1. AP Physics B, Thermodynamics The Laws of Thermodynamics
AP Physics B Chapter 10
The Laws of Thermodynamics
Marcopul-Pandya,

2. Important Distinctions
Thermodynamics – study of processes in which energy is transferred as heat and work.
There is a difference between heat and work:
Heat is energy transferred due to a difference in temperature.
Work is energy transferred that is unrelated to a difference in temperature.
Open System – where mass and energy are exchanged between objects and the environment.
Closed System – where mass does not enter or leave the system, but energy is exchanged with the environment.
Isolated System – closed system in which no energy is exchanged with the environment.

3. The Laws of Thermodynamics
Zeroth Law – If two systems are in equilibrium with a third system, then they are in equilibrium with each other.
Seemingly obvious idea.
Law is fundamental to the next two, but wasn’t realized until after the next two laws.
First Law – The change in internal energy of a closed system is equal to the sum of the heat added to the system and the work done by the system.
This is basically a restatement of the law of conservation of energy.

4. AP Physics B, Thermodynamics
About that First Law…
Internal energy is always present in a system.
NOTE: DU = 3/2nRDT
Work is done on or by a system.
Heat is added or lost from a system.
Sign Convention:
+Q when heat is added to the system.
-Q when heat is lost from the system.
+W when work is done on the system.
-W when work is done by the system.
Interesting Note:
U = 3/2(nRT) – this refers to the total energy of all the molecules within an object.
This depends upon how much material is present (more material, more energy).
KEave = 3/2(R/NA)T – this refers to the average kinetic energy of all the molecules within an object.
This depends only upon the temperature of the material (R & NA are constant).
Marcopul-Pandya,

5. Special Cases with the First Law
AP Physics B, Thermodynamics
Special Cases with the First Law
Isothermal Process – thermodynamic process in which the temperature of the system remains constant.
Occurs often in heat reservoirs.
Adiabatic Process – thermodynamic process in which no heat is exchanged between the system and its environment.
Occurs rapidly or when well insulated.
Isobaric Process – thermodynamic process in which the pressure of the system remains constant.
Isochoric Process – thermodynamic process in which the volume of the system remains constant. P V P V P V
Isothermal Process – T = 0; U = 0, so W = -Q
Adiabatic Process – Q = 0, so U = W
Isobaric Process – W = -P V (if the gas expands, it is doing work, so work is negative)
Isochoric Process - V = 0; W = 0, so U = Q P V
Marcopul-Pandya,

6. One More Law of Thermodynamics
Second Law – can be stated a number of ways.
Heat naturally flows from hot to cold (duh).
Nothing can convert heat entirely into mechanical energy.
Mechanical energy can be easily converted into work, but the opposite is more difficult.
For this, we look at heat engines.
A heat engine is a device that changes the temperature of a material from high to low by doing work on a system.
High Temperature, TH
Engine W
Low Temperature, TL

7. Efficiencies of Heat Engines
The efficiency of any heat engine can be determined using the following equation:
W – work output of the engine
QH – heat added to the engine
For a Carnot Engine (or an ideal engine):
Processes are done slowly, allowing each one to be reversed without any loss in energy.
Efficiency can be determined using the following equation:
This is theoretically the maximum efficiency of any heat engine.

8. Cooling Devices
Refrigerators and air conditioners work by changing the temperature of a material from low to high.
We must do work to accomplish this process.
A cool gas enters a motor where it is heated via compression.
Heat Pumps will take warm air directly from the outside and warm it before sending it inside.
High Temperature, TH
Engine W
Low Temperature, TL

9. AP Physics B, Thermodynamics
Entropy
Entropy is basically a measure of the order or disorder in a system.
The second law of thermodynamics can be most generally stated as the following: Systems tend to go naturally from order to disorder.
In other words, the entropy of a system naturally increases.
This process can be changed by adding work.
Marcopul-Pandya,

10. AP Physics B, Thermodynamics
Sample Problem
4 kg of a O2 gas are compressed isobarically from 5.0 L to 2.0 L at 2 atm of pressure.
If the temperature of the gas increased 3ºC, calculate the heat added to the system.
If the initial temperature of the system was 20ºC, calculate the change in entropy of the system.
Marcopul-Pandya,