Thermodynamics

1st law of thermodynamicsEnergy 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 + WThis 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

ConductionEnergy 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

RadiationEvery 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 LawThe 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 DevicesHeat 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

EFriction with axle

EFriction of tires

with road

E

E EEE

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 J80,000 J

= .375 = 37.5 %

= 10,000 J

Heat Engine EfficiencyEnergy 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

WQC

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