We learned in the previous lesson, that heat flows from a body at a higher temperature
to a body at a lower temperature.
So how exactly does this heat flow occur?
Heat can flow in one of the three ways: through conduction, convection or radiation.
Heat conduction occurs in any material due to increased vibration and collision of molecules.
Heat transfer through convection requires a fluid i.e. either a liquid or gas, and the
fluid molecules transmit and transport the heat.
Radiation, on the other hand, requires no intermediate medium.
In this case, heat energy is in the form of electromagnetic waves that travel from one
surface to another, even in vacuum.
In this lesson, we will learn about these three modes of heat transfer and also about
their various applications.
The first mode of heat transfer is conduction and occurs primarily in solids.
At atomic scales, molecules in a solid are always vibrating with a certain amplitude.
When heat is supplied to some molecules by an external source, these molecules get excited,
and the amplitude of their vibration increases.
Such excited molecules collide with their neighboring molecules, passing on some of
their heat energy.
The neighboring molecules, in turn, get excited and collide with the next set of neighboring
molecules.
Thus, heat energy gets transferred from the hotter portion of the body to the colder portion.
In this course, we do not focus on the changes that occur at molecular level due to heat,
but instead, we study heat transfer at a macroscopic level.
We come across various examples of conduction in our daily lives.
When we place ice cubes in a glass, we observe that the ice starts melting.
This happens because heat flows from the glass to the cold ice cubes through conduction.
On the other hand, when we hold a hot cup of coffee on a cold day, heat flows from the
hot cup to our cold hands making us feel warm.
When we place a pan on the stove, the pan gets hot due to conduction and the food inside
gets cooked.
Different materials conduct heat at different rates.
For example, on a cold day if you touch a metal handrail your hand feels cold suddenly,
but if you touch wooden table you do not feel very cold.
This happens because metal is a good conductor of heat, while wood is a bad conductor.
So, heat flows out of your body at a faster rate when you touch a metal handrail , making
you feel cold.
Doesn’t a breeze feel nice on a hot, summer day?
The reason we feel cool, when the breeze flows, is due to the second mode of heat transfer,
known as convection.
In liquids and gases, together known as fluids, heat transfer is due to movement of the fluid
molecules.
When a fluid is heated the volume of fluid closest to the heat source increases, and
the density of that portion of fluid decreases.
Due to this reduction in density, that portion of fluid flows to the top.
Its place is taken by colder, denser fluid from surrounding regions.
In turn, the cold fluid comes in contact with the heat source, gets heated up, and moves
to the top.
This sets up a current in the fluid, which leads to heating up of the entire fluid.
This method of heat flow, caused by the movement of fluid molecules, is called convection.
Let’s look at some examples of convection.
Ocean water is warm near the equator of the earth, while it is very cold is in polar regions.
This difference in temperatures causes oceans currents, which affect climate and local ecosystems.
You might have heard the sound of cooling fans in your laptop when your laptop gets
heated.
These fans remove the excess heat by convection, thereby preventing any damage to the electronic
components, due to overheating.
Cars have radiators to prevent overheating of their engines.
A coolant liquid circulates between the engine and the radiator, dissipating excess heat
from the engine to the outside air through convection.
Convection is responsible for the “wind chill” that we experience in winter.
When wind blows, it carries away some heat from our body due to convection.
Due to this reason, the temperature that we experience or “feel like” will be colder
than the actual ambient temperature.
Finally, let’s take a look at the third mode of heat transfer: radiation.
The space in between the sun and the earth is vacuum.
So, there is no solid or fluid medium in that vast, empty space, which means conduction
or convection of heat cannot occur.
So then how does heat from the sun reach the earth?
The answer is, through radiation.
All objects with temperatures greater than absolute zero, emit heat in the form of electromagnetic
waves.
These electromagnetic waves do not need a medium to travel through.
Thus, radiation is the transfer of heat energy, in the form of electromagnetic waves.
When we sit around a campfire and feel warm, it is due to the heat from the fire, reaching
us in the form of electromagnetic waves.
Another example of radiation that we see in our day-to-day lives is the toaster oven.
When the toaster oven is switched on, heating elements inside the oven radiate heat which
cooks the food inside.
Since bodies at different temperatures radiate different amounts of heat, thermal imaging
can be used to capture their shape and size.
In fact, this is the working principle behind night vision goggles.
So, in this lesson, we learnt about the three methods of heat transfer: conduction, convection,
and radiation.
The examples illustrated in this lesson, are only a few of their many applications.
We will learn about the mathematical equations, governing each of these three heat transfer
modes, in the following lessons.