Question

The presence of gravitational field is required for the heat transfer by

A. Conduction

B. Stirring of liquids

C. Natural convection

D. Radiation

Answer 1

Hint: Among the above options, stirring of liquid is not a way to transfer heat, while conducting needs the source to be near the sink to transfer heat. In convection and radiation, the source need not be near to the object to transfer heat and radiation does not require medium to transfer heat into a system.

Complete answer:

There are three ways to transfer heat in a system: Conduction, convection and radiation. Stirring of liquids is not a way of transferring heat in a system. In conduction, the heat transfer takes place directly when the source is in contact with the object, then the molecules in the object get excited due to this heat and rise up, while the less heated molecules settle down and then get excited. In such a type of transfer, no gravitational field is needed.

In natural convection, the heat transfer does not need any contact to transfer. The thermal energy travels away from the source towards the object. The temperature gradient exists only in the vertical direction and not in the horizontal direction because of the density variation with the temperature rise. As temperature increases fluid portions become light and move upwards, heavy and more dense fluid portions move downward. So, up and down movement of particles takes place; this depends on the weight and gravity. Thus the presence of a gravitational field is required in the heat transfer through natural convection.

Example of this phenomenon is boiling of water.

Hence, option C is the correct answer.

Note: Radiation does not even require medium to travel. For example, we can feel the warmth of the sun due to the fact that the heat travels through radiation when there is no medium in between the space between earth and the sun that is there is only vacuum in between. Conduction and convection cannot travel through in vacuum.