Interesting thread...

To change the subject a little, from work to energy, I was wondering whether pressure is considered energy. When a book sits on a table, is it transferring energy to the surface of the table in the form of pressure? Since pressure can change the physical/chemical structure of carbon to make diamonds, is this considered work, or a transfer of energy? If so, how do the laws of conservation of energy apply to fields, which seem to be transferring energy eternally?

Interesting thread...

To change the subject a little, from work to energy, I was wondering whether pressure is considered energy. When a book sits on a table, is it transferring energy to the surface of the table in the form of pressure? Since pressure can change the physical/chemical structure of carbon to make diamonds, is this considered work, or a transfer of energy? If so, how do the laws of conservation of energy apply to fields, which seem to be transferring energy eternally?

No, simple pressure does not qualify as energy. However. stored pressure - like compressed air in a tank - would be potential energy, the same as water behind a dam.

Why do you think fields are transfering energy "internally?" There's nothing at all to indicate that.

Read-Only,

No, simple pressure does not qualify as energy. However. stored pressure - like compressed air in a tank - would be potential energy, the same as water behind a dam.


If I heat a tank of gas and the pressure of the gas increases, did I transfer energy to the gas. If I use the pressure of the gas to move a piston, where did the energy come from. What if the gas is pressurized by gravity instead of heat? Is there a transfer of energy in that case?

Why do you think fields are transfering energy "internally?" There's nothing at all to indicate that.

I said eternally, as in forever.

Read-Only,



If I heat a tank of gas and the pressure of the gas increases, did I transfer energy to the gas. If I use the pressure of the gas to move a piston, where did the energy come from. What if the gas is pressurized by gravity instead of heat? Is there a transfer of energy in that case?

In the first case, yes, you transfered energy (in the form of heat) to the gas. In the second question, you are the one who provided the energy initally.

Those last two sentences aren't very clear. If the gas became pressurized as a result of gravity, you would have to move it to a location with less gravity in order to extract energy from it.



I said eternally, as in forever. Ok, I see. But my queston remains: what makes you think there's any transfer of energy? A magnetic field, for example transfers no energy UNLESS you apply energy to it - as in a generator.

Read-Only,

If the gas became pressurized as a result of gravity, you would have to move it to a location with less gravity in order to extract energy from it.

You're correct, but you are still indicating that the pressurized gas, resulting from gravity, has energy in it. Where did the energy come from? If it came from the gravitational field, did the energy of the field decrease?

Let's take it one step further, let's say that gravity pressurizes the gas which results in a chemical reaction, or even a nuclear reaction, in the gas. Wouldn't it be logical to assume that the chemical, or nuclear, reaction resulted from a transfer of energy to that gas?

Read-Only,



You're correct, but you are still indicating that the pressurized gas, resulting from gravity, has energy in it. Where did the energy come from? If it came from the gravitational field, did the energy of the field decrease? It has potential energy - and no, the field did not decrease in energy at all - because it contained none to start with.

Let's take it one step further, let's say that gravity pressurizes the gas which results in a chemical reaction, or even a nuclear reaction, in the gas. Wouldn't it be logical to assume that the chemical, or nuclear, reaction resulted from a transfer of energy to that gas?

No, it would not. The energy was stored in the form of chemical or nuclear energy to begin with.

You are making a very common mistake here - assuming that gravitational or magentic field contains energy while they do not. Many people assume that an ordinary magnet contains energy (usually assumed to be limitless, too) when that's simply not the case. Same with gravity. Both are simply forces, not energy.

Read-Only,

No, it would not. The energy was stored in the form of chemical or nuclear energy to begin with.

But what caused the reaction to occur?

Let me give you two examples:

Example 1: I have a fusion reactor in my basement. I use a large amount of electricity to pressurize and heat hydrogen gas in a container. At a certain point, the pressure and heat of the gas in the container is so great that it fuses and creates helium.

Question: Was energy transferred to the gas to initiate the nuclear reaction? Where did the energy come from? Was energy conserved in this reaction?


Example 2: Hydrogen gas molecules in outer space are being attracted by other hydrogen gas molecules. The gas comes together, and its pressure and temperature increase until fusion occurs.

Question: Was energy transferred to the gas to initiate the nuclear reaction? Where did the energy come from? Was energy conserved in this reaction?


Are you implying that energy was required to start a nuclear reaction in example one, but not in example two?

Read-Only,



But what caused the reaction to occur?

