Chapter 15 The Laws of Thermodynamics

Define: thermodynamics

system

closed system

isolated

It is important to know some of the laws that govern the "flow" of thermal energy. Thermodynamics discusses heat, internal energy and work done by a system.

* The First Law of Thermodynamics is a statement of conservation of energy.

D U = Q - W Q is positive if energy enters the system W is positive if the system does work.

No experiments have observed a violation of this law.

Work in a Thermodynamic System

Work is equal to force times distance (with force and distance parallel).

W = F*d or W = ( P*Area ) * d = P * ( Area *d )

or W = P* D Volume This is only to be used if pressure is constant

In a gas system we can calculate work by multiplying a constant pressure by the change that has occurred in the volume. W is positive when a system expands.

Q and W are thermodynamic processes that change the values of the internal energy, pressure, temperature, volume and perhaps N. A system does not possess certain values of Q or W. A system can be characterized by the values of U, P, T, V and N. U, P, T, V and N are called state variables since they specify the state of the system. Q and W can change the state of a system.

Much of the discussion concerning thermodynamics is referenced to a P V diagram. The PV graph has pressure as a vertical variable and volume as a horizontal variable.

The state of a system is (partially) described by knowing the pressure, temperature, volume and number of moles of gas for the system. Some important processes which take a system of gas from one state to another are: isobaric (constant pressure) isochoric (constant volume) and isothermal (constant temperature).

Consider PV=nRT. If T and n are constant (isothermal, closed system) then PV = constant.

ISOTHERMAL PROCESS

Recall: U = ( 3/2 ) nRT .

Consider a process that changes the state variables. If the temperature is constant during the process what is the value of the change in internal energy? D U = ( 3/2 ) nRD T

Recall D U = Q - W

ADIABATIC PROCESS

In this process there is no energy lost or gained by the system. What is the value of Q?

ISOCHORIC PROCESS

In this process the volume is constant.

ISOBARIC PROCESS

In this process the pressure is constant.

The work done is equal to the area under a line drawn between the initial and final states of a system. The work done on or by a system depends on the type of process(es) which have occurred. Let’s consider two processes.

1) Start the system at 4 liters, 3 atmospheres, 300 Kelvin .Perform an isobaric process that ends when the volume is 2 liters. Perform an isochoric process that ends at 6 atmospheres, 300 Kelvin

2) Start the system at 4 liters, 3 atmospheres, 300 Kelvin. Perform an isothermal process that ends when the pressure is 6 atmospheres and the volume is 2 liters.

Sketch both processes on this PV graph.

Calculate the amount of work for process 2.

For an isothermal process W = nRT ln(V₂/V₁)

What questions do you have for the examples on page 448?

D U = Q - W

Table 15-1 gives metabolic rates (work done) for various activities.

Q for the body is normally a negative number since your body temperature is usually at a higher temperature than your environment. The Q and W values combine to cause a negative value for D U. But, when you eat food you bring internal energy into your system and D U increases.

Calculate the number of food calories required to supply the energy you use during one physics class. Select the appropriate activity from the table on page 449.

The First Law of Thermodynamics does not make any statements about the direction energy will "flow" in a situation. Energy would be conserved if 3,000 J left a cold object and entered a hot object. But, we do not observe this process occur without the input of work. Name a device in a home for which energy leaves a cold region and enters a warm region.

The Second Law of Thermodynamics is a statement as to which processes are allowed and which processes are not allowed.

One statement of the Second Law is due to Clausius: Heat flows naturally from a hot object to a cold object; heat will not flow spontaneously from a cold object to a hot object.

15.5 Heat Engine

Define: Heat Engine

The development of the steam engine in the 1700’s motivated the scientific study of heat engines. The builders of the early steam engines realized that the engines were very inefficient. Part of the motivation for developing the study of thermodynamics was to improve those efficiencies.

A heat engine converts some of the heat energy

leaving a high temperature ( T_H ) reservoir into work.

The energy that is not converted to work moves to

a low temperature reservoir, T_L .

In order to conserve energy, Q_H = W + Q_L

Note that Q_L is taken to be positive even though it represents

the energy leaving the working part of the heat engine.

Cyclic Process When a process completes a cycle the

system has returned to the same state it was in at the

start of the cycle. What is the temperature of the system

What is the relationship between Q_H , Q_L , and W?

e = W/Q_H Is it possible to write the efficiency equation only using Q values?

CARNOT CYCLE

The study of thermodynamic processes in the early 1800’s showed that the maximum efficiency from a heat engine is achieved when the cycle consists of isothermal and adiabatic paths. The Carnot cycle is such a set of paths. The text has a diagram of this cycle on page 454. The efficiency of a Carnot heat engine is

e_ideal = 1 - T_L / T_H .

