Download The principles of electromagnetic induction

Electromagnetic Induction
Chapter 21
Power Plant
Generating Electricity
 A motor is a device for transforming
electrical energy into mechanical
(kinetic) energy
 To generate electricity we need a device
that will do the opposite
 It must transform mechanical energy into
electrical energy
 Use an electric motor in reverse
 The current has been induced and the
motor is acting like a generator
Generating Electricity
 Many different generator designs, some produce direct current
and some produce alternating current
 Some use permanent magnets and others use electromagnets
 A bicycle could be used as a dynamo
 Turbines – a device that is caused to turn by moving air, steam,
or water, often used to generate electricity
 All generators have:
 A magnetic field (magnets or electromagnets)
 A coil or wire (fixed or moving)
 Movement (the coil and magnetic field move relative to one
another)
 When the coil and the magnetic field move relative to one
another a current flows in the coil if it is part of a complete
circuit and this is know as induced current
 If the generator is not connected up to a circuit, there will be
an induced e.m.f across its ends ready to make a current flow
The principles of electromagnetic
induction
 The process of generating electricity from
motion is called electromagnetic
induction
 Michael Faraday invented the idea of
magnetic field lines and drew field lines to
represent it
 Dynamo effect – electricity is generated
when a coil moves near a magnet
 If the coil is part of a complete circuit the
induced e.m.f. will make an induced
current flow around the circuit
The principles of electromagnetic
induction
 Move one pole of the magnet downwards
past the wire and a current flows
 Move the magnet back upwards and a
current flows in the opposite direction
 Alternatively the magnet can be stationary
and the wire can be moved up and down
 In figure b the same effect can happen
moving the magnet in a coil
 Reverse the magnet to use the opposite
pole and the current flows in the opposite
direction
 Hold the magnet stationary next to the wire
or coil and no current flows. They must move
relative to each other
Increasing the induced e.m.f.
 There are 3 ways to increase the e.m.f.
induced in a coil or wire
 Use a stronger magnet
 Move the wire or coil more quickly relative
to the magnet
 Use a coil with more turns of wire
Generating a.c.
 Faraday’s discovery lead to the creation
of bigger and bigger generators
 Alternating current – (a.c.) electric
current that flows one way, then the other
in a circuit
 The frequency of an a.c. supply is the
number of cycles it produces each
second
Induction and field lines
 Picture the field lines coming out of each
pole of the magnets shown in figure 21.5
 If the magnet is stationary there is no
cutting of field lines so no e.m.f. is induced
 As the magnet is moved, the field lines
are cut by the wire, and it is this cutting of
field lines that induces the current
 If the magnet is further from the wire the
field lines are further apart and so fewer
are cut giving a smaller e.m.f.
 This idea helps us to understand the
factors that affect the magnitude and
direction of the induced e.m.f
 If magnet is moved quickly the lines are cut
more quickly and a bigger e.m.f is induced
 A coil gives a bigger effect than a single
wire
Fleming’s right-hand rule
 When a current flows in a magnetic field
there is a force on it so that it moves. The
directions of force, field and current were
given by Fleming’s left-hand rule
 In electromagnetic induction the
directions are given by Fleming's righthand rule
An a.c. generator
 a.c. generator – a device such as a
dynamo used to generate alternating
current
 A d.c. motor working in reverse
 The other difference is the way the circuit
is connected
 Commutator – a device used to allow
current to flow to and from the coil of a
d.c. motor or generator
 Slip rings – a device used to allow current
to flow to and from the coil of an a.c.
motor or generator
An a.c. generator
 Why does this produce alternating current? As the coil
rotates each side of the coil passes first the magnetic
north pole and then the south pole. This means that
the induced current flows first one way and then the
other
 The current flows out through the slip rings. And each
ring is connected to one end of the coil
 The alternating current flows out through the brushes
which press against the rings
 There are four ways to increase voltage
 Turn the coil more rapidly
 Use a coil with more turns of wire
 Use a coil with a bigger area
 Use stronger magnets
 Spin the coil 50 times and the a.c. generator has a
frequency of 50 Hz.
 Page 297
Direction of the induced e.m.f.
 How does an induced current know in
which direction it must flow?
 The answer is that the current has a
magnetic field around it
 This field pushes back against the field that
is inducing the current
 Page 297 & figure 21.5b
Power lines and transformers
 High voltages leave the power station and
carried by power lines to customers
 Power lines – cables used to carry electricity
from power stations to consumers
 National grid – the system of power lines,
pylons and transformers used to carry
electricity around a country
 When the power lines approach the area
where the power is to be used they enter a
local distribution center where the voltage is
reduced
 Transformers typically reduce it to 230V
Why use high voltages?
 Power is carried by cables both above and below
ground.
 Underground cables are much more expensive
 High voltage means low current and this wastes less
energy
 Energy carried is lost through heat because of
resistance
 If current is reduced in half by doubling the voltage
then the losses will be one quarter of their previous
value.
 This is because power losses in cables is proportional to
the square of the current flowing in the cables
 Double current gives 4 times the losses
 Three times the current gives 9 times the losses
Transformers
 Transformers – a device used to change the
voltage of an a.c. electricity supply
 They can be up to 99.9% efficient
 Passing through multiply transformers could
cause significant loss if they were not as
efficient
 Power stations typically generate electricity
at 25kV. This has to be converted to the grid
voltage of 400kV using a transformer.
 We say the voltage is stepped up by a
factor of 16
Construction of transformers
 Primary coil – the incoming voltage 𝑉𝑝 is
connected across this coil
 Secondary coil – this provides the voltage 𝑉𝑠
to the external circuit
 Iron core – this links the two coils
 There is not electrical connection between
the two coils
 The voltages are both alternating and a
transformer does not change a.c. to d.c.
 To step up the input voltage by a factor of
16 there must be 16 times as many turns on
the secondary coil as the primary
 A step up increases voltage (more turns)
 A step down decreases voltage (less turns)
 If voltage is stepped up the current must be
stepped down
How transformers work
 The primary coil has alternating current flowing through it. It
is thus an electromagnet and produces an alternating
magnetic field
 The core transports this alternating field around the
secondary coil
 Now the secondary coil is a conductor in a changing
magnetic field. A current is induced in the coil.
 Secondary coil
 Few turns small e.m.f.
 Lots of turns large e.m.f.
 If a direct current is connected to a transformer there is no
output voltage because the magnetic field produced by
the primary coil is unchanging
 Page 300
 Soft – describes a material that once magnetized can be
easily demagnetised
Calculating current & Energy saving
 Energy saving
 Electricity is transmitted in high voltages
because the current can be small
 Calculating current
 𝑒𝑙𝑒𝑐𝑡𝑟𝑖𝑐𝑎𝑙 𝑝𝑜𝑤𝑒𝑟 𝑃 = 𝐼𝑉
 Example 21.2
 This allows for lower energy loss
 Page 301
 The current flowing in the cables is a flow
of coulombs of charge. At high voltage
we have fewer coulombs flowing, but
each coulomb carries more energy with it
Thinking about power
 If a transformer is 100% efficient no power
is lost in its coils or core
 Well designed transformers waste only
about .1% of the power transferred
through them