Let me give you two examples:

Example 1: I have a fusion reactor in my basement. I use a large amount of electricity to pressurize and heat hydrogen gas in a container. At a certain point, the pressure and heat of the gas in the container is so great that it fuses and creates helium.

Question: Was energy transferred to the gas to initiate the nuclear reaction? Where did the energy come from? Was energy conserved in this reaction?

Yes. You supplied the energy to trigger the reaction - and only to trigger it. Yes, energy is conserved. The energy produced from the reaction AND the energy you supplied are both present.


Example 2: Hydrogen gas molecules in outer space are being attracted by other hydrogen gas molecules. The gas comes together, and its pressure and temperature increase until fusion occurs.

Question: Was energy transferred to the gas to initiate the nuclear reaction? Where did the energy come from? Was energy conserved in this reaction?

No, energy was not transferred in this case. Simple static pressure increase provided the trigger in this example.


Are you implying that energy was required to start a nuclear reaction in example one, but not in example two?

Not just implying, clearly stating some of the basic principles of physics.

Read-Only,

In both examples I have hot, pressurized, gas before the nuclear reaction occurs. Does hot, pressurized, gas have more energy then cold, unpressurized, gas? I heated and pressurized the gas in example one using electricity, what energy was used in example two to heat and pressurize the gas?

Read-Only,

In both examples I have hot, pressurized, gas before the nuclear reaction occurs. Does hot, pressurized, gas have more energy then cold, unpressurized, gas? I heated and pressurized the gas in example one using electricity, what energy was used in example two to heat and pressurize the gas?

I answered that. None. The reaction was brought about by the simple increase in static pressure. That's precisely what "lights" (triggers) all stars. Once the pressure becomes great enough, the hydrogen nuclei will begin to fuse spontaneously. No external energy is needed or applied.

Read-Only,

I answered that. None. The reaction was brought about by the simple increase in static pressure. That's precisely what "lights" (triggers) all stars. Once the pressure becomes great enough, the hydrogen nuclei will begin to fuse spontaneously. No external energy is needed or applied.

Let's forget about the nuclear fusion for a second. Let's look at the gas just before the nuclear reaction occurs. In both examples the gas is heated and pressurized. Do you agree with me that hot gas has more energy than cold gas?

In example one, I heated the gas by pressurizing it using a device powered by electricity. So I transferred energy to the gas from the electricity I used.

In example two the gas is heated and pressurized by a gravitational field. Where did the increased energy in the gas in example two come from?

Read-Only,



Let's forget about the nuclear fusion for a second. Let's look at the gas just before the nuclear reaction occurs. In both examples the gas is heated and pressurized. Do you agree with me that hot gas has more energy than cold gas?

Certainly.

In example one, I heated the gas by pressurizing it using a device powered by electricity. So I transferred energy to the gas from the electricity I used.

Correct.

In example two the gas is heated and pressurized by a gravitational field. Where did the increased energy in the gas in example two come from?

Once again - from the increased static pressure of the atoms being forced/squeezed tighter together. But NO transfer of energy took place. The increase in pressure came simply from gravitational attraction pulling the atoms together. As a result of that, there would be a very small increase in temperature due to confining the Brownian motion (but again, that motion is inherent to the atoms, not because any energy was "added") until the point when the pressure is enough to trigger the nuclear reaction - THAT'S when energy is released from the atoms themselves and a tremendous increase in temperature occurs - not before then.

Read-Only,

Once again - from the increased static pressure of the atoms being forced/squeezed tighter together. But NO transfer of energy took place.

I wouldn't go that far as to say that transfer of energy took place. All I'm saying is that the total energy of the gas increased in both examples, even though in the second example gravity is solely responsible for the increase in the energy of the gas. So if no energy transfer took place, the question remains where did the extra energy in the gas in example two come from?

The increase in pressure came simply from gravitational attraction pulling the atoms together. As a result of that, there would be a very small increase in temperature due to confining the Brownian motion

I disagree with your statement here. Have you ever used a can of compressed air to spray your computer? Did you notice how cold the can got after you used it for a while? That's because in an ideal gas, if you decrease its pressure by two, you'll decrease its temperature (in Kelvin) by two. Also, if you dramatically increase the pressure of gas, you can dramatically increase its temperature. Because of this, in star formation, it would be logical to assume that the intense pressure of the gas would result in very high temperatures even before the nuclear reaction is triggered.

