Why Does Sound Propagate?

[3v0]

Everybody:

I do not know why we are talking about sound in an absolute zero environment.

My point is that a (theoretical) body at absolute zero (the body itself) can have kinetic and potential energy.

Neither of these forms of energy have anything to do with the temperature of the object. I am talking bricks here. Discussions regarding the vibrational speed (heat) of the bricks molecules are a moot point because at abs zero there are none.

3v0

 

[j.friend]

If energy is mass in motion and that's heat....

A mass in motion has energy... it itself is not energy. Also the point that we have being trying to get across is that there are different kinds of energy that are distinct from each other. A molecule may have thermal energy, gravitational potential energy and kinetic energy all at once. One form of this energy may be used. i.e. we drop the molecule and it's gravitational potential energy is lost, however this is converted to kinetic energy. When the molecule hits the ground, the kinetic energy hits the ground it loses its kinetic energy, which is transformed into heat and sound.

Throughout this the thermal energy has remained constant (until maybe at the end where some kinetic energy may be converted to thermal energy).

All energy is not heat!

however most of it can be transformed into heat

 

[crashsite]

Controlled Experiment

for this post, I'm going to zero in on just this one aspect that you bring up.

in the same way you can throw a rock in the air, and it is certainly heavier than the air around it. the resultant in the differing rates of falling is the relative force able to be applied by the object.

In the case of a brick, there is a lot of mass for a relatively small surface area. this means that it is able to exert a lot of force per unit area of surface to displace the surrounding air. This means that it is able to undergo a larger acceleration when moving through the air with the only force being applied by gravity.

When we compare the relative surface area and mass with an air filled balloon hopefully you can see that a much smaller force is able to be exerted per unit area in comparison to the brick.

Rather than thinking of the object applying force to the air, how about thinking about how the air applies force to the object. The object is a solid while the air is a gas so, the air will be more active.

If the air molecules are striking the surface of the brick or balloon due to their thermal activity, what kind of forces are applied?

If an air-filled balloon is set on top of an empty coffee can, you'll have a way to control how the forces are applied to different parts of the balloon.

**broken link removed**

What happens if the air in the can is heated? What happens if the air in the can is cooled? More importantly, why do those things happen? What is the force that makes them happen?

It's easy to say, "oh, it's air pressure". But, what is that air pressure if it's not the result of the molecules moving around...in other words..."heat"?

for some unknown reason, you guys are convinced that it's not heat and try to come up with various stuff to try to make it not be heat.

 

[crashsite]

Real world and vibration vs. linearity

How can you know whether or not sound can propagate (or alternatively not) in an absolute zero environment. Thus far, to the extent of my knowledge, we cannot create an absolute zero environment so we don't know if sound does propagate. please correct me if I am mistaken.

I'm saying that it's not very productive to postulate how sound may propagate in an extreme (even theoretical) condition. There's plenty to consider just in the range of normally expected conditions.

Add to that, how easy it is to get off on tangents in these threads and it becomes even more important not to allow things to stray too far.

when referring to vibrating particles he is referring to their thermal content (i think)

I'm questioning the whole "vibrating" concept.

When energy is added to a molecule and it moves, does it move in a linear manner or does it move in an oscillatory manner?

I don't know the answer to that but, I suspect it moves in a linear manner and the "vibration" is sort of a synthetic effect. If you go way back to my pool ball analogy, the initial state is all the balls moving about randomly at some range of rates (analogous to heat). The balls are bumping into each other and changing direction and speeds depending on the angles of collision. To the casual observer, they can appear to be vibrating. But, really they are not. At least not in an oscillatory sense.

In something like a solid (especially a crystaline material), the molecules may be held in so orderly a manner that they do take on a true oscillatory motion. Like a ball at the end of a string being constrained to a circular orbit when it really wants to go zipping off in a straight line.

 

[q5101997]

Why dosn't sound travel at a certain speed?

Regs Q

 

[j.friend]

It's easy to say, "oh, it's air pressure". But, what is that air pressure if it's not the result of the molecules moving around...in other words..."heat"?

for some unknown reason, you guys are convinced that it's not heat and try to come up with various stuff to try to make it not be heat.

