11.

Two small drops of mercury, each of radius R, coalesce to form a single large drop. The ratio of the total surface energies  before and after  change is

• 1 : 2^1/3

• 2^1/3 : 1

• 2 : 1

• 1 : 2

A black body, at a temperature of  227^°C, radiates heat at  a rate of  20 cal m^-2 s^-1. When its temperature is raised to 727^°C, the heat radiated by it in cal m^-2 s^-1 will be closest to

• 40

• 160

• 320

• 640

Two springs of force constants k and 2k are connected to a mass as shown in figure . The frequency of oscillation of the mass is

• $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{\mathrm{k}}{\mathrm{m}}}$

• $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{2\mathrm{k}}{\mathrm{m}}}$

• $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{3\mathrm{k}}{\mathrm{m}}}$

• $\frac{1}{2\mathrm{\pi }}\sqrt{\frac{\mathrm{m}}{\mathrm{k}}}$

Shown below are the black body radiation curves at temperatures  T₁  and  T₂ ( T₂ >  T₁ ). Which of the following plots is correct?

Assertion: Temperatures near the sea coast me moderate.

Reason:  Water has a high thermal conductivity.

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

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

• If assertion is true but reason is false

• If both assertion and reason are false statements

Assertion: When a beetle moves along the sand within a few tens of centimetres of a sand scorpion, the scorpion immediately turns towards the beetle and dashes towards it. 

Reason:  When a beetle disturbs the sand, it sends pulses along the sand's surface. One set of pulses is longitudinal while the other set is transverse.

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

• if both assertion and reason are true and the reason is not the correct explanation of the assertion

• if assertion and reason is false

• if both assertion and reason are false statements

Assertion: When a bottle of cold carbonated drink is opened, a slight fog forms around the opening. 

Reason:  Adiabatic expansion of the gas causes lowering of temperature and condensation of water vapours.

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

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

• if assertion is true but reason is false

• if both assertion and reason are false statements

Assertion: The size of a hydrogen balloon increases as it rises in air. 

Reason:  The material of the balloon can be easily stretched.

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

• if both assertion and reason are true and the reason is not the correct explanation of the assertion

• if assertion is true but reason is false

• if both assertion and reason are false statements

Assertion: It is hotter over the top of a fire than at the same distance on the sides

Reason: Air surrounding the fire conducts more heat upwards.

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

• if both assertion and reason are true and the reason is not the correct explanation of the assertion

• if assertion is true but reason is false

• if both assertion and reason are false statements

Assertion: The amplitude of an oscillating pendulum decreases gradually with time. 

Reason: The frequency of the pendulum decreases with time.

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

• if both assertion and reason are true and the reason is not the correct explanation of the assertion

• if assertion is true but reason is false

• if both assertion and reason are false statements