Heat Engines and Thermal Efficiency Quiz

Heat Engines and Cycles

• In a Carnot cycle, the thermal efficiency is calculated as the ratio of work output to heat transfer, which is 333 kJ / 500 kJ = 0.666 or 66.6%.
• A system operating on a Carnot cycle has the highest possible thermal efficiency compared to a regular power cycle, making it the most efficient heat engine.
• The first law of thermodynamics addresses the relationship between internal energy, heat transfer, and work in a system.

Heat Engines and Entropy

• During the process of converting heat to work, entropy increases, as some heat is converted to work, and the rest is rejected as waste heat.
• An example of a heat engine is a power plant.

Performance of Heat Engines

• The formula to calculate the net work output for any heat engine is not provided in the text, but it is typically calculated as W = Q1 - Q2, where Q1 is the heat input and Q2 is the heat rejected.
• The desired output of a heat engine is work.
• Thermal efficiency is defined as the ratio of the net work output to the heat input.
• The consequence of the Second Law of Thermodynamics is that heat transfer between reservoirs always occurs from a higher temperature to a lower temperature, never the reverse.

Thermal Equilibrium and Heat Pumps

• Thermal equilibrium is defined as the state where two bodies have the same temperature, and there is no net heat transfer between them.
• When evaluating a heat engine's performance, the quantity that must always be considered is its thermal efficiency.
• According to the conservation of energy principle, the heat input, work input, and heat rejected by a heat pump are related by Q1 = W + Q2.
• The coefficient of performance (COP) of a heat pump indicates its efficiency in transferring heat from a lower temperature to a higher temperature.
• In a Carnot cycle, there is no efficiency higher than Carnot efficiency, making it the most efficient heat engine.
• The relation involving the COP and rate form expresses the efficiency of a heat pump as COP = Q2 / W.
• The implication of the COP of a heat pump always being greater than unity is that a heat pump can transfer more heat than the work input.
• Thermal equilibrium affects the direction of heat transfer between two systems, ensuring that heat always flows from a higher temperature to a lower temperature.