We have seen in nature that solids, liquids and gases, and empty spaces become hot when they are heated. But, to our surprise, they all get heated by different ways. The simple reason behind this difference is the fact that arrangement of particles in all these differ. In solids, particles are very tightly packed. In liquids and gases, the particles are free to move about, and in vacuum, there are no particles.
I have already discussed in my previous blog that solids get heated by the phenomenon called Conduction.
The transfer of heat ( ie. the way things become hotter) in liquids and gases occurs by the phenomenon called CONVECTION. It is the transfer of heat from hotter parts of liquids/ gases to its colder parts due to the movement of freely moving particles in liquids and gases.
The process of Convection in liquids and gases is just the same. Let’s see with examples.
Convection in Liquids
Let’s take water as the liquid. When we heat water in a saucepan from below, first the water at the bottom becomes hot, so it expands (ie. particles gain K.E. due to heat energy, vibrate vigourously and move apart, increasing the space they occupy) and becomes lighter and rises up. The colder denser water above in the saucepan sinks down, taking the space occupied by hot water. In this way, circulation of water is set in the saucepan where hot water rises up, colder water sinks down- then it gets heated and rises up and again the colder water from top sinks down creating a circular motion of water in the saucepan called CONVECTION CURRENTS. So, these convection currents transfer heat from water at the bottom to the water at the top, or in the other words, water is heated by Convection Currents.
A Small Experiment For Readers
Try heating water from top ie. bring the burner on the top of the saucepan containing water.
Does the entire water get heated or is the heat transferred from top to bottom?
You will find that the answer is ‘NO’.
REASON is- the water at the top gets heated- it becomes lighter, so, does not sink down and hence, the water at the bottom remains cold as no Convection Currents are formed.
So, heat is transferred in water by Convection only in the upward direction ie. from bottom to top.
Now you can answer the following Queries:-
Q1. Why we always place the burner below the utensil while boiling, cooking and likewise?
Q2. Why the heating element of an electric kettle placed at the bottom?
Convection in Gases (air)
The heat is transferred from hotter parts of air to colder parts by Convection, the process being exactly the same as in water because particles of air/ gases are also not fixed and can move about freely. So, air gets heated by CONVECTION CURRENTS in the same way as in water.
Now, you will be able to explain how a Room Heater kept at the floor heats the entire room.
It is to be noted that heat can be transferred in air only in the upward direction (because the hot air becomes lighter and rises up) as in liquids.
Where do you find Convection Currents in air in nature?
Ans. We find Convection Currents in the blowing of Sea breeze during the day and Land breeze during the night.
Now, a question comes to our mind- Will the heat be transferred when there are no particles ie. when there is no medium (ie. in vacuum)?
The best example to explain this, is the way by which heat is transferred from the sun to the objects on the earth.
The vast distance between the sun and the earth is mainly empty space or vacuum. Still, sun’s heat rays fall on objects on the earth and heat them. This heating of objects on the earth is not possible by Conduction or Convection as both these processes require a medium. So, this means that the transfer of heat from the sun to the earth takes place through a different process. The invisible heat rays of the sun are Infrared rays. These heat rays from the hot sun transfer heat energy to the colder objects on the earth, without any medium by the process called RADIATION.
So, it is evident, that in Radiation, the heat is transferred from top to bottom (just reverse of Conduction or Convection).
Can you think of examples where heat is transferred by Radiation in the presence of medium?
Ans. YES, see the following situations:- When we keep our hands below or around a glowing bulb or by the sides of a burning candle or around a campfire, we feel the heat on our hands. This heating is not due to Convection because in Convection heat is always transferred upwards, whereas here, it is being transferred in all directions.