Transcript Electricity - Uses of Electromagnetism

KS4 Physics
Uses of
Electromagnetism
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Contents
Uses of Electromagnetism
Electromagnetic devices
Magnetic force of a current
Electromagnetic induction
Transformers
Summary activities
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What is a relay?
A relay allows one circuit to control another circuit by
turning it on and off.
What will happen when the switch in circuit A is closed?
N
coil
steel
switch
M
S
A
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B
As electricity flows in
circuit A, the coil becomes
an electromagnet.
The coil attracts the steel
switch in circuit B, which
then closes and
completes this circuit.
Electricity then flows in
circuit B and operates
the motor.
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How does a circuit breaker work?
A circuit breaker is based on the same principle as the relay.
Current flows through a coil of wire. If the current becomes
dangerously high, the coil creates a magnetic field strong
enough to attract the iron catch.
This iron catch separates from the upper contact and so
breaks the circuit.
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Using electromagnets in switches
Electromagnets can be used to open switches or
to close switches.
Is there a circuit breaker like this in your laboratory?
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How a loudspeaker uses an electromagnet
Varying an a.c. through the wire makes the loudspeaker
cone vibrate back and forth. The vibrations cause sounds.
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Uses of electromagnets
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circuit breaker
Uses one circuit
to turn another
circuit off and on.
relay
Converts electrical
energy into
sound energy.
loudspeaker
Used to break
a circuit if the
current is too high.
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Contents
Uses of Electromagnetism
Electromagnetic devices
Magnetic force of a current
Electromagnetic induction
Transformers
Summary activities
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A current-carrying wire in a magnetic field
A wire carrying a current can be made into an electromagnet
and will behave like a magnet.
If this wire is placed near a permanent magnet, it will
experience a force.
Fleming’s left-hand rule can be used to predict the direction
of this force.
This rule can also be used to predict the direction of any one
of the following (if the the direction of the other two is known):
 the direction of flow of conventional current;
 the direction of the magnetic field (from north to south);
 the direction of the force exerted on the wire.
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Fleming’s left-hand rule
To use Fleming’s left-hand rule, hold the thumb and the first
two fingers of your left hand at right angles to each other.
direction of
force
(thumb)
direction of
magnetic field
(first finger)
direction of
current
(second finger)
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Using Fleming’s left-hand rule
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Increasing the strength of the force
Which of the following would increase the strength of the
force on a wire carrying a current?
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more current
TRUE/FALSE
more resistance
TRUE/FALSE
stronger
magnetic field
TRUE/FALSE
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The DC motor
A DC motor works using the principle of the motor effect.
When a DC current flows in
the coil, the sides of coil are
pushed in opposite
directions because of the
motor effect.
N
S
coil
This results in the coil rotating.
d.c. current
What happens to the coil if the direction of the current or
the magnetic field is reversed?
The coil will spin in the opposite direction.
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Contents
Uses of Electromagnetism
Electromagnetic devices
Magnetic force of a current
Electromagnetic induction
Transformers
Summary activities
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Electromagnetic induction
What happens if a magnet is moved into a coil of wire?
A current is induced in the coil when the magnet is moving.
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Stationary magnet
What happens when the magnet is inside the coil?
Nothing! A current is only induced when the magnet is moving.
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Electromagnetic induction
What happens if the magnet is moved out of the coil of wire?
A current is induced in the coil in the opposite direction.
If the direction of movement is reversed, then the direction
of the induced current is also reversed.
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Inducing current
What will happen if a magnet is moved in and out of a coil
of wire repeatedly?
A current will be induced in the coil. When the magnet is
moved into the coil, the current will flow in one direction.
Then, when the magnet is moved out of the coil, the
direction of the current is reversed and flows in the opposite
direction.
The induced current is constantly changing direction.
What type of current is this?
alternating current
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Increasing the size of the induced current
What are the four ways in which the (induced)
current from an AC generator can be increased?
faster movement
1. _________________________
stronger magnetic field
2. _________________________
3. _________________________
more coils
4. _________________________
larger area of coils
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Contents
Uses of Electromagnetism
Electromagnetic devices
Magnetic force of a current
Electromagnetic induction
Transformers
Summary activities
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Transformers
What is a transformer?
A device used to increase or
decrease voltage.
Where are transformers used?
In the national grid and
household appliances.
What do we call a transformer that increases voltage?
A step-up transformer.
What do we call a transformer that decreases voltage?
A step-down transformer.
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How does electricity get to your home?
power station
national grid
step-up
transformer
homes, factories,
businesses, etc.
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step-down
transformer
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Why are transformers useful?
riction is generated
When electrical current flows, f______
eat
in the conductor which in turn generates h___.
This heat is wasted e_____.
nergy More wasted energy is
oltages than high voltages.
created at low v_______
The voltage of the current is increased before it is
transferred to the n______
ational g___,
rid so that less energy
is lost. Then, when the current gets to homes,
schools or factories, the voltage is stepped d____.
own
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Which type of transformer?
Is this a step-up or a step-down transformer?
secondary
primary
coil
coil
This a step-down transformer because there are
less turns in the secondary coil than the primary coil.
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Which type of transformer?
Is this a step-up or a step-down transformer?
secondary
primary
coil
coil
This a step-up transformer because there are
more turns in the secondary coil than the primary coil.
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Transformer calculations
The size of the output voltage from a transformer depends on
the ratio of the turns on the primary and secondary coils.
This can be calculated using the following formula:
V1
V2
=
N1
N2
In this formula, V is the voltage in a coil and N is the number
of turns in that coil.
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Transformer question (1)
A transformer has 200 turns on its primary coil and 50 turns
on its secondary coil. The input voltage is 920 V.
a) Is this a step-up or step-down transformer? step-down
a) What is the output voltage?
V2
V1
=
V2
=
V2
=
=
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N2
N1
N2
x V1
N1
50
x 920
200
230 V
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Transformer question (2)
A transformer has 100 turns on its primary coil. It has an
input voltage of 35 V and an output voltage of 175 V.
a) Is this a step-up or step-down transformer? step-up
a) How many turns are on the secondary coil?
N2
N1
=
N2
=
N2
=
=
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V2
V1
V2
x N1
V1
175
x 100
35
500 turns
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Contents
Uses of Electromagnetism
Electromagnetic devices
Magnetic force of a current
Electromagnetic induction
Transformers
Summary activities
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Glossary
 circuit breaker – An electromagnetic switch which
breaks the circuit if too much current flows.
 Fleming’s left-hand rule – The rule used to work out
the direction of the force when a current is at right angles to
a magnetic field.
 induction – Generating a voltage in a wire by moving a
magnetic field near the wire.
 motor effect – The effect by which a wire carrying a
current in a magnetic field moves.
 relay – An electromagnetic switch which allows one circuit
to control another.
 transformer – A device which increases or decreases the
voltage of alternating current electricity.
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Anagrams
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Multiple-choice quiz
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