Electronmagnetism lesson notes

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Transcript Electronmagnetism lesson notes

Electromagnetism
Three lessons of fun with
motors and generators
1. Motors
Motor Effect
► If
a current passes
through a wire in a
magnetic field, it feels a
force
► The
force is at right
angles to both the current
and the field lines
Motor Effect
► The
size of force
increases if the current,
field strength or length of
wire increases
► The direction of force
reverses if the current or
field is reversed
► Electrical
energy is
converted to kinetic
Making a motor
► By
coiling the wire we can
 Increase the wire length (and force)
 Get a force on both sides
Making a motor
► We
need to solve a problem …
Making a motor
► Solution:
 a commutator to reverse the current
 and brushes to maintain contact
A real motor
Input mains lead
connects here
Outer coil makes an
electromagnet – wire
looks bare but is
coated in plastic
Commutator has 14
elements – each attaches
to a separate inner coil
Other end of brush
pokes out here
Electromagnet core is
laminated to reduce
heat loss
Spring wire touches
contact for outer coil
Other end of spring
touches one ‘brush’
in here
2. Generators
Starter Question
► Which
direction is the force felt?
Field lines
Wire & current
NONE!! They are not at right angles
Electromagnetic Induction
► If
a conductor cuts through magnetic field
lines a p.d. is generated
► Either the wire or the magnet can move (we
need relative motion)
► If the wire is part of a complete circuit we
get a current generated
► All our electricity is generated this way
Electromagnetic Induction
► The
p.d. is reversed if …
 the movement is reversed (in – out)
 the field is reversed (N-S)
► For




a coil of wire, the p.d. increases if …
the
the
the
the
field is stronger (field lines are closer)
speed of movement is more
number of coils is more
area of the coils is bigger
Practical Use
► Kinetic
energy (turning the handle) becomes
electrical energy
► Slip rings and brushes prevent the external
connections from tangling up
Practical Use
► The
generator produces a.c.
► As the coil turns the direction of current
reverses as shown …
3. Transformers
Starter
1.
2.
How do you increase the voltage from an
a.c. generator (four things)?
Explain the purpose of the slip rings and
brushes
Transformers
►A
transformer is just
an a.c. electromagnet
(primary coil) next to a
generator (secondary
coil)
► The a.c. electromagnet
makes a changing
magnetic field
► This hits the generator
coil making an a.c.
current flow in that coil
Transformers
► Why
use electricity to make electricity?
► The
size of the generated output voltage
can be changed (more coils etc.)
►A
transformer is used to
change an a.c. voltage
Uses
► Step-up
 Car HT coil – increases battery voltage for spark
plugs
 National grid – raise voltage for transmission
► Step-down
 Adaptors – reduce mains a.c. voltage
 National grid –reduce voltage for supply to
homes
Demo
► Note:
the laminated
soft iron core
 Core – helps
‘concentrate’ the field
into the secondary
 Laminated – reduces
heat build-up
 Soft iron – can
magnetise and
demagnetise better
than, say, steel
Transformer Formula
Primary
coils
Secondary
coils
V
Coils on primary
---------------------Coils on secondary
V
=
Voltage on primary
------------------------Voltage on secondary
Homework
► Revise
for test