Transcript Applied.ppsx - DCS Physics

Applied Electricity
• A small current in
one circuit is used
to switch on a
much larger
current in another
circuit.
• When the small
current is
switched on the
electromagnet A
activates and the
iron lever is
attracted to it.
This causes the
arm to move which
closes the contact
B in the other
circuit.
Reed relay
• A The bell push is closed which
allows a current to flow.
• B The electromagnet becomes
magnetized and the armature is
pulled to the magnet this makes
the hammer hits the gong.
• As this happens the contact is
broken so it not a complete
circuit so the electromagnet is
de-magnetized.
• C The armature is no longer
attracted so it jumps back.
• This closes the contact, so
electricity flows again and the
armature is once more attracted
to the electromagnet.The whole
cycle is repeated until you stop
pressing the bell push.
5-18. Electromagnetic Induction
The effect of producing an
induced current is known as
electromagnetic induction.
The direction of the induced
current can be reversed by
reversing the motion of the wire
or reversing the field direction.
The strength of the current
depends on the strength of the
magnetic field and the speed of
the wire's motion.
Electric Motors
An electric motor uses
the sideways push of
a magnetic field to
turn a current-carrying
wire loop. Electric
motors use a
commutator to
change the direction
of the current in the
loop. Alternating
current electric motors
do not use
commutators.
Michael Faraday (17911867) built the first electric
motor and discovered
magnetic induction.
Loudspeaker – Moving Coil
A moving coil is
moved by a
changing input
current.
The paper cone
compresses
and rarefies
the air.
Input
Microphone
A moving coil
activated by
voice vibrations
is used as a
microphone.
The coil induces a
current in the
magnet that can
be amplified or
recorded.
Alternating and Direct Current
Alternating current (ac) is current that flows in a back-andforth manner; household current changes direction 120 times
each second (60 Hz). Direct current (dc) flows in one
direction.
The ac generator (or alternator) produces an ac current and can be
modified to produce dc current by…..
Use of a rectifier which permits current to pass through it in only
one direction.
AC Generator
• As the coil
turns the
direction and
size of the
induced
current is
varied
5-19. Transformers
A transformer is a device composed of two unconnected
coils, usually wrapped around a soft iron core, that can
increase or decrease the voltage of ac current.
Transformers
This how A.C.
changes
voltage up or
down
V In
V Out
=
Turns 2
Turns 1
Better Transformers
• Thick wire to reduce heat loss
• Laminate core to reduce EDDY CURRENTS
• Soft iron reduce the Hysteresis losses as the
core is magnetised and demagnetised
• Figure 8 shape to cut flux leakage
Galvanometer
Device used in
the construction
of ammeters
and voltmeters.
Based on
principle that a
current in a
magnetic field
experiences a
force
Scale
Current
loop or
coil
Radial
Magnetic
Field
Spring
Galvanometer
Magnet forms a
radial field
The springs
balance the
moment of the
force or torque
from the current
in the magnetic
field
Scale
Current
loop or
coil
Radial
Magnetic
Field
Spring
Galvanometer used as Ammeter
• Typical galvanometer have an internal resistance of the
order of 60 Ω - that could significantly disturb (reduce) a
current measurement.
• Built to have full scale for small current ~ 1 mA or less.
• Must therefore be mounted in parallel with a small resistor
or shunt resistor.
Galvanometer
Rp
60 W
Galvanometer
60 W
Rp
• Let’s convert a 60 Ω, 1 mA full scale galvanometer to an
ammeter that can measure up to 2 A current.
• Rp must be selected such that when 2 A passes through the
ammeter, only 0.001 A goes through the galvanometer.
 0.001A 60W   1.999 A Rp
R p  0.03002W
• Rp is rather small!
• The equivalent resistance of the circuit is also small!
Galvanometer used as Voltmeter
• Must mount a large resistor in series to limit the current
going though the voltmeter to 1 mA.
