For a closed system undergoing a process (not a cycle), the first law is expressed as: 

Explanation:

First Law of Thermodynamics:

• The First Law of Thermodynamics is a fundamental principle of energy conservation. For a closed system undergoing a process (not necessarily a cycle), it is expressed mathematically as:

ΔU = Q − W

Here:

• ΔU: Change in the internal energy of the system (measured in joules, J).
• Q: Heat added to the system (measured in joules, J).
• W: Work done by the system (measured in joules, J).

This equation implies that any energy added to a system in the form of heat (Q) is used to either increase the system’s internal energy (ΔU) or perform work on the surroundings (W). In other words, the energy supplied to a system is conserved and cannot be created or destroyed, only transformed from one form to another.

When a closed system undergoes a process:

1. Heat Transfer (Q): Heat may be added to or removed from the system. If heat is added, Q is positive, and if heat is removed, Q is negative. This term represents the thermal energy exchanged between the system and its surroundings.
2. Work Done (W): Work is done by the system on its surroundings or vice versa. If the system does work on its surroundings (e.g., expansion of gas), W is positive. If work is done on the system by the surroundings (e.g., compression of gas), W is negative.
3. Change in Internal Energy (ΔU): Internal energy is the total energy contained within the system, including the kinetic and potential energies of molecules. The change in internal energy reflects the net effect of heat transfer and work done.

For example, in a gas contained within a piston-cylinder arrangement:

• If heat is supplied to the gas, the gas molecules gain energy, causing either an increase in internal energy (ΔU) or expansion (work W).
• During compression, work is done on the gas, increasing its internal energy or causing heat to be rejected to the surroundings.

The equation ΔU = Q − W is valid for all closed systems undergoing a thermodynamic process, whether it involves heating, cooling, compression, or expansion.