Heat is the interaction between systems which
occurs by virtue of their temperature difference when they communicate.
If a
system, at a given temperature is brought in contact with another system (or
surroundings) at a lower temperature, it can be observed that heat is
transferred from the system at the higher temperature to the system at lower
temperature. This heat transfer occurs solely because of the temperature
difference between the two systems. Another important aspect of the definition
of heat is that a body never contains heat. Rather, heat can be identified only
as it crosses the boundary. Similar to work, heat is also a form of energy transfer
occurring at the boundary of the system and is a path function.
Sign Convention of Heat:-
· Heat given into
a system is positive
· Heat coming out
of the system is negative
Modes of Heat Exchange
Conduction, convection and radiation are the
three possible modes of heat transfer between systems and between system and
its surroundings.
Conduction occurs without bulk movement of molecules. Energy transfer in conduction is due to
lattice vibration and free electron movement.
It is the predominant mode of heat transfer in solids.
Convection occurs with bulk movement of molecules and therefore,
occurs in gases and liquids. If the bulk
movement or flow is due to an external device, it is known as forced
convection. In the absence of an
external device the flow is due to the difference in density caused by the
temperature difference. This mode is
known as natural convection.
Bodies separated by a distance may exchange heat in the form of
electromagnetic waves without the participation of the intervening medium. It is known as radiation. It is generally a
surface phenomenon. Sometimes as in the case of gas mixtures containing carbon
dioxide and water vapour it is a volume phenomenon.
Sensible and Latent Heat
It is
known that a substance can exists in three phases namely solid, liquid and gas.
When a substance is heated or cooled temperature of the substance increases or
decreases respectively unless there is any phase change. Quantity of heat added
or removed to change the temperature by unit degree is known as specific heat.
For solids and liquids same quantity of heat is required to cause unit degree
rise for both constant pressure heating as well as constant volume heating as
they are incompressible. But for gases there is appreciable difference in the
quantity of heat required to cause unit difference in temperature between
constant volume and constant pressure processes. Accordingly, they are known as
specific heat at constant volume (CV) and specific
heat at constant pressure (CP). Thus to
increase the temperature of m kg of the given substance by DT degree, amount of heat required is given by
Q = mCVDT at Constant Volume ...(2.5)
Q1 = mCPDT at Constant Pressure …(2.6)
If a
certain single component system is undergoing phase change at constant
pressure, temperature of the system
remains constant during heating or cooling. Quantity of heat removed or added
to cause the change of phase of unit mass of the substance is known as latent
heat. For example latent heat of fusion of water is the amount of heat to be
removed to solidify 1 kg of water into 1 kg of ice at a given temperature.
Let us
consider a process of converting 1 kg of ice at -30°C to system to steam at 250°C at atmospheric pressure. We know that ice melts at 0°C and water evaporates at 100°C at atmospheric pressure.
The total heat required can be obtained as follows:
Q = Qab + Qbc + Qcd + Qde + Qef ...(2.7)
Qab = mCice (tb - tc) ...(2.8)
Qbc = Latent heat of melting of ice at 0°C
Qcd = mCwater (td - tc) ...(2.9)
Qde = Latent heat of evaporation of water at 100°C
Qef = mCPSteam (tf - te) ...(2.10)
Where Cice = Specific heat
of ice
Cwater = Specific heat of water
CPSteam =
Specific heat of steam at constant pressure
Reversible Adiabatic Process
A reversible process during which, the system and the surroundings
do not exchange any heat across the boundary is known as reversible adiabatic
process. For such a process, pressure and volume variation is governed by the
law :
pVg = constant
. ..(2.11)
Where
Cp is the specific heat at
constant pressure
CV is the specific heat at
constant volume
Detailed discussion on these
specific heats is presented in the next chapter.
A wall which does not permit the
heat flow across it is known as adiabatic wall, whereas the wall that permits
the heat is known as diathermic wall. In an adiabatic process the only possible
energy interaction across the boundary of the system is work transfer to or
from the system.
Comparison between work and heat
l Both heat and work are boundary phenomena, that is, they occur only
at the boundary.
l The
interaction due to the temperature difference is heat and all other interactions
are to be taken as work.
l Both work and heat are path functions, that is, they are inexact
differentials.
