Heat - Part IV
heat is transmitted
: We have seen earlier that heat is transmitted from a hotter body to a
colder body, till a thermal equilibrium is reached.
general, heat flows from a hotter to a colder body by one of the three
net flow of heat can sometimes be through more than one of these processes.
Now we will see in details, these processes.
Conduction of heat is defined as the mode of transmission of heat,
through a medium, without actual migration of the atoms or molecules of the
For conduction of heat, a
material medium is necessary. Conduction
occurs mostly in solids. This can be understood if we see how solids, liquids and gases are bound to
their respective atoms or molecules. Solids are tightly bound. The atoms or
molecules cannot migrate from one site of the solid to the other. Although,
the electrons can do so. In liquids and gases, the atoms or molecules
are not that tightly bound, so when the heat has to be transferred or
transmitted, the atoms and molecules themselves move. This is called
convection of heat, which we will see in the next section.
In the experiment of heating a spoon, if instead you
had used a glass
rod, you will notice that the end of the glass rod that you are holding,
does not become hot at all. This shows that heat cannot readily flow in
glass. Glass is a bad conductor of heat. Other bad conductors of heat are
rubber, wood, cork, paper, bakelite, etc. You will notice that the handles
on cooking pans are made of a plastic called bakelite. That is why you can
hold a hot pan. Water is also a bad conductor of heat. Most of the metals
are good conductors of heat. This is because they loose electrons which
can freely move around and conduct heat (as well as electricity). Most of
the ionic compounds are relatively good conductors of heat. Most of the
covalently bonded compounds are bad conductors of heat.
: Keep a vessel of water for boiling. Hold your hand close to
the vessel. You will feel the warmth of the vessel. Similarly, if we sit in
the sun, we feel warm. How is the heat transmitted in this case? The process
cannot be conduction, as there is no direct contact. The process cannot be
convection because there are no material particles moving from the heat
source towards the hand. The
invisible way of transmission of heat is known as radiation.
Radiation is defined as the mode of transfer of heat in the form of
For this no material medium is required.
Heat from the sun is the best example of radiation. The sunís rays reach the earth without having any material medium in between the earth and the sun. The electromagnetic rays carry energy from the sunís surface to the earth and heat up the earth and its atmosphere.
 Light is a form of electromagnetic wave. When electrons are excited to higher orbitals, they can return to lower orbitals by emission of electromagnetic waves. This we will study later. Suffice to know that electromagnetic waves carry energy and do not need any medium for traveling from one place to the other.