21.

An oxide coated filament is useful in vacuum tubes because essentially

• it has high melting point

• it can without and high temperature

• it has good mechanical strength

• it can emit electrons of relatively lower temperature

The Reynolds number of a flow is the ratio of

• gravity to viscous force

• gravity force to pressure force

• inertia force to viscous force

• viscous forces to pressure force

A monoatomic gas supplied the heat Q very slowly keeping the pressure constant. The work done by the gas will be

• $\frac{2}{3} \mathrm{Q}$

• $\frac{3}{5}\mathrm{Q}$

• $\frac{2}{5}\mathrm{Q}$

• $\frac{1}{5}\mathrm{Q}$

A mass m is suspended from the two springs ofspring constants k₁ and k₂ as shown. The time period of vertical oscillations of the mass will be

• $2\mathrm{\pi }\sqrt{\left(\frac{{\mathrm{k}}_1 + {\mathrm{k}}_2}{\mathrm{m}}\right)}$

• $2\mathrm{\pi } \sqrt{\left(\frac{\mathrm{m}}{{\mathrm{k}}_1 + {\mathrm{k}}_2}\right)}$

• $2\mathrm{\pi }\sqrt{\frac{\mathrm{m} \left({\mathrm{k}}_1 {\mathrm{k}}_2\right)}{{\mathrm{k}}_1 + {\mathrm{k}}_2}}$

• $2\mathrm{\pi }\sqrt{\frac{\mathrm{m} \left({\mathrm{k}}_1 + {\mathrm{k}}_2\right)}{\left({\mathrm{k}}_1 {\mathrm{k}}_2\right)}}$

A copper wire and a steel wire of the same diameter and length are connected end to end and a force is applied which stretches their combined length by 1 cm. Then the two wires will have

• the same stress and strain

• the same stress but different strain

• the same strain but different stress

• different stress and strain

Which of the following have higher specific change?

• Positron

• Proton

• He

• None of these

Rain drops are spherical in shape due to

• acceleration due to gravity

• downward motion

• surface tension

• capillary

For molecules of a gas have speeds 1, 2, 3 and 4 km/s. The value of the root mean square speed of the gas molecules is

• $\frac{1}{2}\sqrt{15} \mathrm{km}/\mathrm{s}$

• $\frac{1}{2}\sqrt{10} \mathrm{km}/\mathrm{s}$

• 2.5 km/s

• $\frac{\sqrt{15}}{2} \mathrm{km}/\mathrm{s}$

A simple pendulum of length l has a maximum angular displacement Q. The maximum kinetic energy of the bob is

• mgl (1 - cosθ)

• 0.5 mgl

• mgl

• 2 mgl

A step-up transformer has transformation ratio 3 : 2. The  voltage in the secondary coil, if the voltage in the primary is 30 V will be

• 300 V

• 90 V

• 45 V

• 23 V