31.

2, 4 and 6 S are the conductances of three conductors. When they are joined in parallel, their equivalent conductance will be

• 12 S

• ( 1/12 ) S

• ( 12 /11 ) S

• 11/12 S

Two parallel long wires A and B carry currents i₁ and i₂ ( < i₂ ). When i₁ and i₂ are in the same direction, the magnetic field at a point mid way between the wires is 10 µT. If i₂ is reverse the field becomes 30 µT. The ratio i₁ / i₂

• 1

• 2

• 3

• 4

Two conducting circular loops of radii R₁ and R₂ are placed in the same plane with their centres coinciding. If R₁ > R₂, the mutual inductance M between them will be directly proportional to

• $\frac{{\mathrm{R}}_1}{{\mathrm{R}}_2}$

• $\frac{{\mathrm{R}}_2}{{\mathrm{R}}_1}$

• $\frac{{\mathrm{R}}_1^{ 2}}{{\mathrm{R}}_2}$

• $\frac{{\mathrm{R}}_2^{ 2}}{{\mathrm{R}}_1^{ 2}}$

The most suitable metal for making electromagnets and transformer cores is

• steel

• iron

• copper

• aluminium

An inductive circuit contains a resistance of 10 Ω and an inductance of 2 H, If an AC voltage of 120 V and frequency 60 Hz is applied to this circuit, the current would be nearly

• 0.72 A

• 0.16 A

• 0.48 A

• 0.80 A

The diameter of the plate of a parallel plate condenser is 6 cm. If its capacity is equal to that of a sphere of diameter 200 cm, the separation between the plates of the condenser is

• 4·5 × 10^-4 m

• 2·25 × 10^-4 m

• 6·75 × 10^-4 m

• 9 × 10^-4 m

In a uniform electric field a charge of 3 C experiences a force of 3000 N. The potential difference between two points 1 cm apart along the electric line of force will be

• 10 V

• 30 V

• 300 V

• 100 V

A long straight wire carries 10 A d.c. current an electron travels perpendicular to the plane of this wire at a distance 0.1 m with velocity  5.0 x 10⁶ ms^-1. Force acting on the electron due to current in wire is

• zero N

• 0·6 × 10^-17 N

• 2.4 × 10^-17 N

• 2.2 × 10^-17 N

A particle of charge q and mass m starts moving from the origin under the action of an electric field $\overrightarrow{\mathrm{E}}$ = E₀$\hat{\mathrm{i}}$  and $\overrightarrow{\mathrm{B}} = {\mathrm{B}}_0 \hat{\mathrm{i}}$ velocity  $\overrightarrow{\mathrm{v}} = {\mathrm{v}}_0\widehat{ \mathrm{j}}$. The speed of the particle will become $\frac{\sqrt{5}}{2} {\mathrm{\nu }}_0$ after a time

• $\frac{\mathrm{m} {\mathrm{\nu }}_0}{\mathrm{q} {\mathrm{E}}_0}$

• $\frac{\mathrm{m} {\mathrm{\nu }}_0}{2\mathrm{q} {\mathrm{E}}_0}$

• $\frac{\sqrt{3} {\mathrm{m\nu }}_0}{2\mathrm{q} {\mathrm{E}}_0}$

• $\frac{\sqrt{5} {\mathrm{m\nu }}_0}{2\mathrm{q} {\mathrm{E}}_0}$

Assertion:  Long distance transmission of a.c. is carried out at extremely high voltages. 

Reason:  Because when the distance is large, voltage has to be large.

• If both assertion and reason are true and reason is the correct explanation of assertion.

• If both assertion and reason are true but reason is not the correct explanation of assertion.

• If assertion is true but reason is false.

• If both assertion and reason are false.