Chapter 15 - The Laws of Thermodynamics


Thermodynamics describes the processes whereby energy is transferred as heat and work, e.g., in steam engines, electric power generating plants, energy is transferred from coal (heat as it is burned) to electrical energy which can do useful work.

First Law of Thermodynamics

Isothermal process means temperature, T, is constant. Since internal energy is proportional to temperature, the change in internal energy, DU, for an isothermal process is zero. Thus Q = W.

Adiabatic process means no heat is added or given off, i.e., Q = 0.

Work done by simple system e.g., a piston undergoes expansion or contraction.

Second Law of Thermodynamics - Can be stated in many different forms.


Carnot cycle (ideal or maximum) efficiency

Refrigerators, air conditioners and heat pumps Coefficient of performance - is a measure of how efficient the refrigerator is. The higher the number, the better the performance. For an ideal system,

Change in entropy DS = Q/T

Homework solutions

3. Sketch a PV diagram.

9. Two-step process as shown on the figure below. Heat loss going from a to b at constant volume, and heat added going from b to c where the temperature at c is the same as in a. Calculate, (a) total work, change in internal energy of the gas, and (c) total heat flow into or out of the gas. (Conversion factors 1 atm = 1.0135´105 N/m2, and 1 L = 10-3 m3.)

14. Calculate the metabolic rate for a 24-h activity.

Thus the metabolic rate for a 24 h period is 3380 kcal.

17. Engine does 7200 J of work in each cycle while absorbing 12.0 kcal from a high temperature reservoir. What is the efficiency?

20. Nuclear power plant operates at 75% of its maximum theoretical efficiency between TH = 600°C and TL = 350°C. Plant produces electric energy at 1.1 GW (i.e., 109 W), how much exhaust heat is discharge per hour?

23. Heat engine utilizes a heat source at 550°C with an ideal efficiency of 30%. Find new high temperature if the efficiency is to be increased to 40%.

28. A restaurant refrigerator has coefficient of performance, COP = 5.0. (COP is a measure of how efficient the refrigerator is. The higher the number, the more efficient it is.)

33. 1 kilogram of H20 heated from 0°C to 100°C. Estimate the change in entropy of H20.

38. A 5.0 kg piece of Al at 30°C placed in 1.0 kg of water in a Styrofoam container at room temperature (20°C = 293 K). Calculate the approximate net change in entropy of the system. Need the equilibrium temperature.

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