31.

Two identical long conducting wire AOB and COD are placed at right angle to each other, with one above other such that O is their common point for the two. The wires carry I₁ and I₂ currents, respectively. point P is lying at distance d from O along a direction perpendicular to the plane containing the wires. The magnetic field at the point P will be,

• 
• 
• 
• 

A thin semicircular conducting ring (PQR) of radius r is falling with its plane vertical in a horizontal magnetic field B, as shown in figure. The potential difference developed across the ring when its speed is v, is

• zero

•  is at higher potential
•  and R is at higher potential
•  and R is at higher potential

A transformer having efficiency of 90% is working on 200 V and 3 kW power supply. If the current in the secondary coil is 6 A, the voltage across the secondary coil and the current in the primary coil respectively are: 

• 300 V, 15 A

• 450 V, 15 A

• 450 V, 13.5 A

• 450 V, 13.5 A

Light with an energy flux of 25 x 10⁴ W/m² falls on a perfectly reflecting surfaces at normal incidence. If the surface area is 15 cm², the average force exerted on the surface is,

• 1.25 x 10^-6 N

• 2.50 x 10^-6 N

• 1.20 x 10^-6 N

• 1.20 x 10^-6 N

A beam of light of  = 600 nm from a distant source falls on a single slit 1 mm wide and the resulting diffraction pattern is observed on a screen 2 m away. The distance between first dark fringes on either side of the central bright fringe is,

• 1.2 cm

• 1.2 mm

• 2.4 cm

• 2.4 cm

In the young's double slit experiment, the intensity of light at a point on the screen (where the path difference is ) is K. The intensity at a point where the path difference is  will be,

• K

• K/4

• K/2

• K/2

If the focal length of the objective lens is increased, then the magnifying power of,

• microscope will increase but that of telescope will decrease

• microscope and telescope both will increase

• microscope and telescope both will decrease

• microscope and telescope both will decrease

The angle of a prism is A. One of its refracting surfaces is silvered. Light rays falling at an angle of incidence 2A on the first surface returns back through the same path after suffering reflection at the silvered surface. The refractive index , of the prism is, 

• 2 sin A

• 2 cos A

• 1/2 cos A

• 1/2 cos A

When the energy of the incident radiation is increased by 20%, the kinetic energy of the photoelectrons emitted from a metal surface increased from 0.5 eV to 0.8 eV. The work function of the metal is,

• 0.65 eV

• 1.0 eV

• 1.3 eV

• 1.3 eV

If the kinetic energy of the particle is increased to 16 times its previous value, the percentage change in the de-Broglie wavelength of the particle is,

• 25

• 75

• 60

• 60