AC 1 Flashcards
what is alternating current/voltage
it is the current/voltage whose value changes continuously and its direction reverses with time periodically.
I=Iosinwt=Eo/R sin(wt)
E=Eosinwt
ac voltages
can be easily and efficiently converted from one voltage to the other by
means of transformers. Further, electrical energy can also be transmitted
economically over long distances.
what is the avg value of ac over 1 cycle
Ac is +ve during the first half cycle ( from 0- T/2) and then equally -ve in the next half cycle. So mean/avg value of current over 1 cycle is 0.
what is the avg mean value of current over one half cycle
Avg/mean value of ac over 1 half cycle is defined as that value of steady current that sends the same amount of charge into the circuit in the time of one half cyle as is sent by ac through the same circuit in same time
I avg= 0.637 Io
Io= peak value
mean/avg value of emf over one half cycle
avg/mean emf over one half cycle is that value of constant emf that would send the same amount of charge through a circuit in the time of a half cycle as is sent by an ac emf through the same circuit in same time.
why does dc ammeter/voltmeter/ galvanometer not measure ac current or voltage
Dc ammeter/voltmeter/galvanometer cannot measure ac current voltages
they will record a 0 reading because the average value of ac voltage over one cycle is 0
since avg current is 0, is the heat developed in the cirrcuit also 0?
The fact that the average current is zero, however, does not mean that the average power consumed is zero and
that there is no dissipation of electrical energy. As you
know, Joule heating is given by i
2R and depends on i
2
(which is always positive whether i is positive or negative)
and not on i.
rms value of ac/ efffective current
rms value of ac is defined as that value of steady current whcih would generate the same amount of heat in a circuit in a given time as is done by an ac current when passed through the same circuit for same time
Irms=Io/sqrt(2)
1rms=0.707 Io
Erms=0.707 Eo
rms value of emf
It is defined as that value of steady voltage that would produce the same amount of heat in a given resistance for a given time, as is done by an alternating emf when applied to the same resistance for the same time.
Erms=0.707 Eo
why is the scale of ac ammeter not linear/equally spaced?
AC ammeter is based on the principle of heating of effect of electric current.
since H directly prop to I^2
the relationship between heat generated and current is not linear. hence the non linear scale.
They are called hot wire instruments.
what are the key points of AC
-AC is cheaper than DC
- It can be easily converted to Dc when required using rectifier
- It can be transmitted over long distances at low power loss
- The AC voltage can be increases/decreased easily using transformer
-AC is more dangerous than DC( ac rms value is usually given so it is root 2 times the value hence greater.)
- batteries cannot be charged by ac. electrolysis or electroplating cannt be done.
average power associated with
i)purely resistive circuit
ii)purely inductive circuit
iii)purely capacitative circuit
i)average power over a compltee cycle of ac through the resistor is the product of rms current and rms value
P= IoVo/2= IrmsVrms= (Irms)^2R
ii)average power over a complete cycle is 0. Whatever energy is needed in building up the current in L is returned back during the decay of current.
iii) the avg power over a complete cycle is 0. whatever energy is needed in building up the voltage across capacitor is returned back to the source during discharging of the capacitor.
what is resonance
The phenomenon of resonance is common among systems
that have a tendency to oscillate at a particular frequency. This frequency
is called the system’s natural frequency. If such a system is driven by an
energy source at a frequency that is near the natural frequency, the
amplitude of oscillation is found to be large.
what is resonant freqeucny
. So if w is varied, then at a particular frequency
w0, Xc = XL, and the impedance is minimum .
The amplitude of current is maximum at this frequency.
independent of resistor
graph of current/ w
The graph is bell shaped.
the greater the resistance, the shorter the peak of the curve
application of lcr circuit
Resonant circuits have a variety of applications, for example, in the
tuning mechanism of a radio or a TV set. The antenna of a radio accepts
signals from many broadcasting stations. The signals picked up in the
antenna acts as a source in the tuning circuit of the radio, so the circuit
can be driven at many frequencies. But to hear one particular radio
station, we tune the radio. In tuning, we vary the capacitance of a
capacitor in the tuning circuit such that the resonant frequency of the
circuit becomes nearly equal to the frequency of the radio signal received.
When this happens, the amplitude of the current with the frequency of
the signal of the particular radio station in the circuit is maximum.
