Heating a gas at constant pressure (isobaric heating) In classical thermodynamics for a gas in a piston–cylinder or similar device, when the gas is heated while the pressure is maintained constant, what happens to its state variables?

Introduction / Context:
An isobaric (constant-pressure) process is common in piston–cylinder devices with a sliding weight or regulated outlet. Understanding how temperature and volume respond to heat addition at constant pressure is foundational for analyzing compressors, heaters, and gas turbine components.

Given Data / Assumptions:

• Gas contained in a device that can expand to keep pressure constant.
• Heat is added to the gas.
• Ideal-gas behavior used for intuition; real gases follow the same qualitative trend for ordinary conditions because cp > 0.

Concept / Approach:

At constant pressure, the First law on a per–unit mass basis gives: δq = dh for ideal gases, where h = h(T). Thus heat addition increases enthalpy and therefore temperature. From the ideal-gas relation p * v = R * T at fixed p, any increase in T requires a proportional increase in specific volume v. Hence both T and v rise when heat is supplied isobarically.

Step-by-Step Solution:

Verification / Alternative check:

On a T–v diagram at fixed pressure, isobars slope upward; heating moves the state to higher T and higher v. In practical devices, the piston moves outward while the gas warms, confirming both increases.

Why Other Options Are Wrong:

Only temperature or only volume increases: Inconsistent with p * v = R * T at constant p.No change or temperature decrease: Violates positive cp and heat addition effects for gases.

Common Pitfalls:

Confusing isobaric with isochoric processes. At constant volume, temperature rises but volume does not; at constant pressure, both temperature and volume rise together.

Final Answer:

both temperature and volume will increase