1. Systems and the First Law of Thermodynamics

2. System Boundary For our purposes, the system will almost always be an ideal gas.

3. System Boundary The system boundary controls how the environment affects the system. If the boundary is “closed to mass”, that means that mass can’t get in or out. If the boundary is “closed to energy”, that means energy can’t get in or out.

4. Discussion Consider the earth as a system. What type of boundary does it have?

5. First Law of Thermodynamics U (E int ) W Q

6. Awkward notation WARNING! We all know that U is potential energy in mechanics. However… U is E int (thermal energy) in thermodynamics! Yuk. This means when we are in thermo, U is thermal energy, which is related to temperature. When we are in mechanics, it is potential energy, which is related to configuration or position.

7. More about U U is the sum of the kinetic energies of all molecules in a system (or gas). U = N K ave U = N (3/2 k B T) U = n (3/2 R T) – Since k B = R /N A

8. First Law of Thermodynamics  U = Q + W –  U: change in internal energy of system (J) – Q: heat added to the system (J). This heat exchange is driven by temperature difference. It occurs only if the boundary allows energy transfer. – W: work done on the system (J). Work will be related to the change in the system’s volume. It occurs only if the boundary can change shape in some way. This law is sometimes paraphrased as “you can’t win”.

9. Problem A system absorbs 200 J of heat energy from the environment and does 100 J of work on the environment. What is its change in internal energy?

10. Problem How much work does the environment do on a system if its internal energy changes from 40,000 J to 45,000 J without the addition of heat?

11. Walker, 18.5 A  B B  C C  A QW UU -53 Ja)b) -280 J-130 Jc) d)150 Je)

12. Walker, 18.6 One mole of an ideal monatomic gas is initially at a temperature of 323 K. Find the temperature of the gas if 2250 J of heat are added and it does 834 J of work.