Atoms — Physics STD 12 Science — Question

A plane electromagnetic wave of frequency 28MHz travels in free space along the positive x-direction. At a particular point in space and time, electric field is 9.3V/m along positive y-direction. The magnetic field (in T) at that point is 

A man wearing glasses of focal length +1m cannot clearly see beyond 1m:

1. If he is farsighted.
2. If he is nearsighted.
3. If his vision is normal.
4. In each of these cases.

A charged particle of mass m and charge q describes circular motion of radius r in a uniform magnetic field of strength B. The frequency of revolution is

(a)

(b)

(c)

(d)

An atom of mass number 15 and atomic number 7 captures an α - particle and then emits a proton. The mass number and atomic number of the resulting product will respectively be

(a) 14 and 2

(b) 15 and 3

(c) 16 and 4

(d) 18 and 8

In the figure, three capacitors each of capacitance 6 pF are connected in series. The total capacitance of the combination will be

(a) 9

(b) 6

(c) 3

(d) 2

The average self-induced emf in a 25mH solenoid when the current in it falls from 0.2A to 0A in 0.01 second, is 

The collector plate in an experiment on photoelectric effect is kept vertically above the emitter plate. Light source is put on and a saturation photocurrent is recorded. An electric field is switched on which has a vertically downward direction.

1. The photocurrent will increase.
2. The kinetic energy of the electrons will increase.
3. The stopping potential will decrease.
4. The threshold wavelength will increase.

When NPN transistor is used as an amplifier

(a) Electrons move from base to collector

(b) Holes move from emitter to base

(c) Electrons move from collector to base

(d) Holes move from base to emitter

Which theory is confirmed by the Davisson – Germer experiment?

Capacitance of a capacitor made by a thin metal foil is 2μF. If the foil is folded with paper of thickness 0.15 mm, dielectric constant of paper is 2.5 and width of paper is 400 mm , then length of foil will be

(a) 0.34 m

(b) 1.33 m

(c) 13.4 m

(d) 33.9 m