Heat engine

Definition

A heat engine is a device that converts thermal energy into mechanical work by undergoing cyclic processes. It operates between two reservoirs at different temperatures, absorbing heat from the hot reservoir and partially converting it into work while expelling the remaining heat to the cold reservoir.

5 Must Know Facts For Your Next Test

1. The efficiency of a heat engine is given by $\eta = 1 - \frac{Q_c}{Q_h}$, where $Q_c$ is the heat expelled to the cold reservoir and $Q_h$ is the heat absorbed from the hot reservoir.
2. The Carnot engine represents an idealized heat engine with maximum possible efficiency, given by $\eta_{Carnot} = 1 - \frac{T_c}{T_h}$, where $T_c$ and $T_h$ are the absolute temperatures of the cold and hot reservoirs respectively.
3. Real heat engines always have efficiencies lower than that of a Carnot engine due to irreversibilities like friction and non-ideal gas behavior.
4. The second law of thermodynamics states that no heat engine can be 100% efficient because some energy will always be lost as waste heat.
5. Common examples of heat engines include internal combustion engines in cars and steam turbines in power plants.

Review Questions

• What factors determine the efficiency of a real-world heat engine?
• Explain why no heat engine can ever be completely efficient according to the second law of thermodynamics.
• How does a Carnot cycle maximize the efficiency of a theoretical heat engine?

Related terms

Second Law of Thermodynamics: States that total entropy can never decrease over time for an isolated system, implying that processes occur in a certain direction and that energy has quality as well as quantity.

Carnot Engine: An idealized thermodynamic cycle proposed by Sadi Carnot, which provides maximum possible efficiency between two temperature reservoirs.

Entropy: A measure of disorder or randomness in a system; in thermodynamics, it quantifies how much energy in a system is unavailable for doing work.