  Common Misconceptions

## Thermodynamics

- What you are getting from a statement might not be necessarily true. Let's burst some of the common misconceptions
1
MYTH: Heat is the energy that can be stored in a system.
FACT: Heat energy can not be stored in a system. It is energy in transit.
When the state of the system (specifically temperature) changes, heat energy flows from one part of the system to other. Before the change in state or after the change in state, heat energy does not exist. Thus it is energy in transit.
2
MYTH: The volume of a closed system is constant.
FACT: The volume of a closed system can change.
If a closed system has non-rigid wall then the volume of that system can change. The gas in a cylinder fitted with a piston is a closed system but one can change the volume of the system by simply moving the piston up and down.
3
MYTH: No process can be adiabatic and isothermal simultaneously.
FACT: Free expansion of a gas is an adiabatic as well as isothermal process.
In free expansion of a gas, no heat is exchanged between the system and the surroundings. Thus the process is adiabatic. At the same time the temperature of the system does not alter. Therefore it is an isothermal process too.
4
MYTH: Change in entropy of a system cannot be negative.
FACT: The change in entropy of a system can be negative.
If the final entropy of a system is less than the initial entropy of the system then the change in entropy is negative. For example, entropy change is negative in condensation of vapour because molecules have more degree of disorder in vapour as compared to liquid.
5
MYTH: The total energy of a system at a fixed pressure and volume is the internal energy.
FACT: The total energy of the system at a fixed pressure and volume is enthalpy.
At a fixed pressure and volume a system has the internal energy because of the temperature. But it has a pressure-volume work because of the fixed pressure and volume. Thus the total energy of the system is the sum of the internal energy and the pressure-volume work, i.e. . This total energy of the system is called the enthalpy.
6