THE LAST TWO QUESTIONS ARE SUPPOSED TO BE TRICKY, IF IN DOUBT, PLEASE, DO NOT AN
ID: 1518908 • Letter: T
Question
THE LAST TWO QUESTIONS ARE SUPPOSED TO BE TRICKY, IF IN DOUBT, PLEASE, DO NOT ANSWER AT ALL(NONE OF THE QUESTIONS) Thank you.
A circuit is constructed with an AC generator, a resistor, capacitor and inductor as shown. The generator voltage varies in time as = Va - Vb = msint, where m = 120 V and = 565 radians/second. The values for the remaining circuit components are: R = 97 , C = 330 F. The value for L is unkown. What is known is that the voltage across the generator leads the current in the circuit by = 36 degrees.
1)
What is t1, the first time after t = 0 when the magnitude of the voltage across the inductor is maximum?seconds
2)
What is Z, the impedance of the circuit?
3)
What is L, the value of the inductor?mH
4)
What is VL,max, the magnitude of the maximum voltage across the inductor?V
5)
What is VL = Vb - Vc, the voltage across the inductor, at time t = 0? Note that VL is a signed number.V
6)
In order to make the votage across the generator become in phase with the current in the circuit (i.e., make the phase equal to zero), how would the value of C have to change, keeping all other circuit parameters the same?
The value of C would need to be decreased
The value of C would need to be increased.
It is impossible to bring the voltage across the generator in phase with the current in the circuit by only changing the value of C, keeping all other circuit parameters fixed.
Explanation / Answer
1) t1= (angle)/(omega) The angle will be (90-phi), in radians.
t1 = (90-36)*(2pi/360) / 565 = 1.668*10^-3 sec = 1.668 miliseconds
2) Z = 97 / cos(36) = 126.73
3) Z = 126.73 at 36 degrees.
Z*Sin(36) = XL - Xc, so
Xc = 1/wC = 5.36
XL = 126.73*sin(36) + 5.36 = 79.85 ,
L = XL/ = 79.85 / 565 = 0.141 Henry's
4) VL(max) = Imax*XL = (120/126.73)*79.85 = 75.6 V
5) In a perfect world, XL @ t=0 is infinite, therefore I = 0 and VL = 0 volts also.
6) The value for C would need to be decreased.
Since XL >> Xc here
To make the phase angle = 0
XL = Xc
Hence we need to increase Xc and to do that we need to decrease C
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