _________
resistance
voltage reduces the ________
of the power lines.
WORD SELECTION:
energy
direct
increase
National
400 kV
secondary
resistance
alternating 25 kV
Transformers and the National Grid
Notes questions from pages 260 & 261
1.
2.
3.
4.
5.
6.
7.
8.
9.
(a) Why are transformers used in the National grid? (b) What is the
advantage of using high voltages?
Copy the transformer equation on page 260.
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.
What is the purpose of (a) step-up and (b) step-down transformers?
Explain how the number of turns on the coils of a transformer determine
whether a transformer is step-up or step-down.
State how the currents and voltages associated with the primary and
secondary coils are related to each other with a 100% efficient
transformer.
Copy and answer questions (a) and (b) on page 261.
Copy the ‘Key points’ table on page 261.
Answer the summary questions on page 261.
Transformer - eChalk
Transformers and the National Grid
ANSWERS
In text questions:
(a) 60 turns
(b) (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
More power to you
Notes questions from pages 262 & 263
1. Answer questions 1 and 2 on page 263.
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
More power to you
ANSWERS
1.
2.
(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.
(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)
b)
c)
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.
Height on the X-axis, voltage
on the Y-axis. Axes fully
labelled and plots correctly
plotted.
In part. The voltage
increased as height
increased, but it was not
directly proportional.
d) 0.01V
e) Not at the greater heights.
f) Improve the sensitivity of the
oscilloscope. Repeat his
results.
g) By checking it against other
data/other similar research/get
someone else to repeat his
work or calculate theoretical
relationships.
h) For example: Measuring the
speed of an object through a
tube.