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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

Increase in internal potential energy: relative

configuration

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.

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.

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

dq is given to unit mass =>

T to T+dT

dq/dT is called specific heat

A specific heat at constant volume:

Joule’s law: du depends only on T, so

Physics: cp>cv

Cp=dq/dT,

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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