Magnetism and Matter — Physics STD 12 Science — Question

The electrical conductivity of pure germanium can be increased by:

The capacity of a parallel plate condenser is 10 μF, when the distance between its plates is 8 cm. If the distance between the plates is reduced to 4 cm, then the capacity of this parallel plate condenser will be

(a) 5 μF

(b) 10 μF

(c) 20 μF

(d) 40 μF

In an electromagnetic wave.

A charge "Q" and "2Q" are 0.05 meters apart and isolated. The ratio of the electrostatic force on the charge Q to the force on charge 2Q is:

When a certain circuit consisting of a constant e.m.f. E an inductance L and a resistance R is closed, the current in, it increases with time according to curve 1. After one parameter (E, L or R) is changed, the increase in current follows curve 2 when the circuit is closed second time. Which parameter was changed and in what direction

(a) L is increased

(b) L is decreased

(c) R is increased

(d) R is decreased

The work function of sodium is 2.3 eV. The threshold wavelength of sodium will be

(a) 2900 Å

(b) 2500 Å

(c) 5380 Å

(d) 2000 Å

Light rays from a source are incident on a glass prism of index of refraction μ and angle of prism α. At near normal incidence, the angle of deviation of the emerging rays is

(a) (μ – 2)α

(b) (μ – 1) α

(c) (μ + 1) α

(d) (μ + 2) α

Eight drops of mercury of equal radii possessing equal charges combine to form a big drop. Then the capacitance of bigger drop compared to each individual small drop is

(a) 8 times

(b) 4 times

(c) 2 times

(d) 32 times

Missing lines in a continuous spectrum reveal

(a) Defects of the observing instrument

(b) Absence of some elements in the light source

(c) Presence in the light source of hot vapours of some elements

(d) Presence of cool vapours of some elements around the light source

A photocell is illuminated by a small bright source placed 1 m away. When the same source of light is placed   away, the number of electrons emitted by photo cathode would

(a) Decrease by a factor of 2	(b) Increase by a factor of 2
(c) Decrease by a factor of 4	(d) Increase by a factor of 4