Transcript Electricity – Uses of Electromagnetism

KS4 Electricity –
Uses of Electromagnetism
© Boardworks Ltd 2003
Relays
A relay allows one circuit to control another circuit – turning
it on and off.
What will happen when the switch in circuit A is closed?
N
Coil
The coil becomes an
electromagnet as
electricity flows in
circuit A.
Steel
switch
M
The coil attracts the steel
switch in circuit B and
completes the circuit.
S
A
B
Electricity flows in circuit
B and the motor now
works.
© Boardworks Ltd 2003
Circuit Breakers
Circuit breakers are also based on this principal.
The current flows through a coil of wire.
If the current becomes dangerously high, the coil creates a
magnetic field strong enough to…
…cause the iron or steel switch to open…
…which breaks the circuit.
© Boardworks Ltd 2003
Using electromagnets in switches
Electromagnets can be used to OPEN
switches or to CLOSE switches.
There could be one of these circuit breakers
in your laboratory!
© Boardworks Ltd 2003
Loudspeakers
Varying an AC through the wire makes the cone of the
loudspeaker vibrate back and forth. The vibrations cause sounds.
© Boardworks Ltd 2003
Using electromagnets
Circuit breakers
Relays
Loudspeakers
Convert electrical
energy into sound
energy
Using one circuit to
turn another circuit off
and on
Used to break a circuit
if the electrical current
is too high
© Boardworks Ltd 2003
A current carrying wire in a magnetic field
We know that a wire carrying a current can be made into an
electromagnet. Consequently, such a wire will behave like a
magnet. If this wire is placed near a permanent magnet, it
will experience a force.
We can use Flemings left hand rule to predict the direction of
this force. In fact, we can also use it 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.
© Boardworks Ltd 2003
Flemings Left Hand Rule
Force (thumb)
Field (first finger)
Current (second finger)
© Boardworks Ltd 2003
Animation of a current carrying wire in a
magnetic field
© Boardworks Ltd 2003
How could you increase the strength of the
force?
More current
TRUE/FALSE
More resistance
TRUE/FALSE
Stronger
TRUE/FALSE
magnetic field
© Boardworks Ltd 2003
The DC Motor
Uses Motor effect principle.
How does it work?
Each side of coil pushed
in opposite direction due
to motor effect.
This results in the
coil rotating.
N
S
What happens if you
reverse the direction of
The coil
spin in
current
orwill
magnetic
opposite
direction.
field?
© Boardworks Ltd 2003
Electromagnetic induction
What will happen if you move a magnet into a coil of wire?
A current is induced in the coil.
Only when the magnet is moving.
© Boardworks Ltd 2003
Stationery magnet
What happens when the magnet is in the coil?
Nothing!!
A current is only induced when the magnet is moving.
© Boardworks Ltd 2003
Electromagnetic induction
What will happen if you move the magnet out of the coil of wire?
A current is induced in the coil.
This time in the opposite direction.
If you reverse the direction of the movement, then
you reverse the direction of the current.
© Boardworks Ltd 2003
AC or DC?
What will happen if you move a magnet in and out
of the coil repeatedly?
A current will be induced in the coil, the current will flow
in one direction and then reverse and flow in the
opposite direction.
What type of current is this?
Alternating current
© Boardworks Ltd 2003
Increasing the size of the current
How can you increase the size of the current?
more coils
stronger magnetic field
faster movement
© Boardworks Ltd 2003
AC generator
What are the four ways you can increase the
current from an AC generator?
1.
2.
3.
4.
Faster movement
_____________________
Stronger magnetic field
_____________________
More coils
_____________________
Larger area of coils
_____________________
?
© Boardworks Ltd 2003
Transformers
What is a transformer?
A device used to increase or decrease voltage
Where are transformers used?
On the National Grid, household appliances
What do we call a transformer that
increases voltage?
STEP-UP Transformer
What do we call a transformer that
decreases voltage?
STEP-DOWN Transformer
© Boardworks Ltd 2003
How electricity gets to your home….
Power station
Step-up
transformer
National Grid
Step-down
transformer
Homes, businesses
and factories etc
© Boardworks Ltd 2003
Why bother with transformers?
riction in
When electrical current flows you get f_____
eat
the conductor. With friction you always get h___.
nergy You get more wasted
This heat is wasted e____.
oltages than high voltages. So the
energy at low v______
current is increased in voltage before it is
ational G__.
rid In this way less
transferred to the N_____
energy is lost. Then, when it gets to your home, a
own
school or a factory, the voltage is stepped d___.
© Boardworks Ltd 2003
Step-down
More/less turns on the primary coil than
the secondary coil.
Primary
Secondary
© Boardworks Ltd 2003
Step-up
More/less turns on the primary coil
than the secondary coil.
Primary
Secondary
© Boardworks Ltd 2003
Transformer calculations
The size of the output voltage depends upon the ratio of
the turns on the primary and secondary coils.
Guess what? There’s a formula (it is Physics after all!) -
V – voltage
V1 =
N1
V2 =
N2
N - number of turns
© Boardworks Ltd 2003
Transformer Question 1
A transformer has 200 turns on its primary coil
and 50 turns on its secondary coil. The input
voltage is 920V.
a) Is is a step-up or step-down transformer?
Step-down
a) What is the output voltage?
230V
© Boardworks Ltd 2003
Transformer Question 2
A transformer has 100 turns on its primary coil. It
has an input voltage of 35V and an output
voltage of 175V.
a) Is is a step-up or step-down transformer?
Step-up
a) How many turns are on the secondary coil?
500 turns
© Boardworks Ltd 2003
What happens if you reverse the direction of
current in an electromagnet?
A. The magnetic field is increased
B. The voltage increases
C. The current increases
D. The direction of the magnetic

field is reversed
© Boardworks Ltd 2003
What happens if a wire carrying a current is
placed in a magnetic field?
A. The current is increased
B. The magnetic field is reversed
C. The magnetic field is increased
D. The wire will experience a force

© Boardworks Ltd 2003
Which below will not increase the strength of an
electromagnet?
A. More coils
B. More current
C. Less current

D. Iron core
© Boardworks Ltd 2003
Which use of electromagnets is similar in
function to a fuse?
A. Relays
B. Transformer
C. Circuit breaker

D. Loud speaker
© Boardworks Ltd 2003
Which of the following is not a use of
electromagnets?
A. Fridge magnets

B. Loud speakers
C. Relays
D. Circuit breakers
© Boardworks Ltd 2003
A transformer has 30 turns on its primary coil
and 240 turns on its secondary coil. If the output
voltage is 128V, what is the input voltage?
A. 8V
B. 16V

C. 1024V
D. 512V
© Boardworks Ltd 2003