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Thermodynamics
Mr. Rodichok AP Physics
1st Law of Thermodynamics Definition
Formula
The change in internal energy of a system is related to the heat and work done on and by the closed system. ΔU = internal energy ΔU = Q + W Q = heat W = work
Sign conventions
Q is + for heat added to the system W is + for work done on the system
Typical Format
Most problems are pistons
Work for piston
W=Fx
P= F /A
Solving for F and substituting yields W = - P ΔV Week 11 Thermodynamics
Example of 1st Law Example
In your car’s engine the gas ignites adding 2500 J of heat to the cylinder at the same time 1800 J of work is being done by the gas on the piston in the cylinder. What is the change in internal energy? Q = + 2,500 J W = - 1,800 J
ΔU = Q + W = 2,500 J - 1,800 J ΔU = 700 J
Zeroth Law of Thermodynamics
If two objects are in thermal equilibrium with a third object they are in thermal equilibrium with one another.
Thermal Equilibrium
Objects are at the same temperature.
Week 11 Thermodynamics
PV Diagrams PV Diagram
Pressure is on y-axis and Volume is on x-axis Graphical method of representing the 1st Law (ΔU = Q + W)
Pressure (N / m2 or Pa)
B
C
A
Volume (m3) Process
One curve is represented as gas goes from one state to another state.
Cycle
Multiple process (usually 3 or 4) for the gas to return to the original state. Represents the heat engine’s full cycle. Week 11 Thermodynamics
PV Diagrams (cont’d) B Pressure (N / m2 or Pa) A
Volume changes Work Net Work Temperature
C
Volume (m3) Gas is expanding when a process has a + x direction (-Work) Gas is contracting when a process has a – x direction (+Work)
The area between the process line and the x axis The area contained inside a closed cycle. Temperature increases as the process goes away from the origin of the graph. PV PV Find temperature by using PV = nRT or 1 1 = 2 2 T1 T2 Week 11 Thermodynamics
New Terms for Thermodynamics Isothermal Process Temperature is held constant.
P
ΔU = 0 so Q = - W Isobaric Process
Pressure is held constant.
P
Q = ncΔT amount of heat transferred in process Molar specific heat at constant pressure cp = 5/2 R Isochoric Process Volume is held constant P W = 0 so Δ U = Q Molar specific heat at constant volume cv = 3/2 R Adiabatic Process The heat transfer (Q) is zero, the system is insulated. Very fast process can be approximated as adiabatic. Week 11 Thermodynamics
P
V
V
V
V
New Terms Reversible Process
The process can return to the original state by running the process in reverse.
Irreversible Process
The process cannot return to the original state by running the process in reverse. All real world processes are irreversible.
Entropy
The disorder in a substance. Entropy in a natural process always increases.
Week 11 Thermodynamics
2nd Law of Thermodynamics 2nd Law of Thermodynamics An engine can only use a portion of the available heat to perform work. Applications of 2nd Law
Heat engines: car engines, power plants Heat engines reversed: refrigerators, air conditioners Work (W)
Heat Engine
Hot Temperature Reservoir TH (heat source) QH
Engine
Always need a change in temperature
Cold Temperature Reservoir TC or TL (heat sink) QC or QL
Engine uses the change in internal energy due to the temperature difference to produce work Work output from the engine W = QH – QL Week 11 Thermodynamics
Engines Efficiency of the engine The amount of work compared to the input energy (heat) e = engine efficiency e= W W = work QH QH = input heat (energy) Example
A motorcycle engine has an efficiency of 22% and produces a maximum power rating of 50 hp (1hp = 746 watts). a) What is the rate of heat input into the engine per second? b) What is the heat output from the engine in 10 seconds?