Air pressure is not soley heat. Heat is a contributing factor.

straight out of one of my textbooks:
the kinetic molecular theory states that an increase in the temperature of a gas increases the average kinetic energy of the molecules. the molecules move more rapidly and collide with the walls of the container more frequently with a greater force. this can cause:
- the volume of the gas to increase
-the pressure to increase

i'm also going to bring in one of your hated formulas sorry.
the general gas equation, the equation used to model the behaviour of gases.

P =nRT/V

Now from this you can see that pressure (P) is proportional to temperature (T) and the amount of molecules (n) given a constant volume (v) (R is one of the despised constants found to make it all work nicely)

on a side note: this is one of the equations that you shouldn't mind, it is derived from charles law that P is proportional to T and boyles law that P is inversely proportionate to V. So this equation is taking the two observations and including the other obvious factor (number of molecules)

You can see that you cannot say that pressure is exclusively heat. In many cases in the natural environment pressure changes due to heat.

Rather than thinking of the object applying force to the air, how about thinking about how the air applies force to the object. The object is a solid while the air is a gas so, the air will be more active.

lets also take a look at this, because it is a tricky part of this discussion. So the air exerts a force on the object. When i refer to the object exerting a force on the air to displace it, this is because the air provides a restoring force against the moving object. At any point within the atmosphere. The force due to air pressure will be pushing any object back down towards the earths surface.

The force of 1 kg/squarecm downwards is a considerable force. Of course this can be negated by hot air rising etc.

If the air molecules are striking the surface of the brick or balloon due to their thermal activity, what kind of forces are applied?

I believe that you may find that at any given area the forces applied to the surface of an object will be negated by the force applied from the opposite direction by a different air particle. Again this is if you aren't holding the brick over a bonfire etc, incidentally even then the air pressure around it would be pretty constant.

 

[crashsite]

More defining moments?

A mass in motion has energy... it itself is not energy.

I don't know that's true.

Energy may very well be the "motion" of mass. We do know that energy itself has mass (e=mc^2). Unfortunately, perhaps, I seriously doubt that we are going to resolve that question at this level of discussion.

Perhaps the best we can do is make a layman's definition of energy that serves to give us common ground for discussion. I propose that definition to be the force inherent in a material due to the movement of the molecules in the material and that the amount of that movement is directly related to the nature of the material and its temperature.

Can we constrain ourselves to nominal environmental values that we can reaosnably expect to encounter?

We can talk about an "object" (presumably made up of molecules) having potential or kinetic energy and responding to the force of gravity. But, in the perspective of sound propagation, do all those concepts come into play?

If air molecules are always in motion and if the sound is handed off from molecule to molecule as it propagates, is only kinetic energy an issue? When you start including potential energy, you're back ot waves again and gravity doesn't seem to be a factor that needs to be directly considered (at least when trying to glean a cursory view of sound propagation).

 

[j.friend]

I'm questioning the whole "vibrating" concept.

When energy is added to a molecule and it moves, does it move in a linear manner or does it move in an oscillatory manner?

I don't know the answer to that but, I suspect it moves in a linear manner and the "vibration" is sort of a synthetic effect. If you go way back to my pool ball analogy, the initial state is all the balls moving about randomly at some range of rates (analogous to heat). The balls are bumping into each other and changing direction and speeds depending on the angles of collision. To the casual observer, they can appear to be vibrating. But, really they are not. At least not in an oscillatory sense.

In something like a solid (especially a crystaline material), the molecules may be held in so orderly a manner that they do take on a true oscillatory motion. Like a ball at the end of a string being constrained to a circular orbit when it really wants to go zipping off in a straight line.

this is a hard concept to grasp, but I'm pretty sure that the molecules gain an increasingly oscillatory nature as the force between the molecules increase.

You have gas particles whizzing around. You decrease their velocity, and subsequently Kinetic energy, gradually until the intermolecular forces become comparitively enough to pull the molecules into a liquid state. Now your particles have kinetic motion throughout the liquid, but they have a large attraction to each other. The oscillatory motion is developing now.

