A transformer of efficiency 90% draws an input power of 4  kW. An electrical appliance connected across the secondary draws a current of 6 A. The impedance of the device is

• 60 Ω

• 50 Ω

• 80 Ω

• 100 Ω

42.

The resonance frequency of the tank circuit of an oscillator when $\mathrm{L} = \frac{10}{{\mathrm{\pi }}^2} \mathrm{mH}$ and C = 0.04 µF are connected in parallel is

• 250 kHz

• 25 kHz

• 2.5 kHz

• 25 MHz

An alternating voltage e = 200 sin 100 t is applied to a series combination R = 30 Ω and an inductor of 400 mH. The power factor of the circuit is

• 0.01

• 0.2

• 0.6

• 0.042

A transformer has an efficiency of 80%. It is connected to a power input of 5 kW at 200 V. If the secondary voltage is 250 V, the primary and secondary currents are respectively

• 25 A, 20 A

• 20 A, 16 A

• 25 A, 16 A

• 40 A, 25 A

When a DC voltage of 200 V is applied to a coil of self-inductance $\left(\frac{2\sqrt{3}}{\mathrm{\pi }}\right)$ H, a current of 1 A flows through it. But by replacing DC source with AC source of 200 V, the current in the coil is reduced to 0.5 A. Then the frequency of AC supply is 

• 100 Hz

• 75 Hz

• 50 Hz

• 30 Hz

In a L-R circuit, the value of L is $\left(\frac{0.4}{\mathrm{\pi }}\right) \mathrm{H}$  and the value of R is 30 Ω. If in the circuit, an alternating emf of 200 V at 30 cycles/s is connected, the impedance of the circuit and current will be

• 11.4 Ω, 17.5 A

• 30.7 Ω, 6.5 A

• 40.4 Ω, 5 A

• 50 Ω, 4 A

Which ofthe following statement is incorrect ?

• In LCR series AC circuit, as the frequency of the source increases, the impedance of the circuit first decreases and then increases

• If the net reactance of an LCR series AC circuit is same as its resistance, then the current lags behind the voltage by 45°

• At resonance, the impedance of an AC circuit becomes purely resistive

• At resonance in LCR series AC circuit, the potential drops across inductor and capacitor are equal m magnitude but opposite in sign

If an alternating voltage is represented as E = 141 sin (628 t), then the rms value of the voltage and the frequency are respectively

• 141 V, 628 Hz

• 100 V, 50 Hz

• 100 V, 100 Hz

• 141 V, 100 Hz

A step-down transformer is used on a 1000 V line to deliver 20 A at 120 V at the secondary coil. If the efficiency of the transformer is 80%, the current drawn from the line is  

• 3 A

• 30 A

• 0.3 A

• 2.4 A

For the series LCR circuit shown in the figure, what is the resonance frequency and the amplitude of the current at the resonating frequency ?

• 2500 rad s^-1 and $5\sqrt{2} \mathrm{A}$

• 2500 rad s^-1 and 5 A

• 2500 rad s^-1 and $\frac{5}{\sqrt{2}} \mathrm{A}$

• 250 rad s^-1 and $5\sqrt{2} \mathrm{A}$