Book Reference : Pages 120-122Book Reference : Pages 120-122

1.1. To understand how to generate electricity To understand how to generate electricity using electromagnetic inductionusing electromagnetic induction

2.2. To be able to establish the relative direction To be able to establish the relative direction of the field, motion & induced current by of the field, motion & induced current by using the “dynamo rule”using the “dynamo rule”

With the exception of photovoltaic cells, every other With the exception of photovoltaic cells, every other means of practical electricity generation relies on an means of practical electricity generation relies on an alternatoralternator or or dynamodynamo to convert rotational kinetic to convert rotational kinetic energy in to electricity.energy in to electricity.

The kinetic energy is either available directly : Wind The kinetic energy is either available directly : Wind power, hydroelectric, wave power & tidal powerpower, hydroelectric, wave power & tidal power

Or a fuel is used to produce heat which in turn produces Or a fuel is used to produce heat which in turn produces steam which spins a steam turbine to provide the kinetic steam which spins a steam turbine to provide the kinetic energy.energy.

Xturbgen1.swfXph_energy05.swf

We have seen that a conductor experiences a force We have seen that a conductor experiences a force when it carries current perpendicular to a magnetic when it carries current perpendicular to a magnetic field. This is the basis for the electric motor.field. This is the basis for the electric motor.

What will happen if the same set up is utilised but with What will happen if the same set up is utilised but with no supplied current & with an external force providing no supplied current & with an external force providing perpendicular motion to the wire? perpendicular motion to the wire?

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Supplied Motion

Which factors effect the magnitude & direction of the electricity produced?Which factors effect the magnitude & direction of the electricity produced?

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1.1. The strength of the magnetic fieldThe strength of the magnetic field

2.2. How fast the wire is moved How fast the wire is moved (no movement, no (no movement, no current)current)

3.3. The area of wire in the field (i.e. Make a coil)The area of wire in the field (i.e. Make a coil)

4.4. The direction the wire is moved (Should be The direction the wire is moved (Should be perpendicular to field). perpendicular to field). The wire must “cut” the field The wire must “cut” the field lineslines

5.5. Note motion only needs to be relative, we can move Note motion only needs to be relative, we can move eithereither the coil or the permanent magnet the coil or the permanent magnet

This process is called Electromagnetic Induction and an This process is called Electromagnetic Induction and an induced emf (electromotive force) causes the induced emf (electromotive force) causes the electrons to flow around the circuitelectrons to flow around the circuit

Once again we have a conductor carrying a current Once again we have a conductor carrying a current perpendicular to a magnetic field. What will the perpendicular to a magnetic field. What will the conductor experience?conductor experience?

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Supplied Motion

Motor force on conductor

The current carrying conductor experiences a “motor The current carrying conductor experiences a “motor force” which opposes the supplied motion. “Work” must force” which opposes the supplied motion. “Work” must be done to keep the dynamo spinning. (Assuming no be done to keep the dynamo spinning. (Assuming no losses) the work done spinning the dynamo will equal losses) the work done spinning the dynamo will equal the energy transferred to the circuit (e.g. To light a the energy transferred to the circuit (e.g. To light a lamp)lamp)

The The raterate at which energy is transferred from the source at which energy is transferred from the source of motion is equal to the electrical power supplied to the of motion is equal to the electrical power supplied to the components in the circuit :components in the circuit :

Electrical power =Electrical power = induced EMF x Currentinduced EMF x Current(voltage)(voltage)

Induced EMF is the energy supplied to each unit charge Induced EMF is the energy supplied to each unit charge & current is the charge flow per second& current is the charge flow per second

Electrical Power = Energy transferred per s from sourceElectrical Power = Energy transferred per s from source

We have seen that a charged particle in an electric field We have seen that a charged particle in an electric field experiences a forceexperiences a force

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Magnetic Field Into page

Force

Beam of electrons

Resultantdirection

A conductor can be thought of as a tube containing lots A conductor can be thought of as a tube containing lots of free electrons. If the “tube” crosses a magnetic field, of free electrons. If the “tube” crosses a magnetic field, then the electrons will experience a force which moves then the electrons will experience a force which moves them to one end. Thus one end of the wire becomes them to one end. Thus one end of the wire becomes negative relative to the other. An electromotive force is negative relative to the other. An electromotive force is induced in the wireinduced in the wire

When part of a complete circuit, the induced EMF causes When part of a complete circuit, the induced EMF causes a current to flow in the circuita current to flow in the circuit

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Magnetic Field Into page

Force

Direction ofMovement

Free Electron

- - -

+ + +

Conductor

Relative –ve Relative –ve chargecharge

Relative +ve Relative +ve chargecharge

We have previously used We have previously used “Fleming’s left hand motor “Fleming’s left hand motor rule”. rule”. Since a dynamo is Since a dynamo is effectively an electric motor effectively an electric motor used backwards we can used backwards we can apply a similar rule hereapply a similar rule here

““Fleming's right hand Fleming's right hand dynamo rule”dynamo rule”

Remember CONVENTIONAL Remember CONVENTIONAL CURRENTCURRENT

TThumb humb ((MMotion)otion)

SeSeccond ond ((CCurrent)urrent)

FFirstirst((FField)ield)