Electromagnets

Electromagnets differ from normal magnets in one major way! They are made by passing an electric current through a wire that has been wrapped around iron. The current creates a

magnetic field and magnetises the iron core. When the current is turned off the iron loses its magnetism

LO: Understand the uses of electromagnets

Electric bellLO: Understand the uses of electromagnets

Circuit breakerLO: Understand the uses of electromagnets

Electrical relayLO: Understand the uses of electromagnets

Scrapyard craneLO: Understand the uses of electromagnets

The Motor Effect

If a wire carrying a current is placed into a magnetic field, an interesting thing happens.

LO: Understand how electromagnets can be used to make things move

Fleming’s left hand ruleLO: Understand how electromagnets can be used to make things move

Electric MotorLO: Understand how electromagnets can be used to make things move

An electric motor uses the motor effect of electromagnets to create motion.• The force on one side

of the wire causes it to move up

• The force on the other side of the wire causes it to move down

• The motor rotates!

Electric MotorLO: Understand how electromagnets can be used to make things move

Graphite ‘brushes’ are used to connect the split-ring to the battery. This is used because:• Graphite is an

excellent conductor• It causes very little

friction on the conducting ring

Electromagnetic Induction

KEYWORDS: Electromagnetic induction, Generator, coil, wire, magnet

Understand how generators create

electricity

ALL – State how electricity can be createdMOST – Describe how electromagnetic induction can be increasedSOME – Explain generators work in detail

Electromagnetic induction

Electromagnetic induction is the term used to describe how electricity is created in a wire. This is ONE of only TWO ways that humans know how to generate electricity

LO: Understand how generators create electricity

Electromagnetic inductionBy moving the magnet within the coil of wire, current can be induced within the current. Note, that the current is only induced when the magnet or coil is moving!

How can the amount of current induced be increased?• Increase the number of coils• Increase the speed at which the

magnet/coil moves• Increase the strength of the magnet

LO: Understand how generators create electricity

Electric generatorsLO: Understand how generators create electricity

True or false?Copy the true sentences and change the false sentences to make them true:1) Electricity is induced in a wire using a coil of wire and

an electromagnet2) Electricity can only be generated when a magnet is

stationary within a coil of wire3) The amount of electricity induced can be decreased

by increasing the speed at which the magnet/coil of wire moves

4) The amount of electricity induced can be increased by increasing the number of coils of wire

LO: Understand how generators create electricity

Transformers

KEYWORDS: transformer, core, step up, Step down, primary, secondary, induced

Understand how transformers work

ALL – State the function of step up and step down transformersMOST – Explain how transformers work and perform transformer calculationsSOME – Explain why values from equations are just an approximation

Step-up and Step-downTransformers are an essential part of the national grid. They help to increase the voltage after the power station and help to decrease it again when it gets to your house.

LO: Understand how transformers work

Why do transformersIncrease and decrease

The voltage in this way?

TransformersLO: Understand how transformers work

How do transformers work?

Discuss with the people on your table how transformers work. Remember the following:• They only work with an a.c.

current• They need to have an iron

core• They use electromagnetic

induction to work

LO: Understand how transformers work

How transformers work1. The alternating current in the primary coil makes the iron

core into an electromagnetic2. As the current is alternating, the magnetic field ‘moves’

and also changes direction3. This ‘moving’ magnetic field causes a current to be

induced in the secondary coil4. If the number of coils on the secondary is higher, the

potential difference will increase and it is a step-up transformer

5. If the number of coils on the secondary is lower, the potential difference will decrease and it is a step-down transformer

LO: Understand how transformers work

Make a flow diagram of how a transformer works in your book and draw a diagram to go with each

stage

Transformer calculationsThe following equation links together the voltage and number of coils in a transformer:

LO: Understand how transformers work

𝑉 𝑝

𝑉 𝑠

=𝑛𝑝

𝑛𝑠

Vp = Voltage on primary coil (v)Vs = Voltage on secondary coil (v)np = number of turns on primary coilns = number of turns on secondary coil

Example questionA transformer has 400 turns on the primary coil and 200 on the secondary. If the primary voltage is 300v, what will the secondary voltage be?

LO: Understand how transformers work

Example questionA transformer has 175 turns on the primary coil and 525 on the secondary. If the secondary voltage is 300v, what will the primary voltage be?

LO: Understand how transformers work

Transformer efficiency

Transformers are extremely efficient

devices.

LO: Understand how transformers work

Transformer efficiency

Transformers are usually about 98% efficient. We can round this up and say that this is approximately 100%. Therefore, whatever power the device uses, it will output the same amount of power!

Power in = Power out

LO: Understand how transformers work

Transformer efficiency

Power in = Power out

Vp x Ip = Vs x Is

LO: Understand how transformers work

Vp = Voltage on primary coil (v)Vs = Voltage on secondary coil (v)Ip = Current on primary coil (A)Is = Current on secondary coil (A)

Example questionA transformer has a primary voltage of 30v and a primary current of 2A. If the secondary voltage is 15v, what will the secondary current be?

LO: Understand how transformers work

Example questionA transformer has a primary current of 10A. If the secondary voltage is 2v and the secondary current is 20A, what will the primary voltage be?

LO: Understand how transformers work

Example questions1. A transformer a primary current of 30A. If the secondary

voltage is 10v and the secondary current is 2.4A, what will the primary voltage be?

2. A transformer has a secondary voltage of 15v and a secondary current of 4A. If the primary voltage is 2v, what will the primary current be?

3. A transformer has a primary voltage of 3v and a primary current of 30A. If the secondary current is 1.5A, what will the primary voltage be?

4. A transformer has 300 turns on the primary coil, a primary voltage of 20V and a primary current of 5A. If the secondary coil has 200 turns, calculate the voltage and current on the secondary coil.

LO: Understand how transformers work