31.

A gang capacitor is formed by interlocking a number of plates as shown in figure. The distance between the consecutive plates is 0.885 cm and the overlapping area of the plates is 5 cm. The capacity of the unit is

         

• 1.06 pF

• 4 pF

• 6.36 pF

• 12.72 pF

A 30 V, 90 W lamp is to be operated on a 120 V DC line. For proper glow, a resistor of ....... Ω should be connected in series with the lamp.

• 40

• 10

• 20

• 30

A battery consists of a variable number (n) of identical cells, each having an internal resistance r connected in series. The terminals of the battery are short-circuited. A graph of current (I) in the circuit versus the number of cells will be as shown in figure

In a LCR series circuit, the potential difference between the terminals of the inductance is 60 V between the terminals of the capacitor is 30 V and that across the resistance is 40 V. Then, supply voltage will be equal to

• 50 V

• 70 V

• 130 V

• 10 V

When deuterium and helium are subjected to an accelerating field simultaneously then

• both acquire same energy

• deuterium accelerates faster

• helium accelerates faster

• neither of them is accelerated

A solenoid 1.5 m long and 0.4 cm in diameter possesses 10 turns per cm length. A current of 5 A falls through it. The magnetic field at the axis inside the solenoid is

• $2\mathrm{\pi } \times {10}^{-5} \mathrm{T}$

• $4\mathrm{\pi } \times {10}^{-2} \mathrm{T}$

• $4\mathrm{\pi } \times {10}^{-3} \mathrm{T}$

A wire PQR is bent as shown in figure and is placed in a region of uniform magnetic field B. The length of PQ = QR = l. A current I ampere flows through the wire as shown. The magnitude of the force on PQ and QR will be

• BIl, 0

• 2 BIl, 0

• 0, BIl

• 0,0

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

A current of 6 A enters one corner P of an equilateral triangle PQR having 3 wires of resistances 2 Ω each and leaves by the comer R. Then the current I₁ and I₂ are

• 2 A, 4 A

• 4 A, 2 A

• 1 A, 2 A

• 2 A, 3 A

The twinkling effect of star light is due to

• total internal reflection

• high dense matter of star

• constant burning of hydrogen in the star

• the fluctuating apparent position of the star being slightly different from the actual position of the star