O-Level Physics Notes

1. TRANSFER OF THERMAL ENERGY!
Introduction:
Hey everyone! How are you? Hope you are all good. Today we’re going to study how the thermal energy is
transmitted or transferred. Do you know that thermal energy, that is heat, can only be transferred when there’s a
difference in temperature, and when this happens, it flows from a region of higher temperature to a region of lower
temperature. Too much to absorb? Don’t worry! We’ll make it easy for you.
Definition:
Thermal energy can be transferred by any of the three processes: conduction, convection and radiation. Regardless
of the method of transfer, only heat can be transferred because cold is the absence of heat.
Conduction:
Now first of all, first is conduction? Conduction is the transfer of heat from one molecule to another through a
substance,the energy is transferred without any transfer of material medium.
In this diagram, the heat is transferred through the rod and we can see that there is no transfer of medium. Only heat
is being transferred. Now, the question is: how is heat transferred? When we heat the metal rod from one end, the
atoms on that end start to vibrate as they gain energy and transfer the vibration on the neighbouring atoms.
Consequently, they gain energy too. The entire metal rod becomes hot. This process occurs in non-metals like wood
as well.
But then why are metals better conductors? This is because in metals like copper, there exist free moving electrons
as well. The electrons on the hot end gain energy and move faster as a result. These electrons then diffuse into the
cooler end and when they collide with atoms on cold end, they transfer their energy. This is why metals can transfer
energy much faster than insulators (i.e. non metals).
In liquids and gases, the process of conduction in transfer of thermal energy is insufficient because the atoms are far
apart from each other and they can hardly transfer the vibrations.
Task: Google the experiment which shows that water is a poor conductor of heat.
Convection:
Okay so if conduction is not active in fluids, then how is thermal energy transferred? By convection. Convection is the
process whereby thermal energy is transferred by means of currents. How does it work exactly? Suppose a flak
containing water as shown below is heated.

2. When the water is heated, it will expand. The expanded water is less dense than the cold water and starts to rise.
The cooler regions of water in the upper part of the flask, being denser, sink. This is shown by the arrows in the
diagram above. This movement of the liquids is set up due to the difference in densities, and the current which sets
up is called the convection current. It occurs only in fluids such as liquids and gases but not solids.
To further clear it out, imagine what happens when you turn on the room heater. The air around heater gets heated
up, expand and becomes less dense. So it rises. The cooler air on the upper side is denser and so it sinks. In such a
way, convection currents are created. Easy and logical.
Radiation:
Radiation is the continual emission of infrared waves from the surface of all bodies. Radiation is the only process
which does not require a medium to transmit heat, which means it can occur in vacuum as well.
The best example can be the transfer of heat from Sun to the Earth through vacuum by the process of radiation.
Q1. Why can’t conduction and convection take place to transfer thermal energy from Sun to Earth?
Radiation is affected by three factors, let’s discuss those.
1. Colour and texture of surface. Dark, dull and rough surfaces are better absorbers and emitters of infrared
radiations than shiny, white and smooth surfaces.
2. Surface temperature. The higher the temperature of the surface of the object relative to the surrounding
temperature, the higher is the rate of infrared radiations.
3. Surface area. In two objects with the same mass and material but different surface areas, the one with the larger
surface area will emit and absorb infrared radiations at a higher rate.
Task: Google the applications of the three processes.
Answers:
Q1. Because these two processes
require a medium to take
place. While there is vacuum b/w
Earth and Sun.