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2http://www.eia.doe.gov/oil_gas/petroleum/data_publications/wrgp/mogas_history.html

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Lecture #4 Chapter 3 & 4

2nd Law of Thermodynamics1st Law of Motion

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Newton’s 1st Law of Motion

• A body will stay at rest or in motion at constant velocity and direction unless acted upon by an outside force

• Inertia is the tendency to resist change

• Inertia is the measure of an object’s mass

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Automobile Efficiency

Losses at cruising speeds

(not including engine efficiency)

9Fig. 4-20, p. 118 Heat Engine - OTEC

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2nd Law of Thermodynamics (Natural decrease in usefulness)

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Observations

Processes occur naturally only in one direction→ A cup of hot chocolate always cools down

It will never heat up by taking energy from the colder room

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Observations (cont’d)Pressurized air will always escape from a

container that is puncturedMore air will never push into the pressurized container

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Observations (cont’d)An object held by a string above the floor will

fall when the string is cutThe object will never gain energy from the room and rise or

stay hovering above the floor

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Observations (cont’d)From these examples we see that processes naturally occur in the direction that creates more uniformity of temperature, pressure, etc (i.e, towards equilibrium with surroundings); and therefore less ability to produce work.

→ Heat flows from hot to cold (hot objects cool down)A refrigerator transfers heat the opposite way, but it does not occur

naturally it requires work into a compressor

→ Pressurized air escapes when the container is punctured→ Objects above the floor fall

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Observations (cont’d)

The 1st Law of Thermodynamics does not determine in what direction a process will naturally occur

→For example the 1st law is not violated if a hot object gains more energy from the cold surroundings as long as the energy gained by hot object is equal to the energy lost by cold object

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Observations (cont’d)

For each naturally occurring process there was an opportunity to produce work

→ When the hot object cooled down we could have heated up steam and sent it through a turbine to make work

→ When the compressed air escaped we could have forced it through a turbine to make work

→ When the weight fell to the floor we could have connected the weight to an electrical generator and made work (or raised a slightly lighter weight)

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Observations (cont’d)

We need laws of thermodynamics to predict:

→ The direction a process will naturally take• Simple processes are easy to predict• Complicated processes are more difficult to

predict

→ The amount of work the naturally occurring process could have made

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The 2nd Law of Thermodynamics

The 2nd Law of Thermodynamics predicts in what direction processes will naturally occur→ The direction that creates more energy at ambient

conditions and less ability to produce work

This is useful for both:→ Simple processes where intuitively we know the

direction → For complex processes where we may not know the

final outcome

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The 2nd Law of Thermodynamics

The 2nd Law of Thermodynamics determines:

→ The maximum possible amount of work that can be produced from a process

→ The amount of disorder the process has caused

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Summary of the 2nd Law of Thermodynamics

• Heat flows naturally from hotter to colder • Naturally occurring processes result in more

disorder• Energy has quality as well as quantity• All energy in the form of heat cannot be

converted to work→ A portion must be transferred to a low temperature

sink

• Entropy is a rating of disorder and randomness→ High quality ≈ low entropy→ Low quality ≈ high entropy

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Heat Engines (power cycle)

• Energy from a high temperature source is transferred to the heat engine

• A portion of the high temperature energy is converted to work

• The remaining energy is transferred to low temperature sink

• The efficiency = (work out)/(heat in) is always less than 100%

• Electrical power plants, automobile and jet engines all operate by these principles

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Heat Engines (power cycle)

Usually the heat engine consists of turbines, pistons, etc.

→ For now it will be modeled as a circle or box with: • Energy at high

temperature going in • Work and energy at low

temperature coming out

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Heat Engines – Power Cycles (cont’d)

• Nicolas Leonard Sadi Carnot (1796 – 1832) determined maximum possible efficiency for a heat engine

• Biographical comment:→ “A quiet, unassuming Frenchman who lived during

the turbulent Napoleonic years and had an unspectacular life”

• One of Carnot’s mottos→ “Speak little of what you know, and not at all of what

you do not know”

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Carnot Efficiency

• Max Efficiency = (Th – Tc) / Th

• For % efficiency multiply by 100

• Th = High temperature source

• Tc = Low temperature sink

• All temperatures must be in absolute units (e.g., K or R)

h

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Example of Maximum Efficiency of Heat Engine

Heat engine receives heat from steam at 300°C and exhausts heat to air at 100°C

→ What is the maximum efficiency?

→ Th = 300°C + 273 = 573 K

→ Tc = 100°C + 273 = 373 K

→ Max Efficiency = (573 K – 373 K) / 573 K = 0.35 = 35%

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Actual Efficiencies of Heat Engines – Power Cycles

• The efficiencies of all heat engines are less than 100% because:→ All heat engines cannot operate greater than Carnot

Efficiency even if they were constructed perfectly (no friction, etc.)

→ Losses such as friction even decrease the efficiency to a value lower than the Carnot efficiency

• Less efficient processes are used because:→ Cheaper→ Easier to use

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To Increase Efficiency of Heat Engine – Power Cycle:

• Increase temperature of heat source

• Decrease temperature of heat sink

C

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Why will heat energy never reach 100%?

CARNOT Efficiency

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