Current Electricity — Physics STD 12 Science — Question

Two identical air filled parallel plate capacitors are charged to the same potential in the manner shown by closing the switch S. If now the switch S is opened and the space between the plates is filled with a dielectric of relative permittivity ε_r​, then:

The focal length of a concave mirror is 10 cm. On placing an object at 10 cm in front of the mirror, its image will be formed:

A conducting rod of length l is falling with a velocity v perpendicular to a uniform horizontal magnetic field B. The potential difference between its two ends will be

(a) 2Blv

(b) Blv

(c)

(d)

Two metal plates having charges Q, -Q face each other at some separation and are dipped into an oil tank. If the oil is pumped out, the electric field between the plates will:

1. Increase.
2. Decrease.
3. Remain the same.
4. Become zero.

Two bulbs of 500 watt and 200 watt are manufactured to operate on 220 volt line. The ratio of heat produced in 500 W and 200 W, in two cases, when firstly they are joined in parallel and secondly in series, will be

(a)

(b)

(c)

(d)

When objects at different distances are seen by the eye, which of the following remains constant

(a) The focal length of the eye lens	(b) The object distance from the eye lens
(c) The radii of curvature of the eye lens	(d) The image distance from the eye lens

A source of e.m.f. E = 15 V and having negligible internal resistance is connected to a variable resistance so that the current in the circuit increases with time as i = 1.2 t + 3. Then, the total charge that will flow in first five second will be

(a) 10 C

(b) 20 C

(c) 30 C

(d) 40 C

A man suffering from myopia can read a book placed at 10 cm distance. For reading the book at a distance of 60 cm with relaxed vision, focal length of the lens required will be

(a) 45 cm

(b) – 20 cm

(c) – 12 cm

(d) 30 cm

Magnitude of produced magnetic field at any point inside a solenoid depends on:

Electric charges are distributed in a small volume. The flux of the electric field through a spherical surface of radius 10cm surrounding the total charge is 25V-m. The flux over a concentric sphere of radius 20cm will be: