CHAPTER 14 : HEAT TRANSFER
From observations and experiments it is found that heat energy can be transferred from one
position to another through three different modes: conduction, convection and radiation.
This process occurs most significantly in solids. The atoms or molecules in a solid do not leave
their mean positions, but continue to vibrate about their mean positions. They transfer heat energy
from one atom to another. This happens because of the coupling between them due to mutually
When a solid is heated for a long interval it remains in an unsteady state of heat conduction.
The temperature at each point along its length keeps on increasing with time, whereas the
temperature along its length decreases from the hot to the cold end, i.e. decreasing towards the end that is
away from the source.
After the initial unsteady state, the solid reaches a steady
state of heat conduction, therefore variation in temperature with
time does not occur. However, with regard to its length, the same
condition is maintained i.e. the temperature continues to decrease
with increase in its length, as one moves away from the source of
At our level one can understand the process to be a partial absorption of heat energy during
the unsteady state by each section and a partial transfer to the next. On the other hand, in a steady
state there is no absorption and a complete transfer of heat energy.
A : Area of Cross- section
l : D x = thickness
v1 : Temperature of the left face of the solid where the heat approaches.
v2 : Temperature of the right face from where heat leaves.
Q : Amount of heat
t : time for which the heat is conducted through the slab.
Then in the steady state,
The constant 'K' in the above expression is called the coefficient of thermal conductivity. Its unit
is 1 k-cal / m sec 0K or 1 watt / m 0K.