First law of thermodynamics

Definition

The first law of thermodynamics states that energy cannot be created or destroyed, only transformed from one form to another. This principle emphasizes the conservation of energy in any process, ensuring that the total energy within an isolated system remains constant. Understanding this law is fundamental in analyzing energy transfers and conversions in various physical processes, including heat engines and fluid systems.

5 Must Know Facts For Your Next Test

1. The first law of thermodynamics is mathematically represented as $$ ext{ΔU} = Q - W$$, where $$ ext{ΔU}$$ is the change in internal energy, $$Q$$ is the heat added to the system, and $$W$$ is the work done by the system.
2. In an isolated system, any increase in internal energy must be equal to the amount of heat added minus the work done by the system on its surroundings.
3. This law explains why perpetual motion machines are impossible; they would violate the principle of energy conservation.
4. Applications of the first law can be seen in processes such as heating water, where heat input raises temperature and results in a change in internal energy.
5. The first law is essential for understanding various thermodynamic cycles used in engines, refrigerators, and heat pumps.

Review Questions

• How does the first law of thermodynamics apply to a closed system where heat is added?
  • When heat is added to a closed system, the first law of thermodynamics indicates that this added heat will result in an increase in the internal energy of the system. According to the equation $$ ext{ΔU} = Q - W$$, if no work is done by or on the system, all the added heat contributes directly to increasing internal energy. However, if work is performed by the system while absorbing heat, part of that heat will convert into work instead of increasing internal energy.
• Analyze how the first law of thermodynamics ensures energy conservation during a thermodynamic cycle.
  • In a thermodynamic cycle, the first law of thermodynamics ensures that all forms of energy are accounted for. As a system undergoes various processes such as heating and cooling while performing work, any input or output of heat directly affects its internal energy. For instance, in a heat engine cycle, energy supplied as heat during combustion leads to work done by expanding gases, with leftover heat being expelled. This interplay reflects how energy is conserved throughout each stage of the cycle while transforming between different forms.
• Evaluate the implications of violating the first law of thermodynamics on our understanding of physical systems.
  • Violating the first law of thermodynamics would fundamentally challenge our understanding of physical systems and processes. If energy could be created or destroyed rather than conserved, it would undermine established principles governing mechanics, chemistry, and even cosmology. The consequences could lead to conflicting theories about energy transfer, cause inconsistencies in scientific predictions, and disrupt engineering practices that rely on accurate calculations based on energy conservation. Ultimately, such violations would prompt a reevaluation of all scientific laws based on energy interactions.

Related terms

internal energy:

The total energy contained within a system due to the kinetic and potential energies of its molecules.

heat transfer:

The movement of thermal energy from one object or substance to another, typically occurring through conduction, convection, or radiation.

work: The transfer of energy that occurs when a force is applied to an object, causing it to move a certain distance.