21.

See the diagram. Area of each plate is 2.0 m² and d = 2 x 10^-3 m. A charge of 8.85 x 10^-8 C is given to Q. Then the potential of Q becomes

             

• 13 V

• 10 V

• 6.67 V

• 8.825 V

Three conductors draw currents of 1 A, 2 A and 3 A respectively, when connected in turn across a battery. If they are connected in series and the combination is connected across the same battery, the current drawn will be

• $\frac{2}{7} \mathrm{A}$

• $\frac{3}{7} \mathrm{A}$

• $\frac{6}{11} \mathrm{A}$

• $\frac{5}{7} \mathrm{A}$

In the circuit, R₁ = R₂. The value of E and R₁ are (E - EMF, R₁ - resistance)

• 180 V, 60 Ω

• 120 V, 60 Ω

• 180 V, 10 Ω

• 120 V, 10 Ω

Masses of three wires of copper are in the ratio of 1 : 3 : 5 and their lengths are in the ratio of 5 : 3 : 1. The ratio of their electrical resistances is

• 1 : 3 : 5

• 5 : 3 : 1

• 1 : 15 : 125

• 125 : 15 : 1

For a transformer, the turns ratio is 3 and its efficiency IS 0.75. The current flowing in the primary coil is 2 A and the voltage applied to it is 100 V. Then the voltage and the current flowing in the secondary coil are respectively

• 150 V, 1.5 A

• 300 V, 0.5 A

• 300 V, 1.5 A

• 150 V, 0.5 A

A proton and helium nucleus are shot into a magnetic field at right angles to the field with same kinetic energy. Then the ratio of their radii is

• 1 : 1

• 1 : 2

• 2 : 1

• 1 : 4

Two identical circular coils A and B are kept on a horizontal tube side by side without touching each other. If the current in the coil A increases with time, in response, the coil B

• is attracted by A

• remains stationary

• is repelled

• rotates

In the diagram, I₁, I₂ are the strength of the currents in the loop and straight conductors respectively. OA = AB = R. The net magnetic field at the centre O is zero. Then the ratio of the currents in the loop and the straight conductors is

• $\mathrm{\pi }$

• $2\mathrm{\pi }$

• $\frac{1}{\mathrm{\pi }}$

• $\frac{1}{2\mathrm{\pi }}$

Two tangent galvanometers, which are identical except in their number of turns, are connected in parallel. The ratio of their resistances of the cols is 1 : 3. If the deflections in the two tangent galvanometers are 30° and 60° respectively, then the ratio of their number of turns is

• 1 : 1

• 3 : 1

• 1 : 3

• 1 : 6

A charged particle with a velocity 2 x 10³ ms^-1 passes undeflected through electric field and magnetic fields in mutually perpendicular directions. The magnetic field is 1.5 T. The magnitude of electric field will be

• 1.5 × 10³ NC^-1

• 2 × 10³ NC^-1

• 3 × 10³ NC^-1

• 1.33 × 10³ NC^-1