Energy needed to establish an alternating current I in a coil of self inductance L is
L di/dt
zero
LI2/2
IL2/2
A coil of inductive reactance 31 Ω has a resistance of 8 Ω. It is placed in series with the condenser of capacitative reactance 25 Ω. The combination is connected to an AC source of 110 V. The power factor of the circuit is
0.56
0.64
0.80
0.33
An AC supply gives 30 Vrms which passes through a 10 Ω resistance. The power dissipated in it is
90 √2 W
90 W
45√2 W
45 W
A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is approximately
30%
50%
90%
10%
The reactance of a capacitor of capacitance C is X. If both the freqauency and capacitance be doubled, then new reactance will be
X
2X
4X
X/4
What is the value of inductance L for which the current is maximum in a series LCR circuit with C = 10μF and ω = 1000s-1?
100 mH
1 mH
Cannot be calculated unless R is known
10 mH
The time constant of C - R circuit is
1/CR
C/R
CR
R/C
A 12 Ω resistor and a 0.21 H inductor are connected in series to an a.c. source operating at 20 V, 50 Hz. The phase angle between the current and source voltage is
30o
40o
90o
80o
An alternating voltage (in volts) given by
V = 200 √2 sin (100 t), is connected to a 1 μF capacitor through an AC ammeter. The reading of the ammeter will be
10 mA
20 mA
40 mA
80 mA
The effective value of an alternating current is 5A. The current passes through 24 Ω resistor. The maximum potential difference across the resistor is
10 V
170 V
17 V
1700 V
