In an L-C-R series AC circuit the voltage across L, C and R is 10 V each. If the inductor is short circuited, the voltage across the capacitor would become

• 10 V

• $\frac{20}{\sqrt{2}} \mathrm{V}$

• $20\sqrt{2} \mathrm{V}$

• $\frac{10}{\sqrt{2}} \mathrm{V}$

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

The inductance of a coil in which a current of 0.1 A increasing at the rate of 0.5 As^-1 represents a power flow of $\frac{1}{2}$ W, is

• 2 H

• 8 H

• 20 H

• 10 H

A point source of electromagnetic radiation has an average power output of 1500 W. The maximum value of electric field at a distance of 3 m from this source in Vm^-1 is

• 500

• 100

• $\frac{500}{3}$

• $\frac{250}{3}$

55.

In the following circuit, the current flowing through 1 kΩ resistor is

            

• 0 mA

• 5 mA

• 10 mA

• 15 mA

Electromagnetic waves of frequencies higher than $9\sqrt{2}$ MHz are found to be reflected by the ionosphere on a particular day at a place. The maximum electron density in the ionosphere is

• $\sqrt{5} \times {10}^{12} {\mathrm{m}}^{-3}$

• $\sqrt{2} \times {10}^{12} {\mathrm{m}}^{-3}$

• 2 × 10¹² m^-3

• 5 × 10¹² m^-3

Which one of the following statements is wrong ?

• Radio waves in the frequency range 30 MHz to 60 MHz are called sky waves.

• Radio horizon of the transmitting antenna for space wave is ${\mathrm{d}}_{\mathrm{T}} = \sqrt{\left(2{\mathrm{Rh}}_{\mathrm{T}}\right)}$ (R = radius of earth, h_T = height of transmitting antenna)

• Within the skip distance neither the ground waves nor the sky waves are received

• The principle of fibre optical communication is total internal reflection

The refractive index and the permeability of a medium are respectively 1.5 and 5 x 10^-7 Hm^-1. The relative permittivity of the medium is nearly

• 25

• 15

• 6

• 10

A square wire of side 1 cm is placed perpendicular to the principal axis of a concave mirror of focal length 15 cm at a distance of 20 cm. The area enclosed by the image of the wire is

• 4 cm²

• 6 cm²

• 9 cm²

• 8 cm²

A thin prism P of refracting angle 3° and refractive index 1.5 is combined with another thin prism Q of refractive index 1.6 to produce dispersion without deviation. Then the angle of prism Q is

• 3°

• 4°

• 3.5°

• 2.5°