Thermodynamic Laws

Understanding the laws of thermodynamics and energy conservation

Topics

How do heat engines work?

Heat engines convert thermal energy (heat) into mechanical work by using a working fluid that expands when heated and contracts when cooled. They operate in cycles, taking heat from a hot source, converting some to work, and rejecting waste heat to a cold sink.

How does a heat engine work?

A heat engine works by converting thermal energy into mechanical work through a cyclic process. It absorbs heat from a hot reservoir, converts some of it to work, and rejects waste heat to a cold reservoir. The efficiency is limited by the temperature difference between hot and cold reservoirs.

How does entropy increase?

Entropy increases through irreversible processes that create disorder. When energy spreads out, systems become more disordered, or heat flows from hot to cold, entropy increases. The second law of thermodynamics states that entropy of an isolated system always increases or stays constant, never decreases.

What is entropy?

Entropy is a measure of disorder or randomness in a system. According to the second law of thermodynamics, entropy always increases in isolated systems, meaning systems naturally become more disordered over time.

What is the Carnot cycle?

The Carnot cycle is an idealized thermodynamic cycle that represents the maximum possible efficiency for a heat engine operating between two temperature reservoirs. It consists of four reversible processes: isothermal expansion, adiabatic expansion, isothermal compression, and adiabatic compression.

What is the first law of thermodynamics?

The first law of thermodynamics states that energy cannot be created or destroyed, only converted from one form to another. The change in a system's internal energy equals the heat added minus the work done by the system.

What is the second law of thermodynamics?

The second law of thermodynamics states that entropy (disorder) always increases in isolated systems, and heat cannot spontaneously flow from cold to hot. This law explains why processes are irreversible and why 100% efficient engines are impossible.

What is the third law of thermodynamics?

The third law of thermodynamics states that as temperature approaches absolute zero, the entropy of a perfect crystal approaches zero. This law explains why absolute zero cannot be reached and establishes absolute zero as the reference point for entropy calculations.

Why can't we create a perpetual motion machine?

We can't create a perpetual motion machine because it would violate the laws of thermodynamics. The first law (energy conservation) prevents machines that create energy, and the second law (entropy increase) prevents machines that convert heat completely to work without energy input. Friction and other losses also ensure energy is always lost.

Why can't we have 100% efficient engines?

We can't have 100% efficient engines because the second law of thermodynamics requires that some heat must be rejected to a cold sink. Heat engines need a temperature difference to operate, and some waste heat is inevitable, making perfect efficiency impossible.