Question

1. Real heat engines, like the gasoline engine in a car, always have some friction

between their moving parts, although lubricants keep the friction to a minimum. Would a

heat engine with completely frictionless parts be 100% efficient? Why or why not? Does the

answer depend on whether or not the engine runs on the Carnot cycle? Again, why or why

not?

2.Some critics of biological evolution claim that it violates the second law of thermo-

dynamics, since evolution involves simple life forms developing into more complex and more

highly ordered organisms. Explain why this is not a valid arguement against evolution.

Answer 1

1)

• Heat engines transfer energy from a hot reservoir to a colder reservoir and during this process it converts some energy to mechanical work.
• Carnot's theorem states that all heat engines between two heat reservoirs are less efficient than a Carnot heat engine operating between the same reservoir.
• Efficiency is given by

where,

Temperature of the cold reservoir

Temperature of the hot reservoir

• As you can see if 0 then efficiency is maximum.
• Real gas engines like gasoline engine in a car, due to friction, lose efficiency.
• Heat engine with completely frictionless parts be 100% efficient, the cold reservoir should be at absolute zero temperature.
• Yes the answer depend on whether or not the engine runs on the Carnot cycle because according to carnot theorem the carnot engine is the most efficient engine.

2)

• The second law of thermodynamics says that disorder, or entropy, always increases or stays the same over time, for an isolated system.
• But it is only valid in closed systems with no external source of energy.
• The earth as a system is not closed since it receives energy continuously from the sun. So the second law of thermodynamics cannot be applied.
• The universe on the other hand can be considered as an isolated system hence its total entropy is increasing. The same cannot be said for earth.