1. Physics 2 – Feb 23, 2017
P3 Challenge – A gas expands adiabatically from a volume of 3.80 x m3 to 5.20 x 10-3 m3 from an initial pressure of kPa. What is the new pressure for the gas?
Today’s Objective:
Review
Assignment:
Review problems for Studying
Test on Thurs. March 2
Agenda
PV work derived
Homework Review
Work on Review problems
Get out #30-32 and for HMK check

2. PV Work Derived Part 1 – Piston h signs
Work is defined as a force applied through a distance.
Recall that work done on a system is positive and work done by a system is negative.
Therefore, the work done by an external force for a compression will be positive and the work done by an external force for an expansion will be negative.
Consider a piston on a canister holding a gas. The piston creates an eternal force. Pick a height = 0 point as the initial piston location.
For an expansion, the h of the gas sample is positive (h – 0)
For a compression, the h of the gas sample is negative (-h – 0)
h=h
h=0
h=-h

3. PV Work Derived – Part 2 Work external
Workext for expansion is negative Workext for compression is positive
Expansion, h positive Compression, h negative
Recall that Work requires the displacement to be in the same direction as the force.
Here, our external force is 180 from the direction of h.
Cos 180 = – 1 . Therefore Wext = – F h
Wext, exp = – F h would be negative, Wext,comp = – F h would be positive.
Results as expected above. Verify this.

4. PV Work Derived – Part 3 Gas Pressure
To transform our Wext expression into PV work, we first recognize that the force exerted by the gas pressure is an equal and opposite reaction force to the external applied force.
W done by the gas is the opposite of the Wext
To convert force into P and h into V, multiply by A/A as a form of 1.
Sort the factors and replace by the definitions of P and V.
Ta Da. That’s why the work done by a gas is called PV work.
W for an expansion is positive and W for a compression is negative

5. Exit Slip - Assignment
Exit Slip- On a PV diagram, draw lines representing 4 types of processes all starting from the same initial conditions: a) an isobaric compression, b) an isothermal compression, c) an isovolumetric decrease in pressure and d) an adiabatic compression. Label your lines with the vocabulary words.
What’s Due on Feb 28? (Pending assignments to complete.)
Review problems for Studying. Answers will be posted (w/o complete solutions).
Test on Thurs. March 2
What’s Next? (How to prepare for the next day)
Read B p23-32 about Thermodynamics