ATOC 4720: class The first law of thermodynamics 1. The first law of thermodynamics 2. Joule’s law 2. Joule’s law 3. Specific heats 3. Specific heats 4. Enthalpy 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

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.

Where& Internal Energy Before and after 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.

dW=pAdx=pdV 1 2 State 1, V1 State 2, V2

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:

dq=du Joule’s law: du depends only on T, so

We may also define: Physics: cp>cv

dp=0, constant pressure Cp=dq/dT,

Cv=717 J/deg/kg; Cp=1004 j/deg/kg, Diff? First law of thermodynamics:

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