21.

Three charges, each +q, are placed at the corner of an isosceles triangle ABC of sides BC and AC, 2a. D and E are the midpoints of BC and CA. The work done in taking a charge Q from D to E is.

• 

• 

• zero

• zero

The threshold frequency for a photo-sensitive metal is 3.3 x 10¹⁴ Hz. If light of frequency 8.2 x 10¹⁴ Hz is incident on this metal, the cut -off voltage for the photo-electric emission is nearly

• 2 V

• 3 V

• 5 V

• 5 V

A thin prism of angle 15^o made of glass of refractive index is combined with another prism of a glass of refractive index. The combination of the prism produces dispersion without deviation. The angle of the second prism should be

• 7^o

• 10^o

• 12^o

• 12^o

In the following figure, the diodes which are forward biased, are

Pure Si at 500 K has an equal number of the electron (n_e) and hole (n_h) concentrations of 1.5 x 10¹⁶ m^-3.Doping by indium increases n_h to 4.5 x 1022 m ^-3.The doped semiconductor is of 

• n- type with electron concentration n_e = 5 x 10²² m ^-3

• P- type with electron concentration n_e = 2.5 x 10¹⁰ m ^-3

• n- type with electron concentration n_e = 2.5 x 10²³ m ^-3

• n- type with electron concentration n_e = 2.5 x 10²³ m ^-3

A Zener diode, having breakdown voltage equal to 15 V, is used in a voltage regulator circuit shown in the figure. The current through the diode is

• 10 mA

• 15 mA

• 20 mA

• 20 mA

An electron in the hydrogen atom jumps from excited state n to the ground state. The wavlength so emitted illuminates a photo -sensitive material having work function 2.75 eV. If the stopping potential of the photo-electron is 10 V, the value of n is 

• 3

• 4

• 5

• 5

Two radioactive nuclei P and Q, in a given sample decay into a stable nucleous R. At time t =0, number of P species are 4N_o and that of Q are N_o.Half -life of P (for conversion to R) is 1 min where asthat of Q is 12 min. Initially there are no nuclei of R present in the sample. When number of nuclei of R presnent in the sample would be

• 3N_o

• 9N_o / 2

• 5N_o/2

• 5N_o/2

Out of the following which one is not a possible energy for a photon to be emitted by hydrogen atom according to Bohr's atomic model?

• 1.9 eV

• 11.1 eV

• 13.6 eV

• 13.6 eV

A conversing beam of rays is incident on a diverging lens. Having passed though the lens the rays intersect at a point 15 cm from the lens on the opposite side. If the lens is removed the point where the rays meets will moves 5 cm closer to the lens. The focal length of the lens is 

• -10 cm

• 20 cm

• -30 cm

• -30 cm