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Answers to Readers' Questions on Physics:

Thermodynamics

The following 28 comments and questions have been sent in. They are listed according to date.

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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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Why do some items burn?

Question

May 5, 2005

Why do some ideams burn when others do not?

- USA

6880

Answer

Burning is usually combining with Oxygen. Chemicals that combine with Oxygen will get hot and burn as part of the chemical combination. Many chemicals and materials do not combine with Oxygen. Lead is an example of something that does not combine or burn.

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Heating by reflected light

Question

February 27, 2005

I've been trying to find out from a knowledgeable source if sunlight reflected off a mirror to a surface causes the temperature of surface the sunlight is being reflected on to increase?

Thank you.

Jim - USA

6153

Answer

About 98% of the light hitting a mirror is reflected. The remaining is absorbed, heating the mirror slightly. Some of the reflected light will then be absorbed by the surface it hits, increasing its temperature. How much is absorbed depends on the type of surface and its color. A black, rough surface would absorb very much of the light energy and heat up the most.

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What is happening with entropy?

Question

January 26, 2005

Is it possible to prove or even show that with entropy the universe would eventually ripe itself to pieces? and if so how did it start in the first place? was it creation or evolution? Beacuse with evolution you would have to have something before the big bang to get the ingredients for the big bang. Thanks for letting me ask. JF

Jared - USA

5734

Answer

Entropy is a measure of the state of disorder or randomness of a system. The law of entropy saws that things are getting more random. Another way of looking at it is that the temperature of all things is slowly averaging out. You put hot and cold together and soon you get luke warm. Likewise if you take a firecracker and explode it, the parts become very disordered.

In the big bang theory the universe had little disorder but after the explosion, it has more and more--like the firecracker. But also order was created as stars were formed from gasses.

There are some religions that somehow relate entropy to evolution. They have nothing to do with each other. The theory of evolution states that animals have slowly evolved or developed from one species to another. It does not say how it happened or whether God was controlling the process.

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Why does temperature and volume matter?

Question

December 11, 2004

Hello,
I knew that temperature affect the volume of a contained gas, I also could explain how temperature affect the volume of a contained gas by a experiment, but I don't know why it matters?

Thank you!

Wendy - USA

5316

Answer

These effects are used in air pumps, refrigeration, hot air balloons and racing car tires, to name a few.

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Expansion of iron

Question

November 17, 2004

If a rod is made of iron on one side and wood on the other and it is suspended with a thread. What will happen to the iron part of the rod if it is heated? Will it go up or down?

Sophie - Singapore

5055

Answer

I assume the weight of both are the same and the thread is tied in the middle between them. The the iron will expand more when heated, but its weight will remain the same. Thus the balance will remain the same.

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What happens with entropy?

Question

August 25, 2004

i am a postgraduate in chemistry
my q is as
1.entropy of the universe in increasing that means energy available to do work is decreasing,what will happen when total energyis converted into work

dharam - India

4092

Answer

Kinetic Energy is the movement of matter. Work is movement against a force, such as gravity.

The entropy theory really states that all motion and energy will average out over time. That is unlikely, because the laws of probability state that random motion will always have places of higher and lower energy.

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