Now we bring the temperature down a little more and the intermolecular forces become camparitively strong enough to hold the molecules together permanently next to the same other molecules. You get to this point. Now if the objects still have energy, they must still be moving yes?

but if they are staying next to their neighbors, they are either doing very small circles/ellipses or have adopted a truly oscillatory motion. When we miraculously achieve absolute zero, the molecules have stopped moving in relation to their neighbor.

(for the purposes of previous points, you now chuck the lump of 'stuff' across the room. The particles have zero energy, but the lump has kinetic energy)

The oscillatory movement (hence vibrating) of molecules is proposed as when getting something down to absolute zero, it's in a solid lattice. If you had one single atom, you try and explain how that'd work.

 

[crashsite]

Mach 1

Why dosn't sound travel at a certain speed?

Regs Q

Actually, it does. It travels at Mach 1. However, the speed ofr Mach 1 can change with various conditions (primarily the material the sound is traveling through and it's temperature). In this thread, we are trying to figure out how that works (at least how it works in air...so far).

 

[crashsite]

Zen

(for the purposes of previous points, you now chuck the lump of 'stuff' across the room. The particles have zero energy, but the lump has kinetic energy)

You're trying to make the point that user, 3v0 was making earlier. I can see what you two are saying but, I question whether it's possible to have motion and absolute zero happening at the same time. It almost becomes a Zen type concept. Can you have motion and a cessation of motion in the same object?

I found the Bose-Einstein Condensate link I was looking for. I really like their Q&A approach. Of course, I don't really understand it but, at least I feel like I have some concept of it by following the explanation.

It may give some insight to some of the things being discussed here.

**broken link removed**

 

[3v0]

Stupid insulting title omitted

I am encouraging j.friend to comment on this too.

You're trying to make the point that user, 3v0 was making earlier. I can see what you two are saying but, I question whether it's possible to have motion and absolute zero happening at the same time. It almost becomes a Zen type concept. Can you have motion and a cessation of motion in the same object?

The reason we are putting the moving object at abs zero is so that you can not try to hide energy in moving molecules.

It is key that you understand this. So I will try again.

We are in space where friction is almost zero. Pick 2 large bodies.

By your thinking when they collide the energy released can only come from the heat they already contain.

But the reality is that if they are moving fast enough they will vaporize each other due to the heat generated by the collision.

The energy for this new heat comes from kinetic energy.

At this point you will want to talk about hidden heat we can not measures which is wrong. But if you want to pursue this hidden heat we need to talk about it too.

Examining underlying principals should not be called nit picking, or tangents.

3v0

 

[skyhawk]

The conversation does tend to wander about rather aimlessly. So let's tackle some of the basics.

In relation to defining the energies....

Energy - The capacity of a system to do work.

By this, if it has thermal energy, it is capable to do work in relation to exertion of heat. Likewise kinetic energy, it is capable to do work due to its motion.
etc....

Kinetic energy- the energy an object has due to its motion. Dependant on mass and velocity.

Potential energy- the energy stored within a system that can be released at a later point.

Work - the exertion of energy.

any of these are up for debate

The definition of energy as "the capacity of a system to do work" is a standard textbook definition, at least in high school text books and maybe some basic college texts, but it isn't useful for much of anything other that answering the question on a test, "What is energy?"

As for the definition of work as "the exertion of energy" that's a new one to me. Perhaps it's from a text book, but wherever it comes from it's rather worthless. When combined with the previous definition of energy, it all sounds circular. Work is properly defined as the line integral of a force along a given path. If the force is constant and the direction of the force and the direction of motion are parallel, then the line integral reduces to a common high school level definition of force times distance. Consider some simple cases:

1.The work to lift a mass m to a height h - Gravity is exerting a downward force of mg, so to lift it requires an upward force of mg. Therefore since the force is constant the work is mg times h = mgh.

2. The work to move a mass m a distance of l on a horizontal surface with coefficient of friction u. The normal force is the weight, mg. The force of friction that must be overcome is therefore mgu and the work is mgul.

An important difference between 1 and 2 is that the work done in case 1 is can be recovered by lowering the mass back to its starting point. In case 2 it takes more work to return the mass to its starting point.

The force in case 1 is said to be conservative while the force in case 2 is said to be non-conservative.

