How heat energy is transferred by Conduction?
“The mode of transfer of heat by vibrating atoms and free electrons in solids from hot to cold parts of a body is called conduction of heat.”
The handle of metal spoon held in hot water soon gets warm.But in case of a wooden spoon,the handle does not get warm.Both the materials behave differently regarding the transfer of heat.Both metals and non metals conduct heat.Metals are generally better conductors than non metals.
How Heat transfer in solids?
In solids,atoms and molecules are packed close together.They continue to vibrate about their mean position.What happens when one of its ends is heated?The atoms or molecules present at that end begin to vibrate more rapidly.They also collide with their neighbouring atoms or molecules.In doing so,they pass some of their energy to neighbouring atoms or molecules during collisions with them will increase in their vibrations.These atoms or molecules in tern pass on a part of the energy to their neighbouring particles.In this way some heat reaches the other parts of the solids.This is a slow process and very small transfer of heat takes place from hot to cold parts in solids.
How do metals differ from non-metals in terms of conducting heat?
Metals have free electrons.These free electrons move with very high velocities within the metal objects.They carry energy at a very fast rate from hot to cold parts of the object as they move.Thus ,heat reaches the cold parts of the metals objects from its hard parts much more quickly than non metals.
All metals are good conductors of heat.The substances through which heat does not conduct easily are called bad conductors or insulators. Wood,cork,cotton,wool,glass,rubber,etc are bad conductors or insulators.
Applications of conduction of heat
“The rate of flow of heat across the opposite faces of a meter cube of a substance maintained at a temperature difference of one kelvin is called thermal conductivity of that substance.”
Conduction of heat occurs at different rates in different materials.In metals heat flows rapidly as compared to insulators such as wood or rubber.Consider a solid block :
One of its two opposite faces each of cross sectional area A is heated to temperature T1. Heat Q flows along its length L to opposite face at temperature T2 in t seconds.
“The amount of heat that flows in unit time is called the rate of flow of heat.”
Thus Rate of flow of heat = Q/t……….(1)
It is observed that the rate at which heat flows through a solid object depends upon various factors.
Cross sectional area of the solid:
Larger cross sectional area A of a solid contains larger number of molecules and free electrons on each layer parallel to its cross sectional area and hence greater will be the rate of flow of heat through the solids.Thus:
Rate of flow of heat Q/t ∝ A …..(2)
Length of the solid:
Larger is the length between the hot and cold ends of the solids,more time it will take to conduct heat to the colder end and smaller will be the rate of flow of heat.Thus:
Rate of flow of heat Q/t ∝ 1/L ….(3)
Temperature difference between ends:
Greater is the temperature difference ( T1 – T2 ) between hot and cold faces of the solids,greater will be the rate of flow of heat.Thus:
Rate of flow of heat is Q/t ∝ ( T1 – T2 )….(4)
Combining the above factors,we get:
Q/t ∝ A ( T1 -T2 ) /L
Rate of flow of heat Q/t =K A ( T1 – T2 )/L …..(5)
Here K i s the proportionality constant called thermal conductivity of the solids.Its value depends on the nature of the substance and is different for different materials.From above equation (5),we find K as:
K=Q/t ×L/A ( T1 – T2 ) …..(6)