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8/13/2019 6A AC Theory
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Alternating Currents
Teacher: Mr. B. BurnettDate: January 28, 2013
Class: 6A- Phys1
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Recall Transformer
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A.C Generator
Revisited(Example)
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What is an Alternating
Current?O The term Alternating current (AC) means
that the direction of current flowing in a circuit
is constantly being reversed back and forthcyclically.
O ACis created by generators in power plants,
and other sources. ThisACis delivered to our
homes and businesses by the power lines we
see everywhere.
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AC vs DC
Alternating
Current(AC) Direct Current (DC)
O Current flow is bi-directional (flows back and
forth in two directions) Eg.
Transformer, AC
Generator
O Current flow is uni-diectional (flows in one
direction only). Eg.
Batteries, Car battery, fuel
cells, solar cells etc.
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Important DefinitionsO Peak value (Symbol: V0, I0)
O - This is the maximum value of current or voltage.
O V0= Vrms x 2, I0 = Irmsx 2O
Peak-to-Peak value(Vpp, Ipp)This is the distance fromthe max in the positive
half of the cycle to the min in thenegative half of the cycle. It is doublethe peak value.
O Frequency- This is the number of cycles per second. Thedirection of current is switching back and forth 60times each second (60Hz) in commercial use.
O RMS (Root-mean-squared) value (Effective value)- Therms value of an ac voltage is the DC value that gives thesameamount of heat or light (or produces the same heatingeffect).
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The RMS voltage
RMS (Dc equivalent)
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AC Waveforms
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A.C
1
2
1st
halfcycle
2nd
half
cycle
On every half-cycle, the
polarity of the
alternating voltage
reverses. Hence, the
direction of the current
changes every half
cycle. In the circuit on
the left, terminal 1 is
positive w.r.t terminal 2
for the 1sthalf cycle then
the polarity changes
where terminal 2 is now
positive w.r.t terminal 2
for the next half cycle
and the process is
repeated.
Resistor, R
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Example of AC (Half-wave
rectification)
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Practice Question 1O A sine wave of voltage varies from zero to
a maximum of 100V. How much is the
voltage at the instant of 300
of the cycle?450, 900, 2700 (H.W)?
O V=V0sin
O V= 100 sin 300= 100 x 0.5
O V= 50V
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Advantages of ACO Can be used to transmit powerover long distances to
run AC systems, e.g radio set (via power lines).
O It is less expensive than DC because you can decrease
or increase the AC power very easily.
O you can use high voltageswith small currents to
reduce losses when you transmit power.
O A home wired for AC power is ready for utility or
generator power if / when it is available.
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Example: Hair DryerO a) Calculate the resistance and the peak
in a 1000-W hair dryer connected to a
110-V line in Jamaica?O b) What happens if it is connected to a
220-V line in Britain?
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