21.

If work done in increasing the size of a soap film from 10 cm x 6 cm to 10 cm x 11 cm is 2x10^-4 J, then surface tension is

• 2x10^-2 Nm^-1

• 2x10^-4 Nm^-1

• 2x10^-6 Nm^-1

• 2x10^-8 Nm^-1

A battery of emf 10 V and internal resistance 3 Ω is connected to a resistor. The current in the circuit is 0.5 A. The terminal voltage of the battery when the circuit is closed is

• 10 V

• zero

• 1.5 V

• 8.5 V

A galvanometer coil has a resistance of 15 Ω and gives full-scale deflection for a current of 4 mA. To convert it to an ammeter of range 0 to 6 A

• 10 mΩ resistance is to be connected in parallel to the galvanometer

• 10 mΩ resistance is to be connected in series with the galvanometer

• 0.1 Ω resistance is to be connected in parallel to the galvanometer

• 0.1 Ω resistance is to be connected in series with the galvanometer

A Gaussian sphere encloses an electric dipole within it. The total flux across the sphere is

• zero

• half that due to a single charge

• double that due to a single charge

• dependent on the position of the dipole

A parallel plate air capacitor has a capacitance C. When it is half-filled with a dielectric of dielectric constant 5, the percentage increase in the capacitance will be

• 400%

• 66.6%

• 33.3%

• 200%

The specific charge of a proton is 9.6 x10⁷ C kg^-1  The specific charge of an alpha particle will be

• 9.6 x10⁷ C kg^-1

• 19.2 x10⁷ C kg^-1

• 4.8 x10⁷ C kg^-1

• 2.4 x10⁷ C kg^-1

A gang capacitor is formed by interlocking a number of plates as shown in the 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 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, the supply voltage will be equal to

• 50 V

• 70 V

• 130 V

• 10 V

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

• 2π × 10^-3 T

• 2π × 10^-5 T

• 4π × 10^-2 T

• 4π × 10^-3 T