Potential energy can only be defined for conservative forces. Potential energy is just the work requires to move a system from a reference point to its final point. The reference point is arbitrary. Only differences in potential are meaningful. For gravitational potential near the earth where the earth may be treated as flat the reference point is the surface of the earth. Thus the potential energy of a mass m at a height h is just mgh, the work required to lift it from the surface. When the curvature of the earth is important, as in the case of an orbiting satellite the reference is chosen to be infinity.

Kinetic energy is the energy due to motion. There is both linear and rotational kinetic energy. Linear kinetic energy is given by (mv^2)/2, while rotational kinetic energy is given by (Iω^2)/2 where I is the moment of inertia and ω is the angular velocity.

The energy of a system is simply the sum of its various types of energy with each type of energy calculated in a specific way. The importance of the energy concept and the reason that it is used is that it obeys a conservation law. You calculate the energy of a system at the beginning of a process, then the system goes through some process and at the end when you compute its energy the number is the same as it was at the start. The energy serves to determine what processes can take place out of all imaginable processes. Now some may find this definition of energy unsatisfying; however this is the way that energy is understood in physics and the only reason that it is important.

If only mechanical problems are considered then the energy concept is not needed. Using energy methods can shorten a calculation but they are not necessary. All problems can be solved using forces alone. With the rise of the industrial age when people started burning fuel to make steam to run a steam engine that produced mechanical work, the problem became how to obtain the most work for the least fuel. It was this problem that lead to thermodynamics and the formulation of the law of conservation of energy. The basic equation is one that connects quanties of work, internal energy and heat. At about the same time electromagetism was understood, and the concept of energy in the electromagnetic field was developed. With special relativity yet another term was added to the energy equation.

 

[crashsite]

Apples and Oranges

The reason we are putting the moving object at abs zero is so that you can not try to hide energy in moving molecules.

It is key that you understand this. So I will try again.

We are in space where friction is almost zero. Pick 2 large bodies.

By your thinking when they collide the energy released can only come from the heat they already contain.

But the reality is that if they are moving fast enough they will vaporize each other due to the heat generated by the collision.

The energy for this new heat comes from kinetic energy.

At this point you will want to talk about hidden heat we can not measures which is wrong. But if you want to pursue this hidden heat we need to talk about it too.

Examining underlying principals should not be called nit picking, or tangents.

3v0

There's a number of things to clarify. First is that a mass at absolute zero is still pure energy. But, locally, its motion has been reduced to zero. It's important to note the "local" part.

You don't have to go to an extreme, theoretical example to see the principle. You can have two people asleep on two different airliners. Locally, both seem to be stationary but, in fact, if the two planes collide, head on at 600 mph each, well....

If an observer is aware of the people on the two airplanes, he would not say that those passengers are not moving so they must not have any energy.

At this point you're likely saying, "yes, that's the kinetic energy I was talking about". Relative to each other, there is actual, physical movement of the two masses. But, then I say that just as surely, there is actual, relative movement of two adjacent molecules in a parcel of air. In other words you can't "hide" the energy by simply increasing the distance and throwing in the term, "absolute zero".

You could try the argument that the two masses at absolute zero don't interact until they collide and I'd counter with the notation that the molecules in a parcel of air don't either.

Other than the detail of how two masses, both at absolute zero, can restore molecular thermal motion by colliding, and converting their respecive kinetic energy to heat, I'm not sure what you are trying to demonstrate.

I'm also not sure how the absolute zero scenario plays into how energy is moved along to propagate sound.

 

[crashsite]

Thanks for the info

The conversation does tend to wander about rather aimlessly. So let's tackle some of the basics.

Okay, I'm not going to quote all of that.

That was helpful and not. It was very nicely written. It was helpful in nailing down some of the terms and concepts but, of course, in this thread, I'm always, in the back of my mind, trying to think of how it applies to the topic of sound propagation.

How does sound energy get coupled in and out of the system and moved through it. How do the principles of energy and work perform the tasks needed.

I have a mental image of how, for example, the movement of a speaker cone couples its energy to the air molecules adjacent to it. I hunger for other viewpoints of how that happens; where my views may be validated and more importantly, where they are not. But, I confess that I don't hunger for a mathematical treatise on how it happens.

