Question 1

What happens to temperature when an ideal gas undergoes adiabatic compression?

Accepted Answer

During adiabatic compression, work is done on the gas without heat transfer. According to the first law of thermodynamics, this work increases the internal energy, raising the gas temperature significantly. For example, in a fire piston, rapid compression of air heats it enough to ignite cotton without external heat application.

Question 2

Why does a gas cool down during adiabatic expansion?

Accepted Answer

In adiabatic expansion, the gas does work on its surroundings without receiving heat. This work reduces internal energy, lowering the gas temperature. When opening a carbonated drink bottle, trapped gas expands adiabatically, cooling enough to condense water vapor into a visible cloud.

Question 3

How does adiabatic compression relate to engine knock in automobiles?

Accepted Answer

In car cylinders, gas-air mixtures compress so rapidly that heat exchange with the environment cannot occur, making the process adiabatic. The resulting temperature rise can cause uncontrolled explosions before spark ignition, creating engine knock. Higher-octane gasoline raises the ignition temperature, preventing premature detonation.

Question 4

What is free expansion and why does temperature remain constant?

Accepted Answer

Free expansion occurs when a gas expands into a vacuum without external pressure. No work is done, and the thermally insulated container prevents heat transfer. With both work and heat equal to zero, internal energy remains unchanged. For an ideal gas, constant internal energy means constant temperature.

Question 5

How does the first law of thermodynamics apply to adiabatic processes?

Accepted Answer

In adiabatic processes, heat transfer is zero, so the first law simplifies to: change in internal energy equals negative work done. During compression, negative work increases internal energy and temperature. During expansion, positive work decreases internal energy and temperature. This relationship governs all adiabatic behavior.

Question 6

What distinguishes an adiabatic process from other thermodynamic processes?

Accepted Answer

An adiabatic process involves no heat transfer between the system and surroundings, making it thermally isolated. Unlike isothermal processes where temperature stays constant, or isobaric processes where pressure remains fixed, adiabatic processes allow temperature and pressure to change based solely on work done.

Question 7

Why is the fire piston experiment a practical demonstration of adiabatic compression?

Accepted Answer

The fire piston demonstrates adiabatic compression by rapidly compressing air in a thermally insulated tube. The work done in an adiabatic process increases the gas internal energy and temperature so dramatically that cotton ignites without external heat. This clearly shows the relationship between work, internal energy, and temperature.

理想气体的绝热过程