21.

The potential energy of a molecule on the surface of a liquid compared to one inside the liquid is

• zero

• lesser

• equal

• greater

Two spherical conductors B and C having equal radii and carrying equal charges in them repel each other with a force F when kept apart at some distance. A third spherical conductor having same radius as that of B but uncharged, is brought in contact with B, then brought in contact with C and finally removed away from both. The new force of repulsion between B and C is

• $\frac{\mathrm{F}}{4}$

• $\frac{3 \mathrm{F}}{4}$

• $\frac{\mathrm{F}}{8}$

• $\frac{3 \mathrm{F}}{8}$

In gamma ray emission from a nucleus

• both the neutron number and the proton number change

• there is no change in the proton number and the neutron number

• only the neutron number changes

• only the proton number changes

The resistance of an ammeter is 13 Ω and its scale is graduated for a current upto 100 A. After an additional shunt has been connected to this ammeter it becomes possible to measure currents upto 750 A by this meter. The value of
shunt resistance is

• 20 Ω

• 2 Ω

• 0.2 Ω

• 2 KΩ

The primary and secondary coils of a transformer have 50 and 1500 turns respectively. If the magnetic flux  $\mathrm{\varphi }$ linked with the primary coil is given by  $\mathrm{\varphi } = {\mathrm{\varphi }}_{\mathrm{o}} + 4\mathrm{t}$ where $\mathrm{\varphi }$ is weber, t is time in second and ${\mathrm{\varphi }}_{\mathrm{o}}$ is a constant, the output vltage across the secondary coil is

• 90 v

• 120 V

• 220 V

• 30 V

A transformer is used to light a 100 W and 110 V lamp from a 220 V mains. If the main current is 0.5 A, the efficiency of the transformer is approximately

• 30%

• 50%

• 90%

• 10%

A charged particle (charge q) is moving in a circle of radius R with uniform speed v. The associated magnetic moments given by

• $\frac{\mathrm{q} \mathrm{\nu } \mathrm{R}}{2}$

• qνR²

• $\frac{\mathrm{q} \mathrm{\nu } {\mathrm{R}}^2}{2}$

• qνR

A steady current of 1.5 A flows through a copper voltameter for 10 min. If the electrochemical equivalent of copper is 30x 10^-5 g C^-1 , the mass of copper deposited on the electrode will be

• 0.40 g

• 0.50 g

• 0.67 g

• 0.27 g

In a mass spectrometer used for measuring the masses of ions, the ions are initially accelerated by an electric potential V and then made to describe semicircular paths of radius R using a magnetic field B. If V and B are kept constant the ratio $\left(\frac{\mathrm{charge} \mathrm{on} \mathrm{the} \mathrm{ion}}{\mathrm{mass} \mathrm{of} \mathrm{the} \mathrm{ion}}\right)$ will be proportional to

• $\frac{1}{\mathrm{R}}$

• $\frac{1}{{\mathrm{R}}^2}$

• R²

• R

Three resistances P, Q, R each of 2Ω and an unknown resistance S form the four arms of a Wheatstone's bridge circuit. When a resistance of 6Ω is connected in parallel to S the bridge gets balanced. What is the value of S?

• 2 Ω

• 3 Ω

• 6 Ω

• 1 Ω