# Physics 312 Lecture: "First Law of Thermodynamics" May 8, 1998

• One must keep track of three quantities in a thermodynamic process:
• W = the net work done by the material
• Q = the net heat energy added to the material
• Uf - Ui = the change in internal energy of the material
where U is the internal energy of the material at any time
• for an ideal monatomic gas (such as helium or argon), the internal energy is simply a function of temperature:
```         U = 1.5 nRT
```
• the First Law of Thermodynamics states:
```         Uf - Ui = delta_U = Q - W
```
• a cyclic process restores its material to the original state at its end -- since the temperature is the same, the internal energy must be the same. That means that delta_U = 0, so Q = W
• using the First Law, we see
• adiabatic process: delta_U = -W
• isochoric process: delta_U = Q
• isothermal process: Q = W
• isobaric process: delta_U = Q - P*delta_V
• all heat engines take in heat Qh, discard "wasted heat" Qc, and do some net work W
• the efficiency of a heat engine is
```             e = 1 - Qc/Qh
```
• the Second Law of Thermodynamics states that it is impossible to build a heat engine which wastes no heat; so one cannot attain an efficiency of 100 percent

This lecture discusses material in Chapter 20 and 22 of Serway.