© boardworks ltd 2003 ks4 electricity – uses of electromagnetism

© Boardworks Ltd 2003

KS4 Electricity – Uses of Electromagnetism


Relays

M

A relay allows one circuit to control another circuit – turning it on and off.

A B

What will happen when the switch in circuit A is closed?

N

S

The coil becomes an electromagnet as electricity flows in circuit A.

The coil attracts the steel switch in circuit B and completes the circuit.

Electricity flows in circuit B and the motor now works.

Steel switch

Coil


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.


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!


Loudspeakers

Varying an AC through the wire makes the cone of the loudspeaker vibrate back and forth. The vibrations cause sounds.


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


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.


Flemings Left Hand Rule

Force (thumb)

Field (first finger)

Current (second finger)


Animation of a current carrying wire in a magnetic field


How could you increase the strength of the force?

More current

More resistance

Stronger

magnetic field

TRUE/FALSE

TRUE/FALSE

TRUE/FALSE


The DC Motor

N

How does it work?

Each side of coil pushed in opposite direction due to motor effect.

Uses Motor effect principle.

S

This results in the coil rotating.

What happens if you reverse the direction of

current or magnetic field?

The coil will spin in opposite direction.


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.


Stationery magnet

What happens when the magnet is in the coil?

Nothing!!

A current is only induced when the magnet is moving.


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.


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


Increasing the size of the current

How can you increase the size of the current?

more coils

faster movement

stronger magnetic field


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 ?


Transformers

What is a transformer?

Where are transformers used?

What do we call a transformer that increases voltage?

What do we call a transformer that decreases voltage?

A device used to increase or decrease voltage

On the National Grid, household appliances

STEP-UP Transformer

STEP-DOWN Transformer


How electricity gets to your home….

Step-up transformer

Power station

National Grid

Step-down transformer Homes, businesses

and factories etc


Why bother with transformers?

When electrical current flows you get f_____ in the conductor. With friction you always get h___. This heat is wasted e____. You get more wasted energy at low v______ than high voltages. So the current is increased in voltage before it is transferred to the N_____ G__. In this way less energy is lost. Then, when it gets to your home, a school or a factory, the voltage is stepped d___.

oltages

eatnergy

riction

ational rid

own


Step-down

More/less turns on the primary coil than the secondary coil.

Primary Secondary


Step-up

More/less turns on the primary coil than the secondary coil.

Primary Secondary


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!) -

V1 = N1

V2 = N2

V – voltage N - number of turns


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?

a) What is the output voltage?

Step-down

230V


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?

a) How many turns are on the secondary coil?

Step-up

500 turns


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


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


Which below will not increase the strength of an electromagnet?

A. More coils

B. More current

C. Less current

D. Iron core


Which use of electromagnets is similar in function to a fuse?

A. Relays

B. Transformer

C. Circuit breaker

D. Loud speaker


Which of the following is not a use of electromagnets?

A. Fridge magnets

B. Loud speakers

C. Relays

D. Circuit breakers


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 ks4 electricity – uses of electromagnetism

Documents

direction of current

coil of wire

magnetic field slide

steel switch coil slide

current flows

finger slide

high slide

finger current