Electromagnetism

ElectromagnetismElectromagnetism

GCSE PhysicsGCSE Physics

Learning IntentionsLearning Intentions

Recall the significant link between Recall the significant link between electricity and magnetismelectricity and magnetism

Observe how a magnetic field can Observe how a magnetic field can interact with a conducting wireinteract with a conducting wire

State how electricity can be generatedState how electricity can be generated

Electrical GeneratorsElectrical Generators

Investigate…Investigate…

How does a magnet affect the loop of How does a magnet affect the loop of conducting wire?conducting wire?

What has to happen for the current to flow?

How is the direction of the current flowing related to the magnet?

How is it possible to

increase the amount of current? Is there more

than one way of doing this?

How is it possible for

current to flow without moving

the magnet?

The Not-so-Missing LinkThe Not-so-Missing Link

MagnetismMagnetism is closely related to is closely related to ElectricityElectricity. . These two characteristics combine to make These two characteristics combine to make one of the fundamental forces in physics one of the fundamental forces in physics known as known as ElectroElectromagnetismmagnetism..

Where there is a Where there is a magnetic fieldmagnetic field there is the there is the potential to make an potential to make an electric current flowelectric current flow

Where there is a flow of an Where there is a flow of an electric currentelectric current there is an associated there is an associated magnetic fieldmagnetic field..

Uniform Magnetic Field between poles

Move the wire through the

magnetic field

Making ElectricityMaking Electricity

Move the wireMove the wire downdown cutting throughcutting through the the magnetic fieldmagnetic field

0

Induces a current

Making ElectricityMaking Electricity

Move the wire Move the wire upup cutting throughcutting through the the magnetic fieldmagnetic field

0

Induces a current in the other direction

0

0

No Current Induced when…No Current Induced when…

… the movement of the wire does not does not cut throughcut through the magnetic field lines

… the wire does does not movenot move

Electromagnetic InductioElectromagnetic Induction (pg156)n (pg156)

This time use a wire coil (solenoid) and This time use a wire coil (solenoid) and move the magnet in and outmove the magnet in and out

0 0

Magnet moved in

Magnet moved out

Induced CurrentInduced Current

Move the magnet into the coil and Move the magnet into the coil and a a current is inducedcurrent is induced in one direction in one direction

Move the magnet out of the coil and a Move the magnet out of the coil and a current is induced current is induced in the other directionin the other direction

As soon as the As soon as the motion stopsmotion stops the current the current will stopwill stop

For a current to be induced the magnet For a current to be induced the magnet field lines need to continually be field lines need to continually be cutting cutting throughthrough the wires the wires

Increase the SIZE of Increase the SIZE of current/voltagecurrent/voltage

Increase the speedIncrease the speed of the movement of of the movement of the magnet towards/away from the coilthe magnet towards/away from the coil

Use a more Use a more powerful powerful magnet (more magnet (more magnetic field lines produced)magnetic field lines produced)

Increase the Increase the number of coilsnumber of coils in the wire in the wire (more wire being cut through by magnetic (more wire being cut through by magnetic field lines)field lines)

For current to flow…For current to flow…

The magnetic field lines must continually The magnetic field lines must continually be cut through by the wire (through the be cut through by the wire (through the lines of force)lines of force)

Work is doneWork is done against the force of the against the force of the magnet and an energy transfer occursmagnet and an energy transfer occurs

KineticKinetic ElectricalElectrical


Recall the significant link between electricity Recall the significant link between electricity and magnetismand magnetism

Recognise how a solenoid can induce a Recognise how a solenoid can induce a current in a coil of conducting wire current in a coil of conducting wire

State the need for AC supplied to a solenoid to State the need for AC supplied to a solenoid to continually induce a currentcontinually induce a current

Understand how a transformer can be used to Understand how a transformer can be used to change voltage between two circuitschange voltage between two circuits

How does it work?How does it work?

Alternating Current (AC), one way and then another

The magnetic field lines are cutting through the wires in the secondary coil- inducing a current!

Primary Primary CoilCoil

Secondary Secondary CoilCoil

Pass a current through a coil of wire and a magnetic field is

created.

When a magnetic field is at right angles to a wire, a current is

induced in that wire



Increase the Current and the associated magnetic

field becomes stronger

A stronger magnetic field will induce a larger current in that wire



Decrease the Current and the associated magnetic field

becomes weaker.

A weaker magnetic field will induce a smaller current in that wire



A current which is kept constant in one direction is

known as Direct Current (D.C.)

If the magnetic field does not vary then the current will no longer

be induced and will not flow



A current which flows in one direction and then in

the other direction is called an Alternating

Current (A.C.)

If the magnetic field is changing all the time then a current will

continually be induced.

Transformers (pg 161)Transformers (pg 161)

What is a transformer?What is a transformer?

