Question

Heat is transmitted from higher to lower temperature through actual mass motion of the molecules in
A). Conduction
B). Convection
C). Radiation
D). None of these

Answer 1

Hint: If heat is to be transferred from higher to lower temperature through the actual movement of molecules, the molecules must be relatively free to move about and not be bound by very strong intermolecular forces. This is not possible in solids as the molecules are very closely packed and not free to move about.

Complete step by step answer:
Heat is transferred from a higher temperature to a lower temperature by the actual movement of molecules by the process that the higher temperature molecules have higher energy and move about to reach molecules with lower temperature and transfer the energy to them. This process is known as convection.
This is only possible when the molecules are free to move about and are not bound by high intermolecular forces of attraction. Hence, convection does not happen in solids but can happen easily in liquids and gases.
Hence, the correct option is B) Convection.
Additional information:
Conduction is the process by which heat is transferred within a body due to the temperature difference in different parts of the body. It does not happen by the movement of molecules. The molecules remain in place and only help in transferring the thermal energy to their neighboring molecules by vibration.
Radiation is the process of heat transfer by electromagnetic waves. Electromagnetic waves carry energy and can transfer it to a surface on which they are radiated. This energy presents itself as thermal energy (heat). Radiation does not require a material medium to take place.

Note: Students sometimes get confused between radiation and convention and think that they are the same. However, they must remember that convention is by the actual movement of material molecules. For example a bowl of water gets heated uniformly by convection. The hot water rises up and transfers this energy of the cold water on the surface. However, radiation does not require a medium and there is no movement of material molecules involved. For example, when we sit near a fire, we feel warm because of the radiation coming from the fire. The hot fire does not come to touch us and make our bodies warmer as would be the case if convection were to take place.