# Thermodynamic Processes

Thermodynamic Processes

Illustrate how the 1
st

law of thermodynamics is a
statement of energy conservation

Calculate heat, work, and the change in internal energy
by applying the 1
st

law of thermo

Apply 1
st

law of thermo to cyclic processes

How does the potential energy vary as
the car rolls up and down the track?

The PE depends only on the car’s elevation.

How would a 4
th

bar representing ME
relate to the
U
bars?

The new ME bar should equal KE + PE in each case.
Thus, it will get shorter from (b) to (e) as the
U

bar
gets taller by the same amount.

The First Law of Thermodynamics

Closed system:
Δ
U=Q
-
W

U Internal energy: all the energy of the
molecules

Q : energy transferred to/from system as heat

+ for heat added;
-

for heat lost

W: energy transferred to/from system as work

+ work done by the system;
-

work on system

Signs for heat and work in a system are
summarized as follows…

1
st

law of thermodynamics
mathematically

Δ
U = U
f
-

U
i

Energy conservation requires
that the
total

change in internal energy
from its initial to its
final equilibrium conditions be
EQUAL

to the
transfer of energy as
BOTH HEAT and WORK
.

According to the 1
st

law of thermo, a systems internal energy
can be changed by transferring energy as either work, heat, or
a combination of the two.

KNOW

THIS

CHART

FOR

TEST

Thermodynamic Processes

Isothermal
:
delta T=0; delta U=0
, Q=W

Adiabatic
:
Q=0; delta U
=
-
W

ISOBARIC
(constant P)

ISOVOLUMETRIC

Delta P=0

delta V=0

W=Fd=P
(
ad
)
=P
delta
V

W=0
,
Q=
delta
U

Q=delta U+W=delta U+P delta V

CYCLIC PROCESSES

A thermodynamic process in which a system returns
to the same conditions under which it started.

Final internal energy = initial internal energy

The change in internal energy of a system is ZERO in
a cyclic process

U
net

= 0 + Q
net

= W
net

Q
net

= W
net

CYCLIC PROCESS

A cyclic process represents an isothermal process in
that all energy is transferred as work and heat.

Energy is transferred as heat from the cold interior of
the refrigerator to the even colder evaporating
refrigerant. (Q
cold

or Q
c
)

Energy is also transferred as heat from the hot
condensing refrigerant to the relatively colder air
outside the refrigerator. (Q
hot

or Q
h
)

Cyclic Process

Q
net

= Q
h

Q
c
= W
net

W
net

= Q
h

Q
c

Where Q
h
> Q
c

The colder you want the inside of a
refrigerator to be, the greater the net
energy transferred as heat (
Qh

Qc
)
must be.

Cyclic Process

*
A refrigerator performs work to create a temperature
difference between its closed interior and its
environment.

*Transfers energy from a body at low temperature to
one at a high temperature.

*Uses work performed by an

electric motor to compress

the refrigerant.

Cyclic Process

In each of the 4

steps of a

Refrigeration

cycle, energy is

transferred to or

from the

refrigerant either

by heat or by work.

OPEN YOUR TEXTBOOK
TO PAGE 414

Cyclic Process Frige

Refrigerator Cyclic Process

1: Electrically
-
run compressor does work on the Freon gas, increasing
the pressure of the gas.

High Pressure and High Temperature

2: High Pressure Freon Gas released into external heat enters exchange
coil on the outside of the refrigerator

3: Heat flows from High Temperature gas to the lower
-
temperature air
of the room surrounding the coil. This heat loss causes the Freon to
condense to a liquid releasing heat to outside of the refrigerator.
(space behind the frige)

4: As Freon passes through the expansion valve it expands and
evaporates

now at low pressure and low temperature

5: Evaporating Freon absorbs heat from inside the refrigerator. This
causes the refrigerator to cool as its heat is absorbed by the
evaporating Freon in the internal coils. Thus, temperature inside
refrigerator is reduced

6: When all Freon changes to gas, the CYLCE REPEATS

Cyclic Process

The fact that the refrigerant’s final internal
energy is the same as its initial internal energy
is very important!

(consistent with 1
st

law of thermodynamics)

This is true for all machines that use heat to
do work or that do work to create
temperature differences

Heat Engine

Efficiency (x100%)

e=W/QH

=(QH
-
QL)/QH

=1
-

QL/QH

e<1