Paddle wheel work and expansion of gas into vacuum (free expansion) is a

Explanation:

Paddle wheel work:

• Paddle wheel work is a process involving friction in which the volume of the system does not change at all, and still work is done on the system.
• Friction makes this process irreversible, hence the process is in a non-equilibrium process.
• Work increases the stored energy (internal energy) of the system. Hence the temperature of the system increases in this process.

Free expansion:

• When the partition is removed, the gas rushed to fill the entire volume. The expansion of gas against vacuum is termed as free expansion or unrestricted expansion.

• A free expansion is an irreversible process in which a gas expands into an insulated evacuated chamber. Hence free expansion is also a non-equilibrium process.
• It is also called the Joule expansion.
• During the free expansion, the temperature remains constant this implies there is a drop in the pressure.
• Work done in a free expansion process is zero.
• No heat interaction takes place during free expansion.

So δW = 0

Also, δQ = 0

From the first law of Thermodynamics

δQ = dU + δW

dU = 0

Hence change in internal energy in the free expansion process is also zero.