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ATOC 4720: class 11. 1. The first law of thermodynamics 2. Joule’s law 3. Specific heats 4. Enthalpy. 1. The first law of thermodynamics. A moving air mass: kinetic energy potential energy. Internal energy. Increase in internal kinetic energy: T.

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Atoc 4720 class 11
ATOC 4720: class 11

  • 1. The first law of thermodynamics

  • 2. Joule’s law

  • 3. Specific heats

  • 4. Enthalpy


1 the first law of thermodynamics
1. The first law of thermodynamics

A moving air mass: kinetic energy

potential energy

Internal energy

Increase in internal kinetic energy: T

Increase in internal potential energy: relative

configuration


Assume a body of unit mass:

Takes heat energy q (joules): thermal

conduction or radiation;

External work w:

Excess energy: q-w

If there is no change in macroscopic kinetic

and potential energy, energy conservation

requires that its internal energy must

Increase by q-w.


Internal

Energy

Before and

after

Where

&

In differential form:

This equation states that increment of heat added

to a body is used to do external work by the

body and to increase the internal energy of

the body. This is the first law of

thermodynamics.


State 1, V1

1

dW=pAdx=pdV

State 2, V2

2


2 joule s law
2. Joule’s law

1848, lab experiments

When a gas expands without doing

External work, and without taking in

or giving out Heat, the temperature

of the gas does not change.

Ideal gas.

dq=0, dw=0, ===== > du=0


3 specific heat
3. Specific heat

dq is given to unit mass =>

T to T+dT

dq/dT is called specific heat

A specific heat at constant volume:


dq=du

Joule’s law: du depends only on T, so


We may also define:

Physics: cp>cv



Cv=717 J/deg/kg;

Cp=1004 j/deg/kg,

Diff?

First law of thermodynamics:


4 enthalpy
4. Enthalpy

If heat is added to a material at constant

pressure, so that the

specific volume of the material increases

from a1 to a2, the work done by a unit

mass of the material is p(a2-a1). Therefore, the heat dq added to a unit

mass of the material at constant pressure

is given by


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