Kelvin-Planck Statement of the Second Law: No device is possible whose sole effect is to transform a given amount of heat completely into work.

OR It is impossible for any system to undergo a cyclic process whose sole result is the absorption of heat from a single reservoir at a single temperature and the performance of an equivalent amount of work.

OR 100% efficiency is impossible.

A refrigerator or air conditioner is a heat engine run in reverse. Work is put in and energy is moved to the hot reservoir. The coefficient of performance, CP, for a refrigerator is

CP = Q_L / W or

CP = Q_L / (Q_{H -} Q_L) The maximum value for CP is T_L /( T_H - T_L )

For a heat pump, CP = Q_H / W or CP = Q_H / (Q_H – Q_L )

with a maximum CP of T_H / ( T_H – T_L ) Work problem 31, page 473.

A reversible process is one in which the system changes from one equilibrium state to another equilibrium state by an infinitesimally small change. The system is said to always be in a state of equilibrium. An irreversible process is one in which the system was not in equilibrium at some time during the process (e.g. the free expansion of a gas from a tank into a region which used to be a vacuum; an apple falling off of a tree). During an irreversible process the system moves toward a less ordered state. Entropy is defined to be the state variable that changes in the following way

D S = Q / T for the system moving on a reversible path. For all natural processes D S is either 0 or a positive number. A portion of a system may have a negative D S but the remaining portion of the system will have a D S that has a positive magnitude greater than the magnitude of the negative D S. D S is 0 for an adiabatic process. Why?

Entropy provides another statement of the Second Law: The total entropy of any system plus that of its environment increases as a result of any natural process.

OR The entropy of an isolated system increases in every natural process, and only those processes are possible for which the entropy of the system increases or remains a constant.

Another statement of the Second Law of Thermodynamics: Natural Processes tend to move toward a state of greater disorder

OR An isolated system in a state of relative order will always pass to a state of relative disorder until it reaches the state of maximum disorder, which is thermal equilibrium.

The text has examples of the meaning of disorder.

Theoretically, the universe is moving toward maximum disorder. When (if) it reaches this state, all objects will have the same temperature.

Our textbook is an organized and information-packed book. It was made in a factory from raw materials that were disorganized at one point (vat of ink, wood pulp, etc.) An outside agent acted on the raw materials to create the book. The D S for the book is negative but the D S for the factory and environment is positive such that D S for the universe is positive as the book is made.

We are an organized set of molecules. If you would accumulate the chemical ingredients that match your body composition and put them in a container, shake, and then open the container you would not have a copy of yourself. If evolution is defined as the creation of life from raw materials by random chance arrangement of the molecules and a progression of the arrangement of molecules to form the plants, animals and people we observe today, then I don’t believe in evolution. I believe that an outside agent, God, created life as outlined in Genesis. In my opinion, only an outside agent can bring the order and information into the world that we see today. In my opinion, God created the laws of physics (& chemistry, biology, DNA, etc.) and gave order and life to the universe. Since this paragraph represents my opinion, it will not be used as the basis of a test question. Feel free to visit more with me about this in my office.

skip 15.11 Statistical Interpretation of Entropy

Is an automobile engine a heat engine? (yes) Do heat engines use all of the energy available from the high temperature reservoir? (no) The unwanted release of thermal energy into the environment is known as thermal pollution. What are some examples of thermal pollution? The total energy released as thermal pollution from human activity about 6 x 10^-6 of the energy received by the earth from the sun but it is still a concern in small environments (e.g. temperature increase in a small body of water). How can thermal pollution be reduced?

Why should you be concerned about thermal pollution and the use of energy by our society?

In the winter which nights are the coldest, nights with no clouds or nights with cloud cover present? All other factors being equal, the coldest nights are the nights without cloud cover. Water vapor absorbs some infrared radiation. Clouds occur when there is more water vapor in the atmosphere compared to when skies are clear. As the earth radiates infrared energy towards space, the clouds absorb some of the infrared radiation and radiate a portion back to the earth. This prevents the earth surface from cooling as much as would occur if clouds were not present.

The text briefly discusses the topic of air pollution. The burning of fossil fuels releases gases into our atmosphere that absorb infrared radiation. As the earth radiates energy towards space these extra gases absorb the infrared energy and radiate a portion back towards the earth. There are predictions that the earth’s surface temperature will increase by a few ^oC over the next 100 years due to this "greenhouse" effect of infrared absorption. Why should this concern you?

You should read through the descriptions of energy sources at the end of chapter 15.

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