The reason I responded to this thread is because I believe that Grantywanty has good reason to question why a magnet creates a constant force for a long period of time without running out of "energy". I think that the concept of "energy" is not as simple as we would all like it to be. Actually, I believe that the term "energy" should be completely replaced with terms like "potential difference", "force", etc, even though the concept of conservation of energy is very useful in many situations.

Hi Prosoothus,
You might be interested in this thread: centre of the earth = energy creation?, in which the same concepts were discussed.

In both examples I have hot, pressurized, gas before the nuclear reaction occurs. Does hot, pressurized, gas have more energy then cold, unpressurized, gas? I heated and pressurized the gas in example one using electricity, what energy was used in example two to heat and pressurize the gas?
I answered that. None. The reaction was brought about by the simple increase in static pressure. That's precisely what "lights" (triggers) all stars. Once the pressure becomes great enough, the hydrogen nuclei will begin to fuse spontaneously. No external energy is needed or applied.

I disagree.
The thermal energy of the gas increased as its volume decreased and pressure increased.
This energy came from the gravitational potential energy of the gas before it fell together.

I disagree with your statement here. Have you ever used a can of compressed air to spray your computer? Did you notice how cold the can got after you used it for a while? That's because in an ideal gas, if you decrease its pressure by two, you'll decrease its temperature (in Kelvin) by two. Also, if you dramatically increase the pressure of gas, you can dramatically increase its temperature.
I'm not sure that it's a simple proportional relationship... and there are three variables here, not just two: Pressure, Temperature, and Volume.
For the can of compressed air, the air's volume increased, the temperature decreased, and the pressure decreased.

Now, the cause and effect is a bit tricky... I'm not that great at thermodynamics, but I think that the change in volume causes both a change in pressure and a change in temperature. You can find out more here: adiabatic process (http://en.wikipedia.org/wiki/Adiabatic_process).

Something else to think about is whether the expanding air is doing work on itself or its environment... if it is, then that energy must come from somewhere - like the heat of the air.

With enough pressure you can press two negative sides of a magnet together.
Gravity has a push/pull reaction. Gravity is a neutral spin or two opposites spinning "(+,-)". The two opposites become a neutral force when it is under a spin. Exerting both an inwards and outwards force. Both an attracting and repelling force.

yeah my theory...

I disagree.
The thermal energy of the gas increased as its volume decreased and pressure increased.
This energy came from the gravitational potential energy of the gas before it fell together.

You are quite correct, Pete, sorry - I was on sort of a roll and neglected to acknowledge the role of gravity in that action.

So if no energy transfer took place, the question remains where did the extra energy in the gas in example two come from? As Pete said, from the potential energy of the gravitational fields of each particle;

perhaps, to clarify: temperature is a measure of kinetic energy, and the potential energy of all those particles in each other's gravitational fields at long distances is converted to kinetic energy as they accellerate toward each other.

The total energy of the whole system remains unchanged throughout (even after the fusion starts, accounting for mass as energy as per Einstein).

In general: it sometimes helps me to think of a field - like a gravitational field - as a thing, like a rock, rather than a "force".

Pete,

I'm not sure that it's a simple proportional relationship... and there are three variables here, not just two: Pressure, Temperature, and Volume.
For the can of compressed air, the air's volume increased, the temperature decreased, and the pressure decreased.

Now, the cause and effect is a bit tricky... I'm not that great at thermodynamics, but I think that the change in volume causes both a change in pressure and a change in temperature. You can find out more here: adiabatic process.


You're right, in an ideal gas if you decrease its volume by a multiple of two, you increase its temperature by a multiple of two. Higher pressure is the result of higher temperature and smaller volume, but doesn't have to be directly proportional to either.

iceaura,

As Pete said, from the potential energy of the gravitational fields of each particle;

I guess your right. I just don't like the term "energy", and even less the term "potential energy". :bugeye:

You're right, in an ideal gas if you decrease its volume by a multiple of two, you increase its temperature by a multiple of two.
if the pressure remains constant.

I guess your right. I just don't like the term "energy", and even less the term "potential energy". :bugeye:

Do you understand the concept that a physicist has in mind when they use those terms? Don't get hung up on the labels... look at what they're applied to.

Pete,

Do you understand the concept that a physicist has in mind when they use those terms? Don't get hung up on the labels... look at what they're applied to.

What's your definition of energy? And don't look it up in a book or on the web.... :D