The Second Law of Thermodynamics

Prepared by: SERENE GRACE A. LACEA BSED- PHY SCI 2

UNIVERSITY OF BOHOL

Heat Engines

What is a heat engine?

A heat engine is a device that converts thermal

energy to other useful forms of energy, such as

mechanical and electrical energy.

Two main types of heat engines:

1. Internal Combustion Engines

2. External Combustion Engines

An Internal Combustion Engine is a heat engine

where the combustion of a fuel occurs with an

oxidizer (usually air) in a combustion chamber.

In IC engine, the fuel combustion takes place

inside the engine.

Examples of Internal Combustion Engines:

Gasoline Engine Gas-Turbine Engine

Diesel Engine Rocket- Propulsion System

D:\UB\MAJOR\SCIENCE\2nd yr. summer- thermodynamics\How Diesel Engines Work - Part - 1 (Four Stroke Combustion Cycle).mp4

An External Combustion Engine is a heat engine

where an internal working fluid is compressed and

heated by combustion of an external fuel through

the engine wall or a heat exchanger.

In EC engine, the fuel combustion takes place

outside the engine.

Examples of External Combustion Engines:

Steam Engine

Stirling Engine

D:\UB\MAJOR\SCIENCE\2nd yr. summer- thermodynamics\How Do Steam Engines and Diesel Engines Work_.mp4

A basic heat engine model consists of the

following:

heat is absorbed from a source at a high

temperature,

Work is done by the engine, and

Heat is expelled by the engine to a

source at a lower temperature.

A General Heat Engine Model

Heat Engine

High temperature source

Low temperature reservoir

To calculate the amount of work

done by a heat engine:

Where:

= work done by engine

= energy added as heat

= energy removed as

heat

Sample Problem

A heat engine takes in 4500 J of heat energy, then

does 2750 J of work. How much heat energy does the

engine expel as waste?

Given: Solution:

Required:

Answer:

The goal of a heat engine is to convert ALL of the heat

input into useable work, BUT…

There will always be some waste heat that doesn’t get

converted into useful work.

The less heat wasted, the more efficient the engine.

A measure of how well an engine operates is given by

the engine’s efficiency (eff).

Efficiency is a measure of a machine’s energy

effectiveness. It is the ratio of work done by the engine to

the energy added to the system as heat during one cycle.

Where:

eff = efficiency

Wnet = net work done by engine

Qh = energy added as heat

Qc = energy removed as heat

Efficiencies for different types of engines:

Engine Type eff (calculated maximum

values)

Steam Engine 0.29

Steam Turbine

0.40

Gasoline Engine

0.60

Diesel Engine 0.56

Engine Type eff (measured values)

Steam Engine 0.17

Steam Turbine

0.30

Gasoline Engine

0.25

Diesel Engine 0.35

Notice that efficiency is a unitless quantity that can be

calculated using only the magnitudes for the energies

added to and taken away from the engine.

This equation confirms that a heat engine has 100

percent efficiency (eff = 1) only if there is no energy

transferred away from the engine as heat (Qc = 0).

Unfortunately, there can be no such heat engine, so

the efficiencies of all engines are less than 1.

In other words, a heat engine with perfect efficiency

would have to convert all of the absorbed heat energy

to mechanical work.

One of the consequences of the second law of

thermodynamics is that this is impossible.

In practice, it is found that all heat engines convert

only a fraction of the absorbed heat into mechanical

work.

On the basis of this fact, one form of the second law

of thermodynamics is stated as follows:

No heat engine operating in a cycle can absorb

thermal energy from a reservoir and perform an equal

amount of work.

If the amount of energy added to the system as heat

is increased or the amount of energy given up by the

system is reduced, the ratio of Qc/Qh becomes smaller

and the engine’s efficiency comes closer to 1.

The efficiency equation gives only a maximum value

for an engine’s efficiency.

Friction and thermal conduction in the engine hinder

the engine’s performance, and experimentally

measured efficiencies are usually lower than the

calculated efficiencies.

Efficiencies for different types of engines:

Engine Type eff (calculated maximum

values)

Steam Engine 0.29

Steam Turbine

0.40

Gasoline Engine

0.60

Diesel Engine 0.56

Engine Type eff (measured values)

Steam Engine 0.17

Steam Turbine

0.30

Gasoline Engine

0.25

Diesel Engine 0.35

Sample Problem

Find the efficiency of a gasoline engine that, during

one cycle, receives 204 J of energy from combustion and

loses 153 J as heat to the exhaust.

Given

Qh = 204 J

Qc = 153 J

Required

eff = ?

Solution

Answer

Evaluate

Only 25 percent of the energy as heat is used by

the engine to do work. As expected, the efficiency is less

than 1.

Practice Tests

1. A test model for an experiment gasoline engine does

45 J of work in one cycle and gives up 31 J as heat.

What is the engine’s efficiency?

