Download Physics_A2_37_GeneratingElectricity

Book Reference : Pages 120-122
1.
To understand how to generate electricity
using electromagnetic induction
2.
To be able to establish the relative direction
of the field, motion & induced current by
using the “dynamo rule”
With the exception of photovoltaic cells, every other
means of practical electricity generation relies on an
alternator or dynamo to convert rotational kinetic
Xturbgen1.swf
energy in to electricity.
Xph_energy05.swf
The kinetic energy is either available directly : Wind
power, hydroelectric, wave power & tidal power
Or a fuel is used to produce heat which in turn produces
steam which spins a steam turbine to provide the kinetic
energy.
We have seen that a conductor experiences a force
when it carries current perpendicular to a magnetic
field. This is the basis for the electric motor.
What will happen if the same set up is utilised but with
no supplied current & with an external force providing
perpendicular motion to the wire?
Supplied
Motion
N
S
2_animagalvo1.swf
generator_en.jar
Which factors effect the magnitude & direction of the electricity produced?
1. The strength of the magnetic field
2. How fast the wire is moved (no movement, no
current)
3. The area of wire in the field (i.e. Make a coil)
4. The direction the wire is moved (Should be
perpendicular to field). The wire must “cut” the field
lines
5. Note motion only needs to be relative, we can move
either the coil or the permanent magnet
This process is called Electromagnetic Induction and an
induced emf (electromotive force) causes the electrons
to flow around the circuit
Once again we have a conductor carrying a current
perpendicular to a magnetic field. What will the
conductor experience?
Supplied
Motion
N
S
Motor
force on
conductor
The current carrying conductor experiences a “motor
force” which opposes the supplied motion. “Work” must
be done to keep the dynamo spinning. (Assuming no
losses) the work done spinning the dynamo will equal
the energy transferred to the circuit (e.g. To light a lamp)
The rate at which energy is transferred from the source
of motion is equal to the electrical power supplied to
the components in the circuit :
Electrical power = induced EMF x Current
(voltage)
Induced EMF is the energy supplied to each unit charge
& current is the charge flow per second
Electrical Power = Energy transferred per s from source
We have seen that a charged particle in an electric field
experiences a force
Beam of
electrons
Magnetic Field
Into page
Force
Resultant
direction
A conductor can be thought of as a tube containing lots
of free electrons. If the “tube” crosses a magnetic field,
then the electrons will experience a force which moves
them to one end. Thus one end of the wire becomes
negative relative to the other. An electromotive force is
induced in the wire
Conductor
Relative +ve
charge
Direction of
Movement
+++
Magnetic Field
Into page
-
Free Electron
Force
---
Relative –ve
charge
When part of a complete circuit, the induced EMF causes
a current to flow in the circuit
Thumb
(Motion)
First
(Field)
We have previously used
“Fleming’s left hand motor
rule”. Since a dynamo is
effectively an electric motor
used backwards we can
apply a similar rule here
“Fleming's right hand
dynamo rule”
Second
(Current)
Remember CONVENTIONAL
CURRENT