# First law of thermodynamics

Mechanics

Oct 27, 2013 (4 years and 6 months ago)

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The Laws of Thermodynamics (01 of 38)

Physics Lecture
Notes

The Laws of Thermodynamics

FOUNDRY: It requires about 289 Joules of heat to
melt one gram of steel. In this chapter, we will
define the quantity of heat to raise the temperature
and to change the phase of a substance.

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1796

1832

French Engineer

Founder of the science
of
thermodynamics

First to recognize the
relationship between
work and heat

Thermodynamics

is the study of processes in which energy is
transferred as
heat

and as
work
.

IThe internal energy

is the sum of all the energy of all the
molecules in an object:

random translational kinetic energy

rotational kinetic energy

vibrational energy

intermolecular energy associated with their bonding.

ZEROTH LAW

The
Zeroth law

states that "If bodies
A

and
B

are each
separately in thermal equilibrium with body
C
, then
A

and
B

are in thermal equilibrium with each other."

The common property between
A

and
B

is called
temperature.

Topics

4) Thermodynamic Processes

5) The Second Law of Thermodynamics

6) Heat Engines

8) Entropy

1) The First Law of Thermodynamics

7) Carnot cycle

3) Pressure
-

Volume Graph

2) Work Done on a Gas

The Laws of Thermodynamics (02 of 38)

First Law of Thermodynamics

System

U

Environment

D

-

W

Q

W

The Laws of Thermodynamics (03 of 38)

PV = NkT

Pressure

(Pa)

Volume

(m
3
)

Absolute

Temperature

(K)

Boltzmann’s

Constant

(1.38 x 10
-
23

J/K)

Number of

Molecules

The Ideal Gas Law

Temperature and kinetic Theory13

Thermodynamic Processes

A.
Isobaric Constant Pressure

B.
Iso
-
volumetric Constant Volume

C.
Isothermal Constant Temp.

D.

between systems

The Laws of Thermodynamics (06 of 38)

Pressure
-

Volume Graph

P

V

Pressure

Volume

Isotherms

(lines of constant

temperature)

Area under curve
represents work

T
1

T
4

T
2

T
3

Internal energy

is proportional

to temperature

The Laws of Thermodynamics (05 of 38)

Absorbs heat
Q
hot

Performs work
W
out

Rejects heat
Q
cold

A heat engine is any
device which through
a cyclic process
:

Cold Res. T
C

Engine

Hot Res. T
H

Q
hot

W
out

Q
cold

HEAT ENGINES

THE SECOND LAW OF
THERMODYNAMICS

It is impossible to construct an
engine that, operating in a
cycle, produces no effect other
than the extraction of heat
from a reservoir and the
performance of an equivalent
amount of work.

Not only can you not win (1st law);
you can’t even break even (2nd law)!

W
out

Cold Res. T
C

Engine

Hot Res. T
H

Q
hot

Q
cold

THE SECOND LAW OF
THERMODYNAMICS

Cold Res. T
C

Engine

Hot Res. T
H

400 J

300 J

100 J

A possible engine.

An IMPOSSIBLE
engine.

Cold Res. T
C

Engine

Hot Res. T
H

400 J

400 J

EFFICIENCY OF AN
ENGINE

Cold Res. T
C

Engine

Hot Res. T
H

Q
H

W

Q
C

The efficiency of a heat engine
is the ratio of the net work
done W to the heat input Q
H
.

e = 1
-

Q
C

Q
H

e = =

W

Q
H

Q
H
-

Q
C

Q
H

EFFICIENCY EXAMPLE

Cold Res. T
C

Engine

Hot Res. T
H

800 J

W

600 J

An engine absorbs 800 J and
wastes 600 J every cycle. What
is the efficiency?

e = 1
-

600 J

800 J

e = 1
-

Q
C

-----

Q
H

e = 25%

Question: How many joules of work is done?

EFFICIENCY OF AN IDEAL
ENGINE (Carnot Engine)

maember

perfect engine, the
quantities Q of heat gained
and lost are proportional to
the absolute temperatures T.

e = 1
-

T
C

T
H

e =

T
H
-

T
C

T
H

Cold Res. T
C

Engine

Hot Res. T
H

Q
H

W

Q
C

Work done by engine each cycle

The efficiency of the engine

T
h
= 550 K

T
c

Engine

Q
h

Q
c

W

= 470 J

= 890 J

For the engine

The Carnot Cycle

The Laws of Thermodynamics (26 of 38)

T
h
= 550 K

T
c

Engine

Q
h

Q
c

W = 420 J

= 470 J

= 890 J

Temperature of the cool reservoir

The engine undergoes 22 cycles per second,

its mechanical power output

The Carnot Cycle

The Laws of Thermodynamics (27 of 38)

T
h

T
c

Engine

Q
h

Q
c

W = 350 J

= 900 J

A carnot engine absorbs 900 J of

heat each cycle and provides 350 J

of work

The heat ejected each cycle

The efficiency of the engine

The Carnot Cycle

The Laws of Thermodynamics (28 of 38)

T
h

T
c

= 283 K

Engine

Q
h

Q
c

W = 350 J

= 900 J

A carnot engine absorbs 900 J of

heat each cycle and provides 350 J

of work

The engine ejects heat at 10
o
C

The temperature of the hot
reservoir

=550 J

The Carnot Cycle

The Laws of Thermodynamics (29 of 38)

A carnot engine operates
between a hot reservoir at 650 K
and a cold reservoir at 300 K. If
it absorbs 400 J of heat at the
hot reservoir, how much work
does it deliver?

T
h
= 650 K

T
c
= 300 K

Engine

Q
h

Q
c

W = ?

= 400 J

The Carnot Cycle

The Laws of Thermodynamics (30 of 38)

Entropy is a measure of the disorder of a system. This gives us
yet another statement of the second law:

Natural processes tend to move toward

a state of greater disorder.

Example: If you put milk and sugar in your coffee and stir it,
you wind up with coffee that is uniformly milky and sweet.

No amount of stirring will get the milk and sugar to come
back out of solution.

Entropy

The Laws of Thermodynamics (33 of 38)

Another example: when a tornado hits a building, there is
major damage.

You never see a tornado approach a pile of rubble and leave
a building behind when it passes.

Thermal equilibrium is a similar process

the uniform final state has more disorder than
the separate temperatures in the initial state.

Entropy

The Laws of Thermodynamics (34 of 38)

Another consequence of the second law:

In any natural process, some energy

becomes unavailable to do useful work.

If we look at the universe as a whole, it seems inevitable that,
as more and more energy is converted to unavailable forms,
the ability to do work anywhere will gradually vanish. This is
called the heat death of the universe.

Entropy

The Laws of Thermodynamics (35 of 38)

First law of thermodynamics:

Isothermal process: temperature is constant.

Adiabatic process: no heat is exchanged.

Work done by gas at constant pressure:

Heat engine changes heat into useful work
(requires temperature difference).

Efficiency of a heat engine:

Carnot efficiency:

Summary

The Laws of Thermodynamics (36 of 38)