# (Conservation of Energy)

Μηχανική

27 Οκτ 2013 (πριν από 4 χρόνια και 7 μήνες)

115 εμφανίσεις

First Law of Thermodynamics(I)

(Conservation of Energy)

Lecture Note 7

Physical Chemistry II, 2008

KAIST

Thermodynamic laws are empirical
laws for macroscopic bodies

The approach here is a mixture of

Classical thermodynamics (entirely macroscopic), and

Statistical thermodynamics (starts from atoms and molecules)

Einstein

Thermodynamics

Describes macroscopic properties of equilibrium
systems

Entirely empirical

Built on 4 laws and their “mathematical”
description

0th Law defines Temperature(T)

1
st

Law defines Energy(U)

2
nd

Law defines Entropy(S)

3
rd

Law assigns numerical value to entropy

These laws are
valid without exception

cannot be cheated

System

Surroundings

Heat: (+) input to a system, (
-
) evolve by a system

Work: (+) done on the system by surrounding

(
-
) done by the system

(+) energy of the system increase

(
-
) energy of the system decrease

Work relates to moving mass in the surroundings

a)

b)

We need to know P
ext

everywhere

19
-
2 Work and heat

are not state functions, but
energy is a state function

Differential of a state function: exact differential

When P
ext
is in equilibrium with the P of the system throughout the
process, the process is called a reversible process

(P
ext

infinitesimally larger than P for the compression)

Reversible process:

Reversible isothermal compression : minimum amount of work to be done
on the gas,
w
rev

> 0

Reversible isothermal expansion: maximum work done by gas,

w
rev

< 0

Work: path function,
δ
w: inexact differential

Even reversible w and q are not
state functions: 1
-
> 2
Δ
U=0

For a reversible expansion

And integrate to obtain

Work ~ change in energy

Heat ~ change in probability

Enthalpy: state function related to
heat (at constant pressue)

H for constant P ~ U for constant V

Solid to liquid

~10
-
4

Liquid to gas