Chapter 20: The Second Law of Thermodynamics

What two bodies does the working substance interact with in a heat engine?

1. Hot Reservoir: the heat source; it can give the working substance large amounts of heat at a constant temperature T(h) without appreciably changing its own temperature. 2. Cold Reservoir: can absorb large amounts of discarded heat from the engine at a constnat lower temperature T(c).

What are the steps of the Carnot Cycle?

1. The gas expands isothermally at temperature TH, absorbing heat QH. 2. It expands adiabatically until its temperature drops to TC. It is compressed isothermally at TC, rejecting heat |Qc| 4. It is compressed adiabatically back to its inital state at temperature TH.

What is a heat engine?

A heat engine is any device that transforms heat partly into work or mechanical energy. All heat engines absorb heat from a source at a relatively high temperature, perform some mechanical work, and discard or reject some heat at a lower temperature.

Why is a refrigerator also a heat engine?

A refrigerator is heat engine that's pretty much operating in reverse. While a normal heat engine takes heat from a hot place and gives off heat to a cooler place, a refrigerator does the opposite; it takes heat from a cold place (the inside of the refrigerator) and gives it off to a warmer place). A normal heat engine would have a net OUTPUT of mechanical work, whereas a refrigerator requires a net INPUT in order to carry the heat from cold to hot.

What is entropy?

Entropy is a quantitative measure of disorder.

What is the coefficient of performance?

From an economic point of view, the best refrigeration cycle is one that removes the greatest amount of heat from the inside of the refrigerator for the least expenditure of mechanical work. K = Qc/W

What are examples of irreversible processes?

Heat flow with finite temperature difference, free expansion of a gas, and conversion of work to heat by friction. Also any non-equilibrium processes

How does a practical refrigerator work?

It is a fluid circuit which contains the refrigerant fluid (the working substance). One half of the circuit which is inside the refrigerator is at low temperature and low pressure. The other half is outside the refrigerator and is at high temperature and high pressure. The compressor takes in the cold fluid, compresses it adiabatically, and delivers it to the condenser coil at high pressure. The fluid temperature is then higher than that of the air surrounding the condenser, so the refrigerant gives off heat Qh and partially condense to liquid. The fluid then expands adiabatically into the evaporator at a rate controlled by the expansion valve. As the fluid expands, it cools considerably, enough that the fluid in the evaporator coil is colder than its surrounding. It absorbs heat from its surroundings (inside the refrigerator), cooling them and partially vaporizing. Then the fluid enters the compressor to begin another cycle.

What is a working substance? What is a cyclic process?

It is a quantity of matter inside the engine which undergoes inflow and outflow of heat, expansion and compression, and sometimes change of phase. A cyclic process is a sequence of process that eventually leaves the substance in the same state in which it started. When a system is carried through a cyclic process, its initial and final internal energies are equal. Q = W

Explain what reversible processes are.

Reversible processes are equilibrium processes, with the system always in thermodynamic equilibrium. A reversible process is an idealization that can never be precisely attained in the real world But by making the temperature gradients and the pressure differences in the substance very small, we can keep the system very close to equilibrium states and make the process nearly reversible.

What is the compression ratio? What is the relationship between the compression ration and the thermal efficiency?

The compression ration is the ratio of the maximum and minimum volume in the cylinder of an internal combustion engine. The greater the compression ratio, the better the thermal efficiency.

Compare and contrast the Otto Cycle and the Diesel Cycle.

The main difference between the Otto cycle and the diesel cycle is that in the fuel ignition of the diesel cycle, it heats at a constant pressure.

What is the second law of thermodynamics?

The second law is as follows: It is impossible for any system to undergo a process in which it absorbs heat from a reservoir at a single temperature and converts the heat completely into mechanical work, with the system ending in the same state in which it began.

.What is the thermal efficiency of a heat engine?

Thermal efficiency represents the fraction of the heat that is converted to work. e = W/Qh

What is the total entropy change during any reversible cycle?

Zero.

What are all thermodynamic process that occur in nature?

irreversible processes.

Can the net entropy be negative in a net process?

no