Thermodynamics. 1 st law of thermodynamics Energy may be converted to different forms, but it is...
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Transcript of Thermodynamics. 1 st law of thermodynamics Energy may be converted to different forms, but it is...
Thermodynamics
1st law of thermodynamics
Energy may be converted to different forms, but it is neither created nor destroyed during transformations
Energy from chemical bonds is converted to
kinetic energy and heat (body and friction from
tires)
Amount of energy before and after transformation is the same, only the form of the energy has changed
ENERGY Heat
1st Law (Contd.)
Another way to state the 1st law is mathematically.
DE = Q + W
This equation says that the only way to change the energy of asystem is to add heat to it (Q) or to do work on it (W)
Example: Can makewood hotter by applying fire or hitting
Heat
Heat - the ENERGY transferred between objects of different temperature
While used a lot in our vocabulary, this term is very misunderstood
Heat is NOT temperature. An object CANNOTcontain heat; objects contain thermal energy.
Heat is a very important type of energy transfer
Heat Versus Temperature
Temperature - the property that two objects have in common whenNO heat is transferred between them
Temperature is a relative property. We define it in relationship to other things
T1 > T2T1 = T2
Heat Flow
1. Conduction - energy transfer by next-nearest molecule interaction
2. Convection - energy transfer by mixing; can be naturalor forced (fan, stirring, etc.)
3. Radiation - energy transfer by electromagnetic radiation
Heat can flow via one of three methods
Conduction
Energy transfer by nearest molecules running into each other
Rate of heat transfer depends on
• Temperature difference DT = TH - TC
• Thickness of material L• Thermal conductivity of material k• Surface area A
Q k DT A
t L=
Conduction
Q DT A
t R=
More familiar
If intervening material is made up of more than one substance, add R-values
Rtotal = R1 + R2 + R3 + ….
Problem: How is the rate of heat transfer affected by adding anR-value 8 insulation to an 8’x20’ wall that has an R-value of 12when the temperature difference is 20 oF?
Convection
Heat transfer via mixing; requires some type of fluid (gas, liquid)
Things can naturally convect, especially when density changesand more buoyant materials will rise
Forced convection requires energy input
Radiation
Every object in the universe emitselectromagnetic radiation because ithas a temperature above absolute zero.
Type of radiation depends upon the value of the temperature
Wein’s Law => lmax = .003 m K
T
Problem: At what wavelength do you emit most of your radiation?
Stefan-Boltzmann Law
The rate of heat emission due to radiation depends on size and temperature.
Q/t = e s A T4 where e is the emissivity of the object
Remember, the object will be absorbing radiation while it isemitting. Therefore, the total heat transfer is
Q/t = e s A (Tobject4 - Tsurroundings
4)
Heat Transfer Devices
Heat Pump Heat Engine
Transfers heat from cold to hot using external energy WExample: Refrigerator
Outputs useful energy W byextracting it from heat passingfrom hot to coldExample: Car engine
In both devices,
QH = QC + W
If energy is never created or destroyed, why can’t we keep reusing the same energy source forever?
ANSWER:
Although energy isn’t destroyed, in every energy transfer, some of it will change to a non-usable form
This is a consequence of the 2nd law of thermodynamics
“In a closed system, the total entropy either increases or stays the same”
2nd law of thermodynamics
Second law of thermodynamics
ENERGY WasteHeat
When a chemical bond is broken, you get some high quality ENERGY
capable of doing work, and some low quality
“wasted” energy
No energy was lost or created in the transfer, but the usability of the energy declined in the
transformation.This low quality energy cannot be effectively
harnessed to do any more work, so you cannot use one energy source forever
Example: powering your car
Breaking chemical bonds in gas during combustion yields high quality energy which
produces kinetic energy to move car
Also produces waste energy as heat with little ability to do work
Second law of thermodynamics
Combustion of gasoline
Piston movement
Axleturns
Wheelsturn
Heat loss during combustion
E
Friction with pistons
E
Friction with axle
E
Friction of tires
with road
E
E E
EE
Energy in gasoline
Amount of high quality energy declines with each step (width of orange arrows)
No energy is lost, it simply is converted to low-quality heat that cannot be used for further work
Usable E
Efficiency
A measure of how well energy is converted
Efficiency = useful energy out
total energy input
Examples
Internal combustion engine car is about 10% efficientElectric car is about 20% efficientIncandescent light bulb is about 1% efficient
Efficiency Example
A power plant consumes 80,000 Joules of coal energy to produce 30,000 Joules of electricity. What is the efficiency?
Efficiency = 30,000 J
80,000 J= .375 = 37.5 %
= 10,000 J
Heat Engine Efficiency
Energy input = QH
Usable energy output = W
Efficiency = WQH
Since QH = QC + W => W = QH - QC
Efficiency = 1 - QC
QH
Problem: A car takes in 20,000 J of gasoline and outputs 19,000 J ofheat. What is the efficiency of the car?
Heat Pump COPFor heat pumps, it is not proper to discussefficiency since there is no “usable energyouput”. Instead, define “coefficient of performance” to discuss how much energyit moves per energy paid for.
COPheater =
COPa.c. =
QH
W
QC
W
Note: COPheater is always greater than 1. Why?
Maximum Efficiency
Unfortunately, the 2nd law of thermodynamics limits the maximum efficiency that a device can have. No device will ever be 100% efficient.
For a heat engine, the limit is given by
Maximum efficiency = 1 - TC
TH
where TC is the temperature of the cold reservoir and TH is the temperature of the hot reservoir in the Kelvin temperaturescale
Maximum Efficiency Example
An inventor proposes a heat engine that will produce electricityby extracting heat from ocean surface water at 20oC (293 K) and dumping the waste heat to the deep ocean at 5oC (278 K). What is the maximum efficiency?
Maximum efficiency = 1 - 278 K
293 K= 1 - .95 = .05
At most, this device will be 5% efficient. In reality, it will probably only be about half of this, or 2-3% efficient.
Recapping
2nd LAW:
Energy is transformed from high quality to low quality
1st LAW:
Energy is neither created nor destroyed, only transformed
RESULT:
Low quality heat cannot do substantial work, requiring a new source of high quality energy