ATOC 4720: class 11

1. ATOC 4720: class 11 • 1. The first law of thermodynamics • 2. Joule’s law • 3. Specific heats • 4. Enthalpy

2. 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

3. 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.

4. 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.

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

7. 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

8. 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:

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

10. We may also define: Physics: cp>cv

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

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

13. 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