What is an Isothermal Process? (Thermodynamics)

• Iso means 'Equal' or 'Same'.
• Thermal relates to 'Heat' or 'Temperature'.
• Definition: An Isothermal process is a massive thermodynamic change in a system where the pressure and volume of the gas change heavily, but the Temperature remains absolutely, 100% constant throughout the entire process ($\Delta T = 0$).

Imagine a massive metal cylinder with a piston, filled with gas.

• If you compress the gas by heavily pushing the piston down, physics dictates that the gas MUST violently heat up (like a bicycle pump getting hot).
• However, to keep it 'Isothermal', you must push the piston incredibly, agonizingly Slowly.
• As the massive gas slightly heats up, the cylinder's massive conductive metal walls instantly absorb that tiny heat and release it into the surrounding room. Because the heat escapes instantly, the internal temperature of the gas never actually rises.

• It must be extremely slow: Any massive sudden or violent action (like popping a balloon) will instantly change the temperature, ruining the isothermal rule.
• The walls must be highly conducting: The cylinder walls cannot be insulated. They must freely allow massive heat to travel in and out to constantly balance the temperature.
• Boyle's Law applies: Because temperature is constant, an isothermal process strictly obeys the massive mathematical rule: $P \times V = Constant$. (If you massively double the pressure, the volume will become exactly half).