Isothermal vs Adiabatic Process: Definitions, Examples, and Comparison
The Difference Between Isothermal and Adiabatic Process is a key thermodynamics topic for JEE, NEET, and board exams. Understanding this difference helps students tackle questions involving heat, temperature, and energy relations in gases during various thermodynamic changes.
Definition of Isothermal Process
An isothermal process is a thermodynamic process in which the temperature of a system remains constant throughout the process. For ideal gases, this means that the change in internal energy is zero. For details, refer to Exploring Isothermal Process.
During such a process, the system exchanges heat with its surroundings to maintain the constant temperature as pressure or volume changes. This concept is widely tested in physics exams and is a fundamental part of Thermodynamics Overview.
Definition of Adiabatic Process
An adiabatic process is a thermodynamic process where no heat is transferred into or out of the system. The system is perfectly insulated from its surroundings during the process. To learn more, see Understanding Adiabatic Process.
In the adiabatic process, any change in internal energy results from the work done on or by the system, causing the temperature of the system to change.
Difference Table
| Isothermal Process | Adiabatic Process |
|---|---|
| Temperature remains constant (ΔT = 0) | Temperature changes (ΔT ≠ 0) |
| Heat exchange with surroundings occurs | No heat is exchanged (Q = 0) |
| PV = constant (Boyle’s Law for ideal gases) | PV$^\gamma$ = constant for ideal gases |
| Internal energy remains unchanged (ΔU = 0 for ideal gas) | Internal energy changes (ΔU ≠ 0) |
| Work done comes from heat supplied or removed | Work done leads to change in internal energy |
| Process requires slow change, allowing heat flow | Process is quick or the system is insulated |
| Work done: $W = nRT \ln \dfrac{V_2}{V_1}$ | Work done: $W = \dfrac{P_2V_2-P_1V_1}{1-\gamma}$ |
| Example: Melting of ice at 0°C | Example: Sudden compression in a bicycle pump |
| Maintains equilibrium with a thermal reservoir | System is thermally insulated from surroundings |
| Heat supplied equals work done ($Q = W$) | Change in internal energy equals work done ($\Delta U = -W$) |
| PV diagram curve is less steep (shallow hyperbola) | PV diagram curve is steeper |
| Slow expansion/compression in thermostat bath | Rapid piston movement, such as sound waves in air |
| Phase changes often proceed isothermally | No phase change, temperature varies during process |
| ΔU = 0 for ideal gas | ΔU = nCvΔT |
| Possible only if system can exchange heat easily | Possible only if system is insulated or rapidly changing |
| Heat is a function of temperature and volume | Heat does not flow; energy change is by work only |
| $Q \neq 0$ in the process | $Q = 0$ always in the process |
| Common in laboratory-controlled conditions | Common in natural rapid events, e.g., atmospheric changes |
| Requires external agency to control temperature | No such agency is required during process |
| Used in certain refrigeration cycles | Used in engines and meteorology |
Key Differences
- Isothermal keeps temperature constant throughout process
- Adiabatic does not allow any heat exchange
- Isothermal ΔU = 0 for ideal gases; adiabatic ΔU ≠ 0
- PV = constant for isothermal, PV$^\gamma$ = constant for adiabatic
- Adiabatic curve on PV diagram is always steeper
Examples
An isothermal example is slow expansion of a gas in a thermostat bath, where temperature remains consistent. For comparison, sudden air compression in a bicycle pump is adiabatic because there is no time for heat exchange, as shown in Understanding Adiabatic Process.
Melting ice at 0°C also follows an isothermal process, whereas a rapidly rising air parcel in the atmosphere undergoes an adiabatic process, increasing its temperature with altitude change.
Applications
- Isothermal process used in heat engines and refrigerators
- Adiabatic process found in internal combustion engine cycles
- Isothermal conditions used in phase change studies
- Adiabatic process relevant to atmospheric science and sound waves
- Both processes essential in Thermodynamics Overview questions
One-Line Summary
In simple words, an isothermal process occurs at constant temperature with heat exchange, whereas an adiabatic process occurs without heat exchange and involves temperature change.
FAQs on Difference Between Isothermal and Adiabatic Processes
1. What is the difference between isothermal and adiabatic processes?
Isothermal and adiabatic processes are two major types of thermodynamic changes distinguished by the way heat and temperature behave during expansion or compression.
Key differences:
- Isothermal Process: Temperature remains constant, but heat can be exchanged with surroundings.
- Adiabatic Process: No heat exchange with surroundings; temperature changes during the process.
- In isothermal changes, internal energy does not vary since temperature is constant.
- In adiabatic processes, all energy change appears as temperature change.
2. What is an isothermal process with example?
An isothermal process is a thermodynamic process where the temperature stays constant throughout.
Example:
- Melting of ice at 0°C.
- Slow compression or expansion of an ideal gas in a temperature-controlled environment.
3. What is an adiabatic process with example?
An adiabatic process is when a system does not exchange heat with its surroundings, causing its temperature to change.
- Example: Rapid compression or expansion of air in a piston or the cooling of air as it rises and expands in the atmosphere.
Adiabatic processes are important for understanding real-life applications such as engines and atmospheric science, as included in many CBSE textbooks.
4. What are the main characteristics of isothermal and adiabatic processes?
Isothermal process:
- Temperature remains constant throughout.
- Heat transfer occurs between system and surroundings.
- Change in internal energy is zero.
- No heat transfer (Q = 0).
- Temperature changes during the process.
- Change in internal energy equals the work done.
5. How does pressure change in isothermal and adiabatic expansion?
During isothermal expansion, pressure decreases gradually at constant temperature, while in adiabatic expansion, pressure drops more sharply because the temperature also falls.
Key points:
- Isothermal: PV = constant (Boyle’s Law).
- Adiabatic: PVγ = constant (γ is the ratio of specific heats).
6. Why isothermal process is slow and adiabatic process is fast?
Isothermal processes are slow because sufficient time is needed for heat exchange to maintain constant temperature, while adiabatic processes are fast so that no heat is exchanged with the surroundings.
Key insights for CBSE:
- Isothermal: Slow, allowing for heat transfer.
- Adiabatic: Quick, minimizing heat exchange.
7. What are the equations for isothermal and adiabatic processes?
Isothermal process equation:
- PV = constant (Boyle's Law for ideal gases)
- PVγ = constant where γ = Cp/Cv (ratio of specific heats)
8. What is the significance of isothermal and adiabatic processes in real life?
Isothermal and adiabatic processes are widely observed in natural and industrial phenomena.
Some examples include:
- Isothermal: Ice formation/melting, metal casting, slow gas compression/expansion.
- Adiabatic: Air conditioners, refrigerators (adiabatic expansion/compression), rapid engine cycles, atmospheric cooling/heating.
9. Can a process be both isothermal and adiabatic?
No, a process cannot be both isothermal and adiabatic simultaneously.
Key reasons:
- Isothermal requires heat exchange to keep temperature constant.
- Adiabatic forbids any heat exchange, causing temperature to change.
10. In which thermodynamic process does internal energy remain constant?
In an isothermal process, the internal energy of an ideal gas remains constant because temperature does not change.
For adiabatic processes, internal energy does change since temperature changes.
This point is frequently tested in CBSE and competitive exams for Class 11 and 12.
