March 31, 2009
PHY2054 DiscussionSpring ‘09
Quiz 7 (Chapter 21.122.4)
Name:
UFID:
*1. (2.5pts) An RCL series combination of
R
= 50 Ω, C = 40 μC and L = 200 mH is connected across
an AC generator with
ΔV
rms
= 120 V and
f
= 60.0 Hz. Calculate the rms voltage across the capacitor.
The inductive reactance and the capacitive reactance are
X
L
= 2πfL = 2π×60×0.2 = 75.4 Ω
X
C
= 1//(2πfC) = 1//(2π×60×40×10
6
) = 66.3 Ω
The impedance of the combination is
Z = √[R
2
+(X
L
X
C
)
2
] = √[50
2
+(75.466.3)
2
] = 50.8 Ω
The rms current in the circuit is the same as that in the capacitor. Thus we have
I
C,rms
= I
rms
= ΔV
rms
/Z = 120/50.8 = 2.36 A
The voltage across capacitor is given by
ΔV
C,rms
= I
C,rms
X
C
= 2.36×66.3 = 156 V
***2. (2.5pts) An ideal transformer has a 240turn primary coil and 80turn secondary coil. The
primary coil is connected across an AC source with an rms voltage of 120 V and an internal
resistance of 20.0 Ω. What is the average power consumed in the secondary coil if it is connected
across a 40Ω resistor?
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 Fall '08
 Avery
 Physics, Alternating Current, Angle of Incidence, Snell's Law, Total internal reflection, primary coil

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