31.

When a positively charged particle enters into a uniform magnetic field with uniform velocity, its trajectory can be

(i) a straight line  (ii) a circle  (iii) a helix

• (i) only

• (i) or (ii)

• (i) or (iii)

• any one of (i), (ii) and (iii)

Assertion:  Gauss's law can't be used to calculate electric field near electric an electric dipole.

Reason: Electric dipole don't have symmetrical charge distribution.

• If both assertion and reason are true and reason is the correct explanation of assertion.

• If both assertion and reason are true but reason is not correct explaination of assertion.

• If assertion is true but reason is false.

• If both assertion and reason are false.

Assertion: Magnetic field lines are continuous and closed.

Reason: Magnetic monopole does not exist.

• If both assertion and reason are true but reason is not the correct explanation of assertion.

• If both assertion and reason are true but reason is not the correct explanation of assertion.

• If assertion is true but reason is false.

• If both assertion and reason are false.

Assertion:  Magnetic force between two short magnets, when they are co-axial follows inverse square law of distance. 

Reason:  The magnetic forces between two poles do not follow inverse square law of distance.

• If both assertion and reason are true and reaon is correct explanation of assertion.

• If both assertion and reason are true and reaon is not the correct explanation of assertion.

• If assertion is true but reason is false.

• If both assertion and reason are false.

In an interference, the intensity of two interfering waves are  I  and  4I  respectively. They produce intensity at two points A and B with phase angle of  $\raisebox{1ex}{$\mathrm{\pi }$}\!\left/ \!\raisebox{-1ex}{$2$}\right.$ and  $\mathrm{\pi }$ respectively. Then difference in between them is

• I

• 2 I

• 4 I

• 5 I

In a single slit diffraction with k = 500 nm and lens of diameter 0.1 mm then width of central maxima, obtain on screen at a distance of 1 m will be

• 5 mm

• 1 mm

• 10 mm

• 2.5 mm

Voltage of modulating wave of 5 V with 10 MHz frequency was superimposed on carrier wave of frequency 20 MHz and voltage 20 V then the modulation index is

• 0.25

• 1.25

• 2.43

• 64.0

The area covered by a transmitting antenna of height 50 m is

• 320$\mathrm{\pi }$ km²

• 1440$\mathrm{\pi }$ km²

• 640$\mathrm{\pi }$ km²

• 120$\mathrm{\pi }$ km²

If we assume kinetic energy of a proton is equal to energy of the photon, the ratio of de Broglie wavelength of proton to photon is proportional to

• E

• E^-1/2

• E^1/2

• E^3/2

The ratio of the masses of the elements having their nuclear radii 2 fermi and 1 fermi

• 8

• 2

• 3

• 4