A transformer consists of A transformer consists of two coilstwo coils wrapped around a laminated wrapped around a laminated soft iron soft iron core.core.

Primary Coil

Secondary Coil

Transformer’s SymbolTransformer’s SymbolIron Core

Types of TransformersTypes of Transformers

Primary coils < Secondary coilsPrimary coils < Secondary coils

Step Up

The Iron Core links the two coils magnetically

Step UpStep Up

A step up transformer has A step up transformer has more turns of more turns of wire on the secondary coilwire on the secondary coil, which makes , which makes a larger induced voltage in the secondary induced voltage in the secondary coil.coil.

It is called a step up transformer because It is called a step up transformer because the the voltage output is stepped upvoltage output is stepped up

Step Down TransformerStep Down Transformer

Primary Coils > Secondary CoilsPrimary Coils > Secondary Coils

Step Down

The Core is made of iron so it can be easily magnetised and demagnetised.

Step DownStep Down

A step down transformer A step down transformer has less turns has less turns of wireof wire on the secondary coil, which on the secondary coil, which makes a smaller induced voltage in the makes a smaller induced voltage in the secondary coil.secondary coil.

It is called a step down transformer It is called a step down transformer because the because the voltage outputvoltage outputis stepped downis stepped down..

Transformers EquationTransformers Equation

Voltage Across PrimaryVoltage Across Primary

Voltage Across SecondaryVoltage Across Secondary

Number of turns on PrimaryNumber of turns on Primary

Number of turns on SecondaryNumber of turns on Secondary=

VVpp NNpp

VVSS NNSS

==

Worked ExampleWorked Example

A A voltage of 240Vvoltage of 240V is applied to a primary is applied to a primary coil of coil of 200 turns200 turns. What is the voltage . What is the voltage across the secondary coil if it has across the secondary coil if it has 10 10 turnsturns??

Vp Np

VS NS

=

200

VS 10=240

VS= 240 x10200

= 12 VoltsVS


Recall the significant link between electricity Recall the significant link between electricity and magnetismand magnetism

Recognise how the electrical power generated Recognise how the electrical power generated by a turbine is by a turbine is A.C.A.C.

Recall how electricity is transmitted through out Recall how electricity is transmitted through out the country by the country by The GridThe Grid

State the need for step-up and step-down State the need for step-up and step-down transformers for nationwide transformers for nationwide electricity electricity distributiondistribution

A.C. GeneratorA.C. Generator

Alternating Current Generator (alternator) Alternating Current Generator (alternator) - Page 158- Page 158

http://www.sciencejoywagon.com/physicszone/otherpub/wfendt/generatorengl.htm

Katie Explains AC Katie Explains AC generators-generators-

The National Grid (pg The National Grid (pg 159)159)

Electricity generated a Electricity generated a power power stationstation is distributed to homes, is distributed to homes, shops, schools factories etc across shops, schools factories etc across the country by a network of cables the country by a network of cables called the called the National GridNational Grid..

Step up Transformers are used Step up Transformers are used beforebefore transmissiontransmission on the Grid on the Grid

Local Step down Transformers are Local Step down Transformers are used used before consumptionbefore consumption by by homes, schools etchomes, schools etc

Why use Transformers?Why use Transformers?

Page 160 and 161Page 160 and 161

Let’s get active…Let’s get active…

Match the statements to the pictures to Match the statements to the pictures to make up the process for which electricity make up the process for which electricity is distributed across our beautiful is distributed across our beautiful country…country…

The power is then supplied to the The power is then supplied to the homes, factories, homes, factories, offices etcoffices etc for their own consumption. for their own consumption.

The electrical power is connected to the The electrical power is connected to the National Grid National Grid which is a network of cables stretching over the which is a network of cables stretching over the country. It is important that the voltage is high in these country. It is important that the voltage is high in these cables.cables.

A Transformer A Transformer does not create more energydoes not create more energy by by stepping up the voltage. The power generated is equal stepping up the voltage. The power generated is equal to that consumed. The voltage is increased so that the to that consumed. The voltage is increased so that the current is reduced. This means current is reduced. This means less energy will be lost less energy will be lost as heat in the cablesas heat in the cables (smaller resistance). (smaller resistance).

The voltage needs to be The voltage needs to be stepped downstepped down to a lower to a lower value by local transformersvalue by local transformers

In Power Stations, the alternators are driven by huge In Power Stations, the alternators are driven by huge turbines, spun round by the high pressure steam. A turbines, spun round by the high pressure steam. A large current is producedlarge current is produced..

Transformers are used to Transformers are used to step up the voltagestep up the voltage before before the electrical power is distributed across the countrythe electrical power is distributed across the country

To finish withTo finish with

Page 164Page 164

Questions 11 and 12Questions 11 and 12

Education

Electromagnetism