2. A certain diesel engine performs 372 J of work in each

cycle with an efficiency of 33.0 percent. How much

energy is transferred from the engine to the exhaust and

cooling system as heat?

The second law of thermodynamics states that the state

of entropy of the entire universe, as a closed isolated

system, will always increase over time.

The second law also states that the changes in the

entropy in the universe can never be negative.

What is entropy?

Why is it that when you leave an ice cube at room

temperature, it begins to melt?

Why do we get older and never younger?

And, why is it whenever rooms are cleaned, they

become messy again in the future?

Certain things happen in one direction and

not the other, this is called the arrow of time

and it encompasses every area of science.

The thermodynamic arrow of time is called entropy.

Entropy is the measurement of disorder within a

system. Denoted as , the change of entropy

suggests that time itself is asymmetric with respect

to order of an isolated system, meaning: a system

will become more disordered as time increases.

• The change in entropy of a system, is

equal to the heat, Q, flowing into the system

as the system changes from a state A to a

state B divided by the absolute temperature.

• This definition assumes that the process occurring is

reversible. A reversible process is an idealization in

which the path followed in the change of any

thermodynamic variable, such as P, V, or T, is the

same in the forward and reverse directions.

Sample Problem

Calculate the change in entropy when 300 g of

lead melts at 327°C (600 K). Lead has a latent heat of

fusion of 5.85 cal/g.

Solution:

The amount oh heat added to the lead to melt it is

The entropy change of the lead is

The concept of entropy was introduced into the study of

thermodynamics by Rudolph Clausius in 1865.

One reason entropy became useful and gained wide

acceptance is because it provides another variable to

describe the state of a system to go along with pressure,

volume, and temperature.

The importance of entropy grew tremendously as the field

of statistical mechanics developed because this method of

analysis provided an alternative way of interpreting the

concept of entropy.

In statistical mechanics, the behavior of a substance is

described in terms of the statistical behavior of the atoms

and molecules contained in the substance.

One of the main results of this treatment is that isolated

systems tend toward disorder and entropy is a measure of

this disorder.

For example, consider the molecules of a gas in the air in

your room. If all the gas molecules moved together like

soldiers marching, this would be a very ordered state.

It is also an unlikely state. If you could see the molecules,

you would see that they move haphazardly in all

directions, bumping into one another, changing speed

upon collision, some going fast, some going slow.

This is a highly disordered state, and it is also the most

likely state.

All physical processes tend toward the most likely state,

and that state is always one in which the disorder

increases.

Because entropy is a measure of disorder, an alternative

way of saying this is

the entropy of the universe increases in all natural

processes.

This statement is yet another way of stating the second

law of thermodynamics.

This tendency of nature to move toward a state of

disorder affects the ability of a system to do work.

Consider a ball thrown toward a wall. The ball has kinetic

energy, and its state is an ordered one. That is, all of the

atoms and molecules of the ball move in unison at the

same speed and in the same direction.

When the ball hits the wall, however, part of this ordered

energy is transformed to disordered energy. The

temperature of the ball and the wall both increase slightly

as part of the ball’s kinetic energy is transformed into the

random, disordered, thermal motion of the molecules in

the ball and the wall.

Before the collision, the ball is capable of doing work. It

could drive a nail into the wall, for example.

When part of the ordered energy is transformed to

disordered thermal energy, this capability of doing work is

reduced.

That is, the ball rebounds with less kinetic energy than it

had originally because the collision is inelastic.

Various forms of energy can be converted to thermal

energy, as in the collision between the ball and the wall,

but the reverse transformation is never complete.

In general, if two kinds of energy, A and B, can be

completely interconverted, we say that they are of the

same grade.

However, if form A can be completely converted to form B

and the reverse is never complete, then form A is a higher

grade of energy than form B.

In the case of a ball hitting a wall, the kinetic energy of the

ball is of a higher grade than the thermal energy

contained in the ball and the wall after the collision.

Therefore, when a high-grade energy is converted to

thermal energy, it can never be fully recovered as high-

grade energy.

This conversion of high-grade energy to thermal energy is

referred to as the degradation of energy.

The energy is said to be degraded because it takes on a

form that is less useful for doing work. In other words, in

all real processes where heat transfer occurs, the energy

available for doing work decreases.

Finally, it should be noted that the statement that entropy

must increase in all natural processes is true only when

one considers an isolated system.

There are instances in which the entropy of some system

may decrease but that decrease takes place at the

expense of a net increase in entropy for some other

system.

When all systems are taken together as the universe, the

entropy of the universe always increases.

Ultimately, the entropy of the universe should

reach a maximum. At this point, the universe will

be in a state of uniform temperature and density.

All physical, chemical, and biological processes

will have ceased because a state of perfect

disorder implies no energy available for doing

work.

This gloomy state of affairs is sometimes

referred to as an ultimate “heat death” of the

universe.

Thank you