Thermodynamics: Types of Systems and Entropy
Three different thermodynamic systems are under study. Their analysis is given below:
(a) Type of system in Case 1
Case 1: Water is boiling in an open container with continuous supply of heat.
Type of system: Open system
Reason: An open system allows exchange of both matter and energy with surroundings. Heat enters the system continuously and water vapour escapes from the container.
(b) Case in which entropy increases continuously
Answer: Case 1
During boiling, liquid water changes into vapour. Continuous heat supply and phase change increase molecular randomness, hence entropy increases continuously.
(c) Expression for internal energy change in Case 1
For an open system, internal energy change is given by:
dU = δQ − δW + Σhindmin − Σhoutdmout
Where δQ is heat supplied, δW is work done, h is enthalpy and dm represents mass flow.
(d) System in which entropy decreases with time
Answer: Case 3
In freezing, liquid changes into solid. Molecular motion decreases and order increases, therefore entropy decreases with time.
Summary Table
| Case | Type of System | Entropy Change |
|---|---|---|
| Case 1 (Boiling, open) | Open system | Increases continuously |
| Case 2 (Boiled water, closed) | Closed system | Nearly constant |
| Case 3 (Freezing, closed) | Closed system | Decreases |
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