# Basic thermodynamics PPT

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27 Οκτ 2013 (πριν από 4 χρόνια και 8 μήνες)

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1

Outline

Define terms and conventions

Introduce 1
st

law of thermodynamics

Contrast state and non
-
state properties

Describe the Carnot cycle

2

System and environment

System

= what we wish to study

View as
control mass
or
control volume

Control mass (CM)

Define some mass, hold fixed, follow it around

Control volume (CV)

Define and monitor a physical space

Environment

= everything else that may
interact with the system

3

System states

Systems may be
open

or
closed
to mass

Open systems permit mass exchange across system
boundaries

Our CVs are usually open

Strictly speaking, a CM is closed

Closed systems may be
isolated

or
nonisolated

Isolated systems do not permit energy transfer with
environment

Closed, isolated system = environment doesn’t matter

4

Lagrangian

vs.
Eulerian

CM is the
Lagrangian

viewpoint

Powerful, desirable but often impractical

Total derivatives

Freeway example

CV is the
Eulerian

viewpoint

Observe flow through volume

Partial derivatives

5

Air parcel

Our most frequently used system

CM (usually!)

Lagrangian

concept

Monitor how T, p, and V change as we follow
it around

6

Conventions

We often use CAPITAL letters for
extensive

quantities, and lower case for
specific

quantities

Specific = per unit mass

Example:

U is internal energy, in Joules

u is specific internal energy, in J/kg

Unfortunately, “u” is also zonal wind velocity

Exceptions:

Temperature T is essentially specific, but capitalized
(and isn’t per unit mass anyway)

Pressure p is fundamentally extensive, but lower case

7

Energy and the 1
st

law

Total energy = KE + PE + IE

Conserved in absence of sources and sinks

Our main use of 1
st

law: monitor changes in
internal energy (IE or u) owing to sources and
sinks

How do we change system u? With energy
transfer via

heat Q or q

w
ork W or w

Caveat: w is also vertical velocity, and q will be
reused (briefly) for water vapor specific humidity

8

Work

Work = force applied over a distance

Force: N, distance: m

Work: Nm = J = energy

Our principal interest: CM volume
compression or expansion (
dV
) in presence of
external pressure (p)

W > 0 if
dV

> 0

9

Work

10

W > 0 when system expands against

environment

Heat

Diabatic

heat

Diabatic: Greek for “passable, to be passed
through”

Internal energy exchanged between system and
environment

q > 0 when energy flow is INTO system

= system is
isolated

: impassable, not to be passed through

11

Caution on nomenclature

We should use
diabatic

when the energy exchange is
between system and environment

But, what if the heat source or sink is
inside

the system?

That’s adiabatic, but q ≠ 0

Our interior heat source will be water changing phase

: q = 0

No heat source, outside OR inside

“dry” really means no water phase changes

: q ≠ 0, but heat source/sink is
inside
system

“moist” implies water phase change

Can also be referred to as “diabatic”!

12

1
st

law

In the absence of ∆KE and ∆PE

Other ways of writing this

13

Most of my examples will be per unit mass.

State properties

Internal energy u is a
state property

Changes in state properties are not path
-
dependent

Other state properties include m, T, p,
r
, V,
etc.

14

State properties

15

Path
-
dependence

Work and heat are
path
-
dependent

16

Path
-
dependence

A
cyclic

process starts
and ends with the same
state property values

… but the cyclic process
can have
net heat
exchange

and do
net
work

17

Path
-
dependence

18

Path
-
dependence

19

Carnot cycle

4
-
step piston cycle on a CM

2 steps of volume expansion, 2 of volume
compression

2 steps are isothermal, 2 are (dry) adiabatic

Warm and cold thermal reservoirs external to
system

Start and end with temperature T
1

and
volume V
1

20

Carnot

Step 1

21

Isothermal volume expansion

A

from warm

reservoir

T
2

= T
1

V
2

> V
1

Carnot

Step 2

22

No heat exchange

T
3

< T
2

V
3

> V
2

Carnot

Step 3

23

Isothermal volume compression

Lose heat Q
B

to cold thermal

reservoir

T
4

=

T
3

V
4

<

V
3

Carnot

Step 4

24

No heat exchange

T
1

> T
4

V
1

<

V
4

Returned to original state T
1
, V
1
.

Cycle is complete.

25

Apply 1
st

law

26

Carnot on T
-
V diagram

27

Carnot on T
-
V diagram

28

Carnot on T
-
V diagram

29

Carnot on T
-
V diagram

30

Carnot on T
-
V diagram

31

Carnot on T
-
V diagram

32

Carnot on T
-
V diagram

33

No net ∆V

But did net W

Conceptual summary

34

Heat flow diverted

t
o do work

Question for thought #1

35

The isothermal expansion (Q
A
) occurred at a

higher temperature than the

Isothermal compression (Q
B
).

What does this imply for the work?

Q
B

is waste heat.

What does this imply for the

efficiency of this heat engine?

Is there a limit to efficiency?

Is the limit found in the 1
st

law?

Question for thought #2

36

Can you design a cyclic process that does
no net work?

What would it look like on a T
-
V diagram?

Summary

1
st

law says, in essence, if you can’t take the
heat, you can’t do the work

Work and heat are path
-
dependent

Carnot cycle illustrates isothermal and (dry)

Heat diverted to do work, but some is wasted

W = Q
A

-

Q
B

37