1. Conservation

2. Work Expanded
Mechanical work involves a force acting through a distance.
Work can involve a change in internal energy.
Temperature may change
Temperature did not cause the work
Work is an energy transfer that is not due to a temperature difference.
Heat is an energy transfer that is due to a temperature difference.

3. First Law of Thermodynamics
Internal energy can only change by heat transferred to and work done by a system.
Heat added is +
Heat lost is –
Work by a system is +
Work on a system is -
When work is defined as positive on a system it is DU = Q + W.

4. PV Diagram The ideal gas law links pressure, volume, and temperature.
PV = nRT
For fixed quantity of gas, any two of the three determine the third.
Graph of pressure vs volume describes the system. P V

5. Slow Motion
During rapid change the macroscopic state variables are not well defined.
On a PV diagram it is assumed that the system changes slowly.
This is a quasi-static change.

6. Measuring Work The mechanical work can be measured on the PV diagram.
The work done is the area under the PV curve describing the system. P V
work done

7. Constant Temperature
Walls and top are insulating
Changes often involve holding one variable constant.
Moving a piston slowly changes pressure and volume.
Temperature remains the same.
Base is kept at a constant temperature

8. Isothermal Process
For an ideal gas at constant temperature DT = 0, P is inversely proportional to V.
P = nRT(1/V)
The work done by an ideal gas can be found with calculus. P V

9. Internal Energy Conserved
An ideal gas only changes internal energy with temperature change.
DU = 0
From the first law, work done equals heat flow.
DU = Q – W
Q = W
Work done by gas
Heat flow into gas

10. Underwater
A scuba diver is swimming at a depth of 25 m where the pressure is 3.5 atm. The exhaled air forms bubbles 8.0 mm in radius. The bubbles stay at 300 K as they rise.
How much work is done by each bubble as it expands reaching the surface?
The ideal gas law is used to find the amount of gas.
P and V are related.
This is an isothermal process.
W = 0.95 J.