Air is a bad conductor of heat or partly conducts heat. Still, a vacuum is to be placed between the walls of the thermos flask because:

1.	it is difficult to fill the air between the walls of the thermos flask.
2.	due to more pressure of air, the thermos can get cracks.
3.	by convection, heat can flow through the air.
4.	on filling the air, there is no advantage.

While measuring the thermal conductivity of a liquid, we keep the upper part hot and the lower part cool, so that:

Mode of transmission of heat, in which heat is carried by the moving particles, is 
(1) Radiation           

(2) Conduction

(3) Convection         

(4) Wave motion

In heat transfer, which method is based on gravitation ?

(1) Natural convection               

(2) Conduction

(3) Radiation                             

(4) Stirring of liquids

When fluids are heated from the bottom, convection currents are produced because -

(1) Molecular motion of fluid becomes aligned

(2) Molecular collisions take place within the fluid

(3) Heated fluid becomes more dense than the cold fluid above it

(4) Heated fluid becomes less dense than the cold fluid above it

If a liquid is heated in weightlessness, the heat is transmitted through 

(1) Conduction

(2) Convection

(3) Radiation

(4) Neither, because the liquid cannot be heated in weightlessness

The rate of loss of heat from a body cooling under conditions of forced convection is proportional to its (A) heat capacity (B) surface area (C) absolute temperature (D) excess of temperature over that of surrounding : state if

(1) A, B, C are correct               

(2) Only A and C are correct

(3) Only B and D are correct     

(4) Only D is correct

In which of the following process, convection does not take place primarily ?

(1) Sea and land breeze

(2) Boiling of water

(3) Warming of glass of bulb due to filament

(4) Heating air around a furnace

On a clear sunny day, an object at temperature T is placed on the top of a high mountain. An identical object at the same temperature is placed at the foot of the mountain. If both the objects are exposed to sun-rays for two hours in an identical manner, the object placed on the top of a mountain will register a temperature:

Heat travels through vacuum by 
(1) Conduction                 

(2) Convection

(3) Radiation                     

(4) Both (a) and (b)