Physics Lecture Notes
Chapter 10: Thermodynamic Processes
ACOS #5 Explain the concept of entropy as it relates to heating and cooling, using the laws of
thermodynamics.
a. Using qualitative and quantitative methods to show the relationship between changes
in heat energy and changes in temperature
A process is defined as a change in state, or the thermodynamic coordinates of a system either P,V, or T
would change. Therefore when we indicate a thermodynamic process we’re saying that this is a process at
which heat/energy is causing a change in one of these variables.
Objective #1: What is meant by the term thermodynamics?
 Thermodynamics is the study of the connection between heat and work and the conversion of
one into the other.
Objective #2: What is the relationship between work and heat?
 The two of them are simply energy transferred to or from a system.
 This study is important because many machines and modern devices change heat into work (such
as an automobile engine) or turn work into heat (or cooling, as in a refrigerator).
Objective #3: What is the difference between a system and an environment?
 A system examines the interactions of a confined space whereas an environment takes into
consideration the system and the outside components. These components could be biotic or
abiotic.
 In the example, Figure 1 p336, the balloon does work on the surrounding air. It pushes the air
back as the system does work on its surroundings.
Objective #4: Understand the work formula for a gas
 W=PV
 This formula simply explains the work done on or by a gas.
 It expresses this work as the product of pressure and volume
 W=PAd
 If the gas is compressed, V is negative
 If the gas is expanded then V is positive
 Work is only done if the volume changes. This concept is true when you consider the original
work formula that states that movement must take place in order for work to be done on the
system
Objective #5: Understand the three thermodynamic processes

The isovolumetric process

The isothermal process

The adiabatic process

In each case, it’s rare to see a process where work was done, but heat wasn’t present or vice
versa. Usually one entity is more dominant
Objective #6: Understand what is meant by the term, “isovolumetric”
 Iso refers to same …. Therefore based on the word, it would mean that volume is consistent
 An iso-volumetric process is a process that changes the internal energy of an object
through a transfer of heat only.
 No work is done on or by the system
 Deals with the volume of gases
Objective #7: Understand what is meant by the term, “iso-thermal process”

The temperature of the system remains constant.

Internal energy does not change when energy is transferred to and from the system as heat or
work

As heat is released as work, it is replaced thus keeping temperature constant

Heat is added to the system, to keep temperature constant, pressure and volumen changes to
keep the system constant.
Objective #8: Understand what is meant by the term adiabatic process”

No heat is transferred to or from the system

Adiabatos is Greek for “impassable”

This can happen if the process happens so quickly that there is no time to transfer heat, or if
the system is very well insulated from its surroundings.

We can only approximate this type of process (See fig 6 p341)
Objective #9: Be able to explain the three laws of thermodynamics (The Laws of Heat Power)

Thermo refers to heat; Dynamic refers to power

Be able to understand Table 1 p343
1st Law of Thermodynamics:

Energy can be changed from one form to another, but it cannot be created or destroyed. The
total amount of energy and matter in the Universe remains constant, merely changing from
one form to another. The First Law of Thermodynamics (Conservation) states that energy is
always conserved; it cannot be created or destroyed. In essence, energy can be converted
from one form into another.

U=Q-W (SI Units must be the same in order for this formula to work – the SI Unit is the joule)
2nd Law of Thermodynamics:
The Second Law of Thermodynamics has several variations, which will be explained below.
Some heat is wasted in conversion

One version of the Second Law of Thermodynamics states that some heat is wasted when
converting heat into mechanical energy. In other words, in a car engine, not all of the heat
created from the exploding gasoline is used in turning the engine or moving the car. Some of
the heat simply heats the engine. The percentage of heat turned to work is called the
thermal efficiency of the engine.
Heat flows from high to low

The Second Law of Thermodynamics also states that heat normally flows from high
temperature to low temperature. For example, when you heat the end of a metal rod, the
heat will gradually travel to the cool end and heat it up.
Heat sink


Another example of this part of the Second Law of Thermodynamics concerns waterbeds.
The reason waterbeds use heaters to warm the water is because otherwise the heat from
your body (at 98.6 degrees F) will flow to the cooler water (at room temperature of 72
degrees F). Since there is so much water in a waterbed, it would take much energy from your
body to heat the water to body temperature. Thus, you can feel chilled from the loss of body
heat.
Objective #10: Be able to explain what is meant by the term, “cyclic process”
 In a cyclic process, the system starts in a particular state and returns to that state after
undergoing a few different processes.
 It is a process where the system returns to the same conditions that it started
 The system’s properties at the beginning of the process are identical to those where it ended.
 Unet = 0 and Qnet = Wnet
 An example of this is a refrigerator
Objective #11: Be able to explain and apply the efficiency formula

Efficiency = net work done by engine/energy added to engine as heat = energy added as heatenergy removed as heat/energy added as heat = 1 – energy removed as heat/energy added as
heat.

Eff = Wnet/Qh = Qh – Qc/Qh = 1 – Qc/Qh
Objective #12: Be able to explain the concept of entropy
 It is a measure of the randomness or disorder of a system