1. Work in Thermodynamic Processes
Energy can be transferred to a system by heat and/or work
The system will be a volume of gas always in equilibrium
Consider a cylinder with a movable piston
As piston is pressed a distance Δy, work is done on the system reducing the volume
 W = -F Δy = - P A Δy

2. Work in Thermodynamic Processes – Cont.
Work done compressing a system is defined to be positive
Since ΔV is negative (smaller final volume) & A Δy = V
 W = - P ΔV
Gas compressed  Won gas = pos.
Gas expands  Won gas = neg.

3. Work in Thermodynamic Processes – Cont.
Can only be used if gas is under constant pressure
An isobaric process (iso = the same) P1 = P2
Represented on a pressure vs. volume graph – a PV diagram
Area under any curve = work done on the gas
If volume decreases – work is positive (work is done on the system)

4. THERMODYNAMICS First Law of Thermodynamics Energy is conserved
Heat added to a system goes into internal energy, work or both
ΔU = Q + W
Heat added to system  internal energy  Q is positive
Work done to the system  internal energy  W is positive (again)

5. First Law – Cont.
A system will have a certain amount of internal energy (U)
It will not have certain amounts of heat or work
These change the system
U depends only on state of system, not what brought it there
ΔU is independent of process path (like Ug)

6. Isothermal Process Temperature remains constant
Since P = N kB T / V = constant / V
An isotherm (line on graph) is a hyperbola

7. Isothermal Process – Cont.
Moving from 1 to 2, temperature is constant, so P & V change
Work is done = area under curve
Internal energy is constant because temperature is constant
 Q = -W
Heat is converted into mechanical work

8. Isometric (isovolumetric) Process
The volume is not allowed to change
V1 = V2
Since no change in volume, no work is done
 ΔU = Q
Heat added must go into internal energy  it
Heat extracted is at the expense of internal energy  it

9. Isometric Process – Cont.
The PV diagram representation
No change in volume
Area under curve = 0
That is, no work done
The process moves from one isotherm to another

10. Isobaric Process
As heat is added to system, pressure is required to be constant
The ratio of V / T = constant
Some of the heat does work and the rest causes a change in temperature
Thus moving to another isotherm
Recall: changes in temperature = changes in internal energy
 ΔU = Q + W

11. Adiabatic Process No heat is transferred into or out of system Q = 0
 ΔU = W
All work done to a system goes into internal energy increasing temperature
All work done by the system comes from internal energy & system gets cooler

12. Adiabatic Process – Cont.
Either system is insulated to not allow heat exchange or the process happens so fast there is no time to exchange heat

13. Adiabatic Process – Cont.
Since temperature changes, we move isotherms

14. The Second Law of Thermodynamics & Heat Engines
Heat will not flow spontaneously from a colder body to a warmer
OR: Heat energy cannot be transferred completely into mechanical work
OR: It is impossible to construct an operational perpetual motion machine

15. Heat Engines Any device that converts heat energy into work
Takes heat from a high temperature source (reservoir), converts some into work, then transfers the rest to surroundings (cold reservoir) as waste heat

16. Heat Engines – Cont. Consider a cylinder and piston
Surround by water bath & allow to expand along an isothermal
The heat flowing in (Q) along AC equals the work done by the gas as it expands (W) since ΔU = 0
To return to A along same isothermal, work is done on the gas and heat flows out
Work expanding = work compressing

17. Heat Engines – Cont. A cycle naturally can have positive work done
In going from A to B work is done by gas, temperature  (ΔU ) and heat enters system
B to C
No work done, T , ΔU , & heat leaves system
C to A
ΔU = 0, heat leaving = work done
The work out = the net heat in (ΔU = 0)

18. Heat Engines – Cont. Thermal Efficiency
Used to rate heat engines
efficiency = work out / heat in
 e = Wout / Qin
Qin = heat into heat engine
Qout = heat leaving heat engine
For one cycle, energy is conserved
 Qin = W + Qout
Since system returns to its original state ΔU = 0

21. The Carnot Engine
Any cyclic heat engine will always lose some heat energy
What is the maximum efficiency?
Solved by Sadi Carnot (France) (Died at 36)
Must be reversible adiabatic process

22. The Carnot Engine – Cont.
Carnot Cycle
A four stage reversible process
2 isotherms & 2 adiabats
Consider a hypothetical device – a cylinder & piston
Can alternately be brought into contact with high or low temperature reservior
High temp – heat source
Low temp – heat sink – heat is exhausted

23. The Carnot Engine – Cont.
Step 1: an isothermal expansion, from A to B
Cylinder receives heat from source
Step 2: an adiabatic expansion, from B to C

24. The Carnot Engine – Cont.
Step 3: an isothermal compression, C to D
Ejection of heat to sink at low temp
Step 4: an adiabatic compression, D to A
Represents the most efficient (ideal) device
Sets the upper limit

26. Entropy A measure of disorder
A messy room > neat room
Pile of bricks > building made from them
A puddle of water > ice came from
All real processes increase disorder  increase entropy
 of entropy of one system can be reduced at the expense of another

27. Entropy – Cont. Entropy of the universe always increases
The universe only moves in one direction – towards  entropy
This creates a “direction of time flow”
Nature does not move systems towards more order

28. Entropy – Cont. As entropy , energy is less able to do work
The “quality” of energy has been reduced
Energy has “degraded”
Nature proceeds towards what is most likely to happen