121.

A lamp in which 10 amp current can flow at 15 V, is connected with an alternating source of potential 220 V. The frequency of source is 50 Hz. The impedance of choke coil required to light the bulb is

• 0.07 H

• 0.14 H

• 0.28 H

• 1.07 H

An L.C. circuit is in the state of resonance. If C = 0.1F and L = 0.25 Henry. Neglecting ohmic resistance of circuit. What is the frequency of oscillations?

• 1007 Hz

• 100 Hz

• 100 Hz

• 500 Hz

When temperature of an ideal gas is increased from 27°C to 227°C, its rms speed is changed from 400 m/s to v_s Then, v_s is

• 516 m/s

• 450 m/s

• 310 m/s

• 746 m/s

In a L-C-R circuit, the capacitance is made one-fourth, then what should be changed in inductance, so that the circuit remains in resonance?

• 8 IMES

• 1/4 times

• 2 times

• 4 times

In a circuit, L, C and R are connected in series with an alternating voltage source of frequency f. The current leads the voltage by 45°. The value of C is

• $\frac{1}{2\mathrm{\pi f} \left(2\mathrm{\pi f} \mathrm{L} + \mathrm{R}\right)}$

• $\frac{1}{\mathrm{\pi f} \left(2\mathrm{\pi f} \mathrm{L} + \mathrm{R}\right)}$

• $\frac{1}{2\mathrm{\pi f} \left(2\mathrm{\pi f} \mathrm{L} - \mathrm{R}\right)}$

• $\frac{1}{\mathrm{\pi f} \left(2\mathrm{\pi f} \mathrm{L} - \mathrm{R}\right)}$

In a series L-C-R circuit, resistance R = 10 Ω and the impedance Z = 10 Ω. The phase difference between the current and the voltage is

• 0^o

• 30^o

• 45^o

• 60^o

The impedance of a circuit, when a resistance R and an inductor of inductance L are connected in series in an AC circuit of frequency f, is

• $\sqrt{\mathrm{R} + 2 {\mathrm{\pi }}^2 {\mathrm{f}}^2 {\mathrm{L}}^2}$

• $\sqrt{\mathrm{R} + 4{\mathrm{\pi }}^2 {\mathrm{f}}^2 {\mathrm{L}}^2}$

• $\sqrt{{\mathrm{R}}^2 + 4 {\mathrm{\pi }}^2 {\mathrm{f}}^2 {\mathrm{L}}^2}$

• $\sqrt{{\mathrm{R}}^2 + 2{\mathrm{\pi }}^2 {\mathrm{f}}^2 {\mathrm{L}}^2}$

A choke is preferred to a resistance for limiting current in AC circuit because

• choke is cheap

• there is no wastage of power

• choke is compact in size

• choke is a good absorber of heat

Induced emf in the coil depends upon

• conductivity of coil

• amount of flux

• rate of change of linked flux

• resistance of coil

In an L-C-R circuit inductance is changed from L to L/2.To keep the same resonance frequency, C should be changed to

• 2 C

• $\frac{\mathrm{C}}{2}$

• 4 C

• $\frac{\mathrm{C}}{4}$