21.

Assertion: Machine parts are jammed in winter. 

Reason:  The viscosity of lubricant used in machine parts increase at low temperatures.

• 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 the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

Assertion:  A brass tumbler feels much colder than a wooden tray on a chilly day. 

Reason: The thermal conductivity of brass is more than the thermal conductivity of wood.

• 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 the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

Assertion:  In free expansion of an ideal gas, the entropy increases. 

Reason:  Entropy increases in all natural processes.

• 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 the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

Assertion:  Sound waves cannot travel in vacuum but light can travel in vacuum. 

Reason: Sound waves are longitudinal waves and they cannot be polarised but  electromagnetic waves are transverse and they can be polarised.

• 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 the correct explanation of assertion

• If assertion is true but reason is false

• If both assertion and reason are false

With the decrease of current in the primary coil from 2 amperes to zero value in  0.01s  the emf generated in the secondary coil is   1000 volts. The mutual inductance of the two coils is

• 1.25 H

• 2.50 H

• 5.00 H

• 10.00 H

In a case of infinite long wire electric field is proportional to

• $\frac{1}{\mathrm{r}}$

• $\frac{1}{{\mathrm{r}}^2}$

• $\frac{1}{{\mathrm{r}}^3}$

• r⁰

What is the magnetic field at a distance R from a coil of radius r carrying current ?

• $\frac{{\mathrm{\mu }}_0 \mathrm{I} {\mathrm{R}}^2}{2 {\left( {\mathrm{R}}^2 + {\mathrm{r}}^2 \right)}^{{\displaystyle \raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}}$

• $\frac{{\mathrm{\mu }}_0\mathrm{I} {\mathrm{R}}^2}{2 {\left( {\mathrm{R}}^2 + {\mathrm{r}}^2 \right)}^{{\displaystyle \raisebox{1ex}{$3$}\!\left/ \!\raisebox{-1ex}{$2$}\right.}}}$

• $\frac{{\mathrm{\mu }}_0 \mathrm{I}}{2\mathrm{r}}$

• $\frac{{\mathrm{\mu }}_0 \mathrm{I}}{2\mathrm{R}}$

In the following diagram, which particle has highest e/m value?

• A

• B

• C

• D

What is the dimensions of impedance ?

• [ M L² T^-3 A^-2 ]

• [ M^-1 L^-2 T³ A² ] 

• [ M L³ T^-3 A^-2 ]

• [ M^-1 L^-3 T³ A² ]

Which wavelength of sun is used finally as electric energy?

• Radio waves

• Infrared waves

• Visible light

• visible light