### Which Physical Law Underlies the First Law of Thermodynamics?

Which Physical Law Underlies the First Law of Thermodynamics?

Thermodynamics is a branch of physics that deals with the study of energy and its transformations. The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed, only transferred or transformed from one form to another. It is a fundamental principle that governs the behavior of energy in various systems. But what physical law underlies this fundamental principle? Let’s explore.

The first law of thermodynamics is based on the principle of the conservation of energy, which is derived from one of the most fundamental laws in physics – the law of conservation of mass-energy. This law, also known as the principle of mass-energy equivalence, states that mass and energy are two forms of the same entity and are interchangeable. It was famously expressed by Albert Einstein’s equation E=mc², where E represents energy, m represents mass, and c represents the speed of light.

The law of conservation of mass-energy tells us that the total amount of mass-energy in a closed system remains constant over time. This means that energy cannot be created or destroyed but can only change its form. When applied to thermodynamics, this law forms the basis of the first law.

The first law of thermodynamics can be expressed mathematically as follows:

ΔU = Q – W

Where ΔU represents the change in internal energy of a system, Q represents the heat added to the system, and W represents the work done by the system. This equation tells us that the change in internal energy of a system is equal to the heat added to the system minus the work done by the system.

The underlying physical law behind this equation is the principle of the conservation of energy, which states that the total energy of a closed system remains constant. In other words, the energy input (heat) to a system must be equal to the energy output (work) plus the change in internal energy.

This principle holds true for various systems, whether it’s a simple gas in a cylinder or a complex chemical reaction. For example, consider a gas confined within a cylinder with a moving piston. If heat is added to the gas, it increases the internal energy of the gas, causing the gas molecules to move faster. This increase in energy can be observed as an increase in temperature.

FAQs:

Q: Can energy be destroyed according to the first law of thermodynamics?
A: No, the first law of thermodynamics states that energy cannot be destroyed. It can only be transformed from one form to another.

Q: How is the first law of thermodynamics related to the conservation of mass-energy?
A: The first law of thermodynamics is derived from the principle of the conservation of mass-energy. It states that the total amount of energy in a closed system remains constant.

Q: Are there any exceptions to the first law of thermodynamics?
A: The first law of thermodynamics is a fundamental principle that holds true for all systems. However, it is important to note that in some cases, energy transfer or transformation may be difficult to measure accurately, leading to apparent violations of the first law. Nonetheless, the underlying principle remains intact.