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Answers to Readers' Questions on Physical Science:

Thermodynamics

The following 24 comments and questions have been sent in. They are listed according to date.

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Heating a waterbed warmer than your body temp

Question

July 17, 2009

My waterbed should be slightly higher than body temperature?

My body creates heat - constantly. BUT it insists on keeping its temperature constant. It seems to me that it is very unhappy, if it does not loose the heat it produces. It loses the heat it produces to the environment: the waterbed and the air surrounding my body.

If the air is particularly cold, a warm(er than body temperature) waterbed would be nice. But if the air is "warm", then a slighter cooler (than body temperature) waterbed would seem more comfortable.

John - USA

18033

Answer

When you are lying on your waterbed, there is a tendency for both to reach an average temperature. Since the mass of the waterbed is much greater than your body, it acts as a "heat sink" in that is seems to suck heat from your body.

Ideally, the waterbed should be at your body temperature. But since it loses some heat to the air, having it slightly warmer would keep it comfortable. On a hot day, a slightly cooler waterbed might be more comfortable. That is why they usually have heaters with an adjustable thermostat. Then you can adjust the temperature to what is comfortable.

The big issue, though, is that many people don't realize that the water must be heated. Water at room temperature would be uncomfortable in a waterbed.

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Zeroth Law of thermodynamics

Question

March 28, 2009

WHAT IS ZEROTH LAW OF THERMODYNAMICS?

BIJULAL - India

17519

Answer

A system is said to be in thermal equilibrium when its temperature does not change over time. It results from the definition and properties of temperature.

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What is Gibbs free energy?

Question

December 1, 2008

What is GIBB'S FREE ENERGY CHANGE??
Is this theorem has any relation with THIRD LAW OF THERMODYNAMICS??

Manpreet Singh - India

16879

Answer

The Gibbs free energy is the maximum amount of non-expansion work that can be extracted from a closed system. It is a potential that measures the "useful" work obtainable from an isothermal, isobaric thermodynamic system.

It is more related to the first law of thermodynamics

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How can an automobile engine heat the car?

Question

May 7, 2008

how does the heat generated in automobile is put to use in cold region(heat transfer )

kenny - Nigeria

15897

Answer

The heat generated by an automobile engine can be used to warm the inside of the car by the use of a heater. The engine heats water by conduction, which in turn heats air moving through the heater, which moves throughout the car by convection.

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What are applications of thermodynamic law?

Question

January 12, 2008

what are the application of thermodynamic law?

tina - Philippines

15111

Answer

The transfer of heat and converting heat into work are applications of the Laws of Thermodynamics.

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How to solve a difficult problem?

Question

July 30, 2007

Saturated CO2 vapor expands (in an isentropic/adiabatic expansion) from 60F (I'm using English units) and 747.4 psi to 40F and 567.3 psi. The enthalpy decreases by 4.9 btu/lb (from 132.2 to 127.3), resulting in 62.5% vapor and 37.5% liquid. Is the decrease caused by the attraction of the molecules resisting the expansion, and is this attraction of the molecules an electromagnetic force? Conversely, if the gas was compressed from the lower pressure and temperature to the higher, would there not be an increase in enthalpy in excess of the enthalpy of the lower pressure gas (which is 136.8btu/lb), caused by the attraction of the molecules assisting the compression,a nd would this increase in enthalpy not take the form of a much higher temperature than 60F? Finally, can the attraction of the molecules be considered to be a form of potential energy which is converted into kinetic when the gas expands?

If you cannot answer this question can you please direct me to who can?.

rannoch - Canada

14131

Answer

I'm sorry, but that is beyond the scope of our material. And I don't know of any online sources would be able to help you that problem. It is something that a Physics professor might show you how to do. I know that doesn't help you out, but best wishes in your efforts.

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What does message entropy mean?

Question

July 9, 2007

In the field of communications, can you explain the meaning of the term 'message entropy' and is there a connection with the word 'entropy' used in thermodynamics

Alexander - UK

14043

Answer

In information theory, message entropy or information entropy is a measure of the uncertainty associated with a random variable. The inspiration for adopting the word entropy in information theory came from the close resemblance between equations in information theory and thermodynamics. But at the practical level, the similarity between information entropy and thermodynamic entropy are not close.


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Trying to burn paper wrapped around metal pipe

Question

July 21, 2006

I am trying to find information about a experiment I am going to do. It is wrapping a piece of paper around a metal pipe and lighting it with a tourch. The paper should not burn but I can't figure out how to explain why the paper won't burn. The other part of the experiment is wapping the paper around a piece of wood and lighting it on fire. It should burn and I need to explain why it burns. I have searched the web and can't find any information on this. If you could help me I sure would appreciate it very much.
PS: I am a 50 year old student trying to figure out what I am doing.

Kim - USA

11520

Answer

The flame might scorch the paper wrapped around the metal pipe, but it will not set it on fire. The reason is because the iron acts as a heat sink, quickly absorbing the heat on the paper, such that the temperature of the paper never gets to the ignition point of paper (451 F or 233 C). Metal is a good conductor of heat, so it draws off the heat from the paper. If you kept heating the metal to over 451 degrees, the paper would then start burning.

Now, wood is a poor conductor of heat, such that it would absorb only a little of the energy from the paper. Thus the paper would start burning soon.

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Effects of gravity on molecules in a plasma state

Question

October 24, 2005

I just stumbled across your site while looking for some information on the effects of gravity on a huge body of molecules in a plasma state. I wonder if you could offer me some guidance upon this question:

In a huge body of simple molecules (hydrogen and helium for example) super heated to a plasmatic state, would the kinetic energy of the molecules (whether individually or corporately) be stronger or weaker than the gravitational attraction between them (whether individually or corporately)? As per this question, these elements would have no external forces acting upon them.

Also:
Can trace minerals exist in an environment of 2000+k?

If an outside source of radiation WERE to emit from a large source upon a single side of a superhot nebula, what would you expect the reaction of these molecules to be to that outside force? (assume that the radiation is in the form of the entire short wave band)

Thanks.

Mark - USA

8676

Answer

Gravity is a very weak force at the atomic level. A huge body of superheated H and He, such as the Sun will exert gravity to outside masses, but the force of attraction between the atoms themselves would be so much smaller than their KE, that there would be no real effect.

There have been trace metals detected on stars with temperatures around 6000 K.

I'm not sure of the last question.

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Is a tomato breaking down entropy?

Question

September 2, 2005

Dear Ron,

I know, or think I know, that entropy is part of the Second Law of Thermodynamics. As I understand it the more disorder of a system the more entropy. The less disorder the less entropy. My question to you is this:

If I have a beautiful red tomato. The tomato seems very organized. I put the tomato out on a table and leave it there. Gradually the tomato starts breaking down and becomes ugly and a mass of glob. Is this entropy?

Phoenix, Arizona

Harry - USA

7984

Answer

Entropy really concerns the averaging out of energy. If one side of a metal rod is hot and the other side is cold, it will soon reach an average temperature.

Although people apply the idea of entropy to different things--like the example of a tomato decaying--it is really just a metaphor. The tomato breaks down because bacteria is working on it.

I've never liked the use of entropy as some property, just like the idea that heat is a fluid. It is better to look at it in terms of movement of molecules.

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