Download why alternating current??

REVISION
MOTORS & GENERATORS
ELECTRODYNAMICS
Study of the relationship between electricity,
magnetism and mechanical phenomena
ELECTROMAGNETIC INDUCTION
When a conductor and a magnet move relative to
each other, it causes a changing magnetic field around
the conductor. The changing magnetic field causes
(induce) an emf and the emf causes charge to flow
FARADAY’S LAW
The induced emf in a conductor is directly
proportional to the rate of change in die
magnetic flux in the conductor.

 mk   N
t
MAGNETIESE VLOED
The number of magnetic field lines cutting
through an area
  BA cos 
LENZ’S LAW
The direction of the induced current is such that it
withstands cause thereof.
WHY IS CURRENT INDUCED?
• When the magnet is moved close to the coil, the
magnetic field lines come into contact with the electrons
in the coil.
• This creates a potential difference. This potential
difference causes the electrons to move (what we call
current)
• The coil now has its own magnetic field which opposes
the applied magnetic field
• MOTORS convert
electric energy into
mechanical energy
FLEMING’S LEFT-HAND
RULE FOR MOTORS
• Your middle finger points in the direction of the
current
• Your index finger points in the direction of the
magnetic field
• Your thumb represents the direction of the force
FLAT RIGHT-HAND RULE
FOR MOTORS
• Your thumb points in the direction of motion of a
POSITIVE charge (or conventional current)
• your fingers point in the direction of the magnetic field
• and the direction of the force is given by the direction
your palm would push something
ELECTRODYNAMICS
AC MOTORS
SLIP RINGS & CARBON
BRUSHES
ELECTRODYNAMICS
DC MOTORS
COMMUTATORS &
CARBON BRUSHES
GENERATORS
Convert mechanical
energy into electrical
energy
FLEMING’S RIGHT-HAND
RULE FOR GENERATORS
• Your middle finger points in the direction of the
current
• Your index finger points in the direction of the
magnetic field
• Your thumb represents the direction of the force
FLAT RIGHT-HAND RULE
FOR GENERATORS
• Your thumb points in the direction of the force
• Your fingers point in the direction of the magnetic field
• The direction of the current comes out of your palm
IMPROVING A GENERATOR

 mk   N
t
  BA cos 
ELECTRODYNAMICS
AC GENERATORS
SLIP RINGS & CARBON
BRUSHES
USES OF AC GENERATORS
• Most generators are AC generators
• AC is preferred, due to the fact that the power can easily be
increased through the use of transformers
• Power is delivered over power lines with AC, because less energy is
lost
WHY ALTERNATING CURRENT??
• Electrical energy is sent through power cables so that we
can use it
• The equations for power (rate of work) is
•
2
P  IV
PI R
2
V
P
R
• So to increase the power, we need to increase the current
and/or potential difference
WHY ALTERNATING CURRENT??
W  VIt
• but
• So when current increases, the amount of energy lost as
heat also increases. Large current also means thicker
cables, because thin cables will melt
• therefore
• Potential difference is increases at the power stations (with
step-up transformers)
• And when it reaches substations in our neighborhood, it is
decreases to safe magnitudes by step-down transformers
ELECTRODYNAMICS
DC GENERATORS
COMMUTATORS & CARBON
BRUSHES
USES OF DC GENERATORS
• Electroplating and electro-refining of
materials
• Charging battery & normal lighting
purposes
• Generators for welding and arc
welding
• .
EQUATIONS
• ORDINARY CIRCUITS
FOR
vs
ALTERNATING
CURRENT
ALTERNATING CURRENT
EQUATIONS
FOR
ALTERNATING
CURRENT
• 𝑉𝑚𝑎𝑥 is the maximum voltage that your house receives
• 𝑉𝑎𝑣𝑒 is the average value of household voltage
Vave   Vmax
2
•
Vrms 
Vmax
2
𝑉𝑅𝑀𝑆 is the effective voltage (square root of the average square
voltage)
EQUATIONS
FOR
ALTERNATING
CURRENT
• 𝐼𝑚𝑎𝑥 is the maximum current your house receives
• 𝐼𝑎𝑣𝑒 is the average value of household current
I ave   I max
2
I rms 
I max
2
• 𝐼𝑅𝑀𝑆 is the effective current (square root of the average square
tension)
a  RT  R1  R2
 48
 12
Vwgk  I wgk RT
b
I wgk 
I maks
c  Pgemid  I 2 wgk R
2
3
I maks
 32  4 
I maks
I1  I wgk
36  I wgk 12 
 3A
I wgk  3 A
 36 J
2
 4.24 A
d  The power usage of the 4 is dubble the
power usage of the 8 loadspeaker
Pgemid 
V 2 wgk
R
Power is inversely proportional to resistance
when potential difference remains constant
(The potential difference across R1 and R 2 is the same)
Loudspeaker 1
36V rms
Loudspeaker 2
A source supplies an rms-potential difference of 36V
to a 4Ω and 8Ω loudspeaker that is connected in series.
Determine the following
a  rms-current through the 4Ω loudspeaker
b  Peak current through each loudspeaker
c  Average power usage of the 4Ω loudspeaker
d
Without any further calculation, how will the
average power usage of the 4Ω loudspeaker
compare the average power usage of the 8Ω
loudspeaker? Give a reason for your answer