31.

Magnetic field intensity H at the centre of a circular loop of radius r carrying current I emu is 

• $\frac{\mathrm{r}}{\mathrm{I}} \mathrm{oersted}$

• $\frac{2\mathrm{\pi I}}{\mathrm{r}} \mathrm{oersted}$

• $\frac{\mathrm{I}}{2\mathrm{\pi r}} \mathrm{oersted}$

• $\frac{2\mathrm{\pi r}}{\mathrm{I}} \mathrm{oersted}$

Which of the following materials is the best conductor of electricity ?

• Platinum

• Gold

• Silicon

• Copper

To write the decimal number 37 in binary, how many binary digits are required ?

• 5

• 6

• 7

• 4

A junction diode has a resistance of 25 Ω when forward biased and 2500 Ω when reverse biased. The current in the diode, for the arrangement shown will be

• $\frac{1}{15} \mathrm{A}$

• $\frac{1}{7} \mathrm{A}$

• $\frac{1}{25} \mathrm{A}$

• $\frac{1}{180}\mathrm{A}$

If the electron in a hydrogen atom jumps from an orbit with level n₁ = 2 to an orbit with level n₂ = 1 the emitted radiation has a wavelength given by

• $\mathrm{\lambda } = \frac{5}{3\mathrm{R}}$

• $\mathrm{\lambda } = \frac{4}{3\mathrm{R}}$

• $\mathrm{\lambda } = \frac{\mathrm{R}}{4}$

• $\mathrm{\lambda } = \frac{3\mathrm{R}}{4}$

What is the particle X in the following nuclear reaction ?

    ${}_4{}^9\mathrm{Be} + {}_2{}^4\mathrm{He} \stackrel{}{\to } {}_6{}^{12}\mathrm{C} + \mathrm{X}$

• Electron

• Proton

• Photon

• Neutron

The distance between an object and a divergent lens is m times the focal length of the lens. The linear magnification produced by the lens is

• m

• $\frac{1}{\mathrm{m}}$

• m + 1

• $\frac{1}{\mathrm{m} + 1}$

A 2.0 cm object is placed 15 cm in front of a concave mirror of focal length 10 cm. What is the size and nature of the image ?

• 4 cm, real

• 4 cm, virtual

• 1.0 cm, real

• None of these

A beam of monochromatic blue light of wavelength 4200 $\stackrel{\circ }{\mathrm{A}}$ in air travels in water of refractive index 4/3. Its wavelength in water will be.

• 4200 $\stackrel{\circ }{\mathrm{A}}$

• 5800 $\stackrel{\circ }{\mathrm{A}}$

• 4150 $\stackrel{\circ }{\mathrm{A}}$

• 3150 $\stackrel{\circ }{\mathrm{A}}$

Two identical light waves, propagating in the same direction, have a phase difference 6. After they superpose the intensity of the resulting wave will be proportional to

•  $\cos \mathrm{\delta }$

• $\cos \left(\frac{\mathrm{\delta }}{2}\right)$

• ${\cos }^2 \left(\frac{\mathrm{\delta }}{2}\right)$

• ${\cos }^2\mathrm{\delta }$