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Anonymous Poster

Enthalpy and Entropy

04/08/2007 11:46 AM

sir

pl define enthalpy and entropy with some examples

gemrafic

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#1

Re: enthalpy and entropy

04/08/2007 12:41 PM

ENTHALPY (Heat Content) is the sum of the internal and external energies. (Btu/lb or Btu)

Energy - of a substance is it's capacity, either latent or apparent, to exert a force through a distance, that is to do work.

External energy - is that energy represented by the product of pressure and volume. It may be regarded as the energy a substance possesses by virtue of the space it occupies.

Internal energy - is that energy which a substance possesses because of the motion and configuration of it's atoms, molecules, and subatomic particles.

ENTROPY is a measure of the unavailability of energy in a substance. (Btu/lb/°F)

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#2
In reply to #1

Re: enthalpy and entropy

04/08/2007 4:43 PM

Enthalpy is the amount of energy in a system capable of doing mechanical work..

Entropy is the amount of energy in a system that is no longer available for doing mechanical work...short version

When you're young, you're enthalpically over-endowed...

When you get as old as I am, you become entrophically overweighted...

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#3
In reply to #2

Re: enthalpy and entropy

04/08/2007 11:32 PM

I like (feel and fully understand) that!!!!

Dick

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#4
In reply to #3

Re: enthalpy and entropy

04/09/2007 12:04 AM

The three laws of thermodynamics:

1. You can't win. (Conservation of Energy within a system)

2. You can't break even. (Energy transfer occurs from higher state to lower state of availability)

3. You can't get out of the ball game!

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#5
In reply to #4

Re: enthalpy and entropy

04/09/2007 12:17 AM

You're forgetting the fourth law of thermodynamics:

4. Conservation of Problems.

For example...

Horses suck for everyday use. We make automobiles. Automobiles suck for everyday use.

Another?

Typewriters suck for everyday use. We make computers. Computers suck for everyday use.

Is this clear?

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#6

Re: Enthalpy and Entropy

04/09/2007 3:34 AM

Enthalpy is the "thermodynamic potential" useful in the chemical thermodynamics of reactions and non-cyclic processes. Enthalpy is defined by

H = U + PV (3)

where

H = enthalpy

U = internal energy

P = pressure

V = volume

Enthalpy is then a precisely measurable state variable, since it is defined in terms of three other precisely definable state variables.

When a liquid evaporates its go through a process where

· the liquid heats up to the evaporation temperature

· the liquid evaporate at the evaporation temperature by changing state from fluid to gas

· the vapor heats above the evaporation temperature - superheating

The heat transferred to a substance when temperature changes is often referred to as sensible heat. The heat required for changing state as evaporation is referred to as latent heat of evaporation.

The most common vapor is evaporated water - steam or moist.

Enthalpy

Enthalpy of a system is defined as the mass of the system - m - multiplied by the specific enthalpy - h - of the system and can be expressed as:

H = m h (kJ) (1)

where

H = enthalpy (kJ)

m = mass (kg)

h = specific enthalpy (kJ/kg)

Specific Enthalpy

Specific enthalpy is a property of the fluid and can be expressed as:

h = u + p v (kJ/kg) (2)

where

u = internal energy (kJ/kg)

p = absolute pressure (N/m2)

v = specific volume (m3/kg)

The second law is concerned with entropy (S), which is a measure of disorder. The second law says that the entropy of the universe increases. An increase in overall disorder is therefore spontaneous. If the volume and energy of a system are constant, then every change to the system increases the entropy. If volume or energy change, then the entropy of the system actually decrease. However, the entropy of the universe does not decrease.

For energy to be available there must be a region with high energy level and a region with low energy level. Useful work must be derived from the energy that would flows from the high level to the low level.

· 100% of the energy can not be transformed to work

· Entropy can be produced but never destroyed

· For the universe as a whole the entropy is increasing!

· Entropy is not conserved like energy!

The Third Law of Thermodynamics states that

· the entropy of any pure substance in thermodynamic equilibrium approaches zero as the temperature approaches zero (Kelvin), or conversely

· the temperature (Kelvin) of any pure substance in thermodynamic equilibrium approaches zero when the entropy approaches zero

I hope this is quit enough for you but you can explore more in books!!

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#7
In reply to #6

Re: Enthalpy and Entropy

04/09/2007 7:18 AM

Just to complete:

Entropy is a mathematical and virtual entitity defined as:

S=Integral dQ/T

where

S is the entrophy

Q is the heat developed during a process

T is the temperature

For each situation one musta evaluate this integral, by substituting dQ and T for theri respresctive equations.

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#8

Re: Enthalpy and Entropy

04/09/2007 7:34 AM

My Brain Hurts~~But I just wanted to jump in and say thanks I need this information for a project that I am working on also. Its amazing what you can find when you not looking for it.

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#9

Re: Enthalpy and Entropy

04/09/2007 7:46 AM

Good stuff.. nice jokes

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#10

Re: Enthalpy and Entropy

04/09/2007 12:35 PM

You can find a good explanation to your questions in this site:

http://hyperphysics.phy-astr.gsu.edu/hbase/therm/entrop2.html#c1

Good luck !

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#11

Re: Enthalpy and Entropy

04/09/2007 3:30 PM

I seem to recall reading about some research, once back in the 90's and again within the last year or two, in which the ambient heat energy of a surface is collected by a microscopic array of solid state lasers sitting on the surface and, by way of complimentary interference, generate light at higher wavelengths than the heat.

I've googled all the keyword and phrases I can think of but all I can find at the moment are articles about the use of lasers to supercool atoms. That's not what I'm refering to.

Another example of converting normally useless heat energy into potentially useful energy is sonoluminescence. The accustics of these lab devices are likely generating more heat then is being converted by the little bubble, however, a single bubble is a very in efficient use of all that accustic energy so, there's plenty of room for improving efficiency. I'm thinking mankind could employ these technologies to at least reduce the rate that the entropy of the universe increases or, at most, reverse it.

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#12
In reply to #11

Re: Enthalpy and Entropy

04/09/2007 10:51 PM

Are you a Dr. Who fan by any chance???

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#13
In reply to #11

Re: Enthalpy and Entropy

04/09/2007 10:57 PM

Speaking of tiny bubbles...

A few years ago, there was an article in Sci Am that was about certain oceanographers that study only the top centimeter of the oceans. They stated that when really small bubbles pop, incredible velocities and pressures are put into motion, but for only really short periods of time. We're talking droplets at well over a thousand mile per second and tons of force! Weird.

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