the first law of thermodynamics statement and limitations

The first law of thermodynamics, also known as the law of energy conservation, states that energy cannot be created or destroyed in an isolated system; it can only be transformed from one form to another. Mathematically, it can be expressed as:

$$\mathrm{\Delta }U=Q-W$$

where:

• $\mathrm{\Delta }U$
• $Q$
• $W$

In essence, the energy added to the system (through heat) minus the energy lost (through work done) results in a change in the system’s internal energy. This principle forms the foundation for understanding thermodynamic processes and the behavior of energy in various physical systems.

Statements of the First Law of Thermodynamics

1. Energy Conservation: The total energy of an isolated system remains constant. It can change forms but cannot be created or destroyed.

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Limitations of the First Law of Thermodynamics

1. Does Not Specify Process Direction: The first law does not indicate the direction of energy transformations or the spontaneity of processes. It cannot determine whether a process will occur, only that energy is conserved.

2. Internal Energy Measurement: The internal energy of a system cannot be measured directly; only changes in internal energy can be determined relative to a reference state.

3. Excludes Non-Conservative Forces: The first law applies to closed systems and does not take into account energy losses due to friction or dissipative forces unless these are explicitly included in the work term.

4. Non-equilibrium Processes: The law does not provide insight into the dynamics of non-equilibrium processes or the speed at which equilibrium is reached.

5. Does Not Account for Quality of Energy: While it recognizes energy conservation, it does not consider the quality of energy. For example, converting high-quality energy (like electricity) to low-quality energy (like heat) often leads to less useful energy for work.

related: second law of thermodynamics statement

Examples of the First Law of Thermodynamics

1. Heating a Gas in a Cylinder: When heat is added to a gas in a cylinder, the internal energy of the gas increases. If the gas expands and does work on the piston, some of the energy from heat input is converted into work.

2. Refrigerator: In a refrigerator, work is done on the refrigerant to remove heat from the interior (lower temperature) to the exterior (higher temperature), demonstrating energy transfer according to the first law.

3. Fire and Water: When you throw cold water into a fire, the water absorbs heat (Q), transforming the energy from the fire to increase the internal energy of the water, which may cause it to evaporate, doing work against atmospheric pressure (W).

4. Chemical Reactions: In a chemical reaction, reactants may absorb heat (endothermic) or release heat (exothermic), changing their internal energy in accordance with the first law. For instance, in the combustion of fuel, chemical energy is converted into thermal energy.

related: third law of thermodynamics

These principles exemplify how the first law of thermodynamics governs energy interactions in both simple and complex systems, illustrating its fundamental role in physics and engineering.