• Must also have a large resistance to avoid disturbing
circuit when measured in parallel.
Rs
Galvanometer
60 W
Rs
Galvanometer
60 W
Maximum voltage across galvanometer:
Vmax   0.001A 60W  0.06V
Suppose one wish to have a voltmeter that can measure
voltage difference up to 100 V:
100V   0.001A  Rp  60W 
Rp  99940W
Large resistance
Photodiode
A photodiode is a
PN junction in reverse
bias.
When a photon of
sufficient energy
strikes the diode, it
excites an electron,
thereby creating a
mobile electron
and a positively
charged electron hole.
LED
• While all diodes release
light, most don't do it
very effectively. In an
ordinary diode, the
semiconductor material
itself ends up absorbing
a lot of the light energy.
LEDs are specially
constructed to release a
large number of photons
outwards.
Bipolar Transistor
• The small current between the base and
emitter controls the large current from
collector to emitter
Transistor as amplifier
• Input signal controls
the larger voltage
output
• Looks exactly like
the input but bigger
amplitude
Transistor as switch
• Small change in base voltage turns on
larger lamp
• Warnings, alarms and computer circuits
Transistor as switch in ECU
Voltage Inverter
• Input is opposite of
the output
• Standard set up for
an amplifier as well
Boolean Algebra to Logic Gates
• Logic circuits are built from components
called logic gates.
• The logic gates correspond to Boolean
operations +, *, ’.
OR
+
AND
*
NOT
’
AND
Logic Gate:
A
B
A
A∩B
Truth Table:
B
A
0
0
1
1
Series Circuit:
A∩B
B A∩B
0
0
1
0
0
0
1
1
OR
Logic Gate:
A
AUB
B
A
0
0
1
1
A
Parallel Circuit:
Truth Table:
B
AUB
B AUB
0
0
1
1
0
1
1
1
NOT
Logic Gate:
A
A’ or Ā
(also called an inverter)
Single-throw
Double-pole
Switch:
Truth Table:
A
0
1
A
A’ or Ā
Ā
1
0
Computing Inputs
1101100101
0100110111
10001111
00111100
110001
1101110111
00001100
001110
A bit of binary humor
Full Wave (Bridge) Rectifier
Full Wave Rectifier
Smoothing
• Add capacitor
Faraday's Law
• Any change in
the magnetic
environment
(flux) of a coil of
wire will cause a
voltage (emf) to
be "induced" in
the coil.
Faraday's Law
Lenz’s Law
• http://courses.science.fau.edu/~rjordan/rev_note
s/movies/MFA07AN1.MOV
• gives the direction of the induced electromotive
force (emf) and current resulting from
electromagnetic induction. The law provides a
physical interpretation of the choice of sign in
Faraday's law of induction, indicating that the
induced emf and the change in flux have
opposite signs.
Induction Motor
• No brushes to wear out
• Induction motors use shorted
wire loops on a rotating
armature and obtain their
torque from currents induced
in these loops by the changing
magnetic field produced in the
stator (stationary) coils
Bars
are
really
loops
Practical Induction Motor
Bars
are
really
loops
• This implies that there
is no current supplied
to the rotating coils.
These coils are closed
loops which have
large currents induced
in them because of
their low resistance.
Induction Coil
• Father Nicholas Joseph
Callan (1799 – 1864) was
a Irish priest and scientist
from Darver, Co. Louth,
Ireland. He was Professor
of Natural Philosophy in
Maynooth College from
1834, and is best known for
his work on the induction
coil.
Induction Coil
Induction Coil
It is a type of
electrical
transformer used to
produce high-voltage
pulses from a lowvoltage DC supply.
To create the flux changes necessary to induce voltage
in the secondary, the DC current in the primary is
repeatedly interrupted by a vibrating mechanical
contact called an interrupter.
As in the relay the
coil becomes an
electro-magnet
This time it
disconnects the
supply.
• This turning off
and on creates a
large current in
the greater
windings of the
secondary