Thermodynamics and Heat Engines

Question 1

Explain ∆U = Q - W.

Answer 1

∆U is the change in internal energy. This is the first law of thermodynamics, and is based upon the laws of conservation of energy. Q is the heat transfer to the system, and W is the work done by the system.

Question 2

What is an adiabatic change?

Answer 2

An energy transfer where no heat transfer occurs (heat energy does not enter or leave the system). Note that this does not mean at a constant temperature.

Question 3

What is an isothermal change?

Answer 3

An energy change where the change in temperature is zero (but heat energy DOES enter and leave the system).

Question 4

State the second law of thermodynamics.

Answer 4

There is always an increase in entropy for processes to occur without work being done on the system.

Question 5

State the third law of thermodynamics.

Answer 5

Absolute zero (0 K) is unattainable.

Question 6

Give an easy way to remember the first, second and third laws of thermodynamics.

Answer 6

Gambling with bad luck:

1. You cannot win (get something for nothing)
2. You can’t break even (you cannot return to the same energy state, because there is always an increase in disorder; entropy always increases)
3. You cannot get out of the game (absolute zero is impossible).

Question 7

Give the four cycles in a petrol engine, and briefly explain each one.

Answer 7

Compression (air in cylinder compressed adiabatically), Ignition (a spark ignites the mixture raising the pressure and temperature very rapidly), Power (high pressure in the cylinder forces the piston outward; work is done by the expanding gas), and Cooling (the gas is cooled rapidly at a constant volume until the pressure is reduced to the initial pressure by contact with the internal walls of the cylinder block).

Question 8

Give the four cycles in a diesel engine and briefly explain each one.

Answer 8

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