Question

The first law of thermodynamics is usually written as

ΔEth = Q+W
You need to look at the signs carefully; some other disciplines (chemistry, for example) and some other textbooks may use other signs, which implies other definitions of these quantities.
A)Which physical law underlies the first law of thermodynamics?

Answer 1

The conservation of energy is the physical law underlying the first law of thermodynamics. According to this law, the alteration in the internal energy (ΔEth) within a closed system equals the net heat transfer (Q) into the system minus the net work done (W) by the system.

Step-by-step explanation:

The foundational principle supporting the first law of thermodynamics is the conservation of energy.

According to this law, the alteration in the internal energy (ΔEth) within a closed system equals the net heat transfer (Q) into the system minus the net work done (W) by the system.

In practical terms, if heat is introduced into the system and work is performed by the system, the resultant change in internal energy will be positive, signifying an overall increase in the system's energy content.

This fundamental law underscores the interconnectedness of heat transfer, work done, and changes in internal energy within a closed system, providing a foundational framework for understanding energy transformations and exchanges.

The conservation of energy serves as a guiding principle, emphasizing the preservation of total energy in a system as it undergoes various thermodynamic processes.

Answer 2

The conservation of energy is the physical law that underlies the first law of thermodynamics.

Step-by-step explanation:

The first law of thermodynamics, also known as the law of conservation of energy, states that energy cannot be created or destroyed, only transferred or transformed from one form to another. This principle is a fundamental law of physics that applies to all forms of energy, whether it be mechanical, electrical, chemical, or thermal.

The mathematical expression of the first law, ΔEth = Q+W, represents the change in internal energy (ΔEth) of a system as a result of heat (Q) and work (W) transferred to or from the system. The sign conventions for Q and W are important to understand. Heat is defined as the transfer of thermal energy between systems at different temperatures, and is represented by a positive value if it is added to the system and a negative value if it is removed from the system.

Work, on the other hand, is defined as the transfer of energy due to a force acting over a distance, and is represented by a positive value if it is done on the system (i.e., work is being done by the system) and a negative value if it is done by the system (i.e., work is being done on the system).

The conservation of energy principle has many practical applications in various fields, such as engineering, chemistry, and physics. For example, in engineering, this principle is used to design efficient machines and systems that minimize waste and maximize output. In chemistry, it explains why certain chemical reactions occur spontaneously while others do not.

In physics, it helps to explain phenomena such as conservation of angular momentum and conservation of linear momentum.

In summary, the conservation of energy principle is a fundamental physical law that underlies the first law of thermodynamics. It helps us understand how energy is transferred and transformed between systems and explains many important phenomena in various fields of study.