Same with the way the sound energy gets transmitted through a medium.

Same for how the energy is extracted on the other end.

I'm especially interested in how sound acts around barriers, such as how sound is received from a direct path between the emitter and receptor and how it is received indirectly (as you might envision a ear pointed toward a speaker and the other ear pointed away from the speaker but, receiving the same sound).

I have opined fairly extensively on how I see these things happening (way too extensively as noted by some).

 

[3v0]

Stupid insulting title omitted

Crash:

But, then I say that just as surely, there is actual, relative movement of two adjacent molecules in a parcel of air. In other words you can't "hide" the energy by simply increasing the distance and throwing in the term, "absolute zero".

That was never my point. It was you who wanted to place imaginary heat energy into objects. And you have just admitted to the existence of kinetic energy.

You could try the argument that the two masses at absolute zero don't interact until they collide and I'd counter with the notation that the molecules in a parcel of air don't either.

I would not say that. Their gravity fields interact regardless of temperature.

Other than the detail of how two masses, both at absolute zero, can restore molecular thermal motion by colliding, and converting their respecive kinetic energy to heat, I'm not sure what you are trying to demonstrate.

That kinetic energy exists and is not heat. You keep denying the existence of kinetic and potential energy, or saying they are heat which they are not.

I'm also not sure how the absolute zero scenario plays into how energy is moved along to propagate sound.

It has exactly the same right in the conversation as does heat.

3v0


​

 

[3v0]

Skyhawk:

The conversation does tend to wander about rather aimlessly.

To the contrary. It has come down to illustrating the flawed nature of crashsite's physics. He has tried to simplify the nature of energy to the point where it makes no sense.

We need to get crashsite past "All energy is heat" and have him understand that there are various forms of energy and they can be converted to the heat form of energy.

3v0

 

[crashsite]

Hail to the chief

That kinetic energy exists and is not heat. You keep denying the existence of kinetic and potential energy, or saying they are heat which they are not.
​

I never denied kinetic energy but, I did deny potential energy (or, at least strongly questioned it). This is what I said:

"But, I suggest that there is no such thing as "potential energy".

"You can sigh and roll your eyes all you want....

"The instant I realized that sound is propelled by heat, a whole bunch of things suddenly clicked into place. I only regret that it didn't happen when I was much younger. I have wasted a lifetime thinking about these things all wrong!

This may all sound nit picky but, if the intent is to nail down definitions, then it needs to be advanced.

"There's no potential energy. That compressed spring. That brick on the upper shelf. That drawn bow with a nocked arrow. They are all in as vigorous motion as when the spring is rebounding and the brick is falling and the arrow is being launched. The motion is simply at a scale that we can't see it so, we teach high school students that the motion is not there....it's just potentially there.

"The energy is there in moving molecules. Those molecules are bouncing off each other in an essentially Newtonian way (action and reaction). What's more, they are doing it as vigorously when something is at rest as they are when it is in motion (maybe, even more so)."

But, kinetic energy is kinetic energy whether it is between adjacent molecules or between masses at absolute zero temperature. Two masses, both at absolute zero, of which neither has any kinetic energy also has no potential for there to be energy either. So, I stand by that part.

Unless and until matter or energy actually does something, you can't conclusively say what that will be.

In the case of trying to define, "potential energy", there's a question of whether it's even an object or the environment around it has the power to do work. To say that a brick on an upper shelf has more potential energy has no meaning if the brick itself is exactly the same as when it was on a lower shelf.

Thinking of potential energy as, "energy" is like trying to prove a negative. It just can't be done. You can set up hypothetical situations and postulate how a negative (or the potential of something) may act in response but, it's always just a guess.

I, myself have the potential to be the President of the United States of America. Technically, I possess all the criteria of age and citizenship. But, you know...I don't really have the potential to be elected to that office. Still, within the right scenario, I could be the President. There could be a plague that kills every person on Earth except me and I would be not just the US President but, the king of the world.

I need to defer defining kinetic energy as heat but, only because of naming conventions. I think maybe part of the confusion about all this is not equating kinetic energy to heat. I'll have to ponder further on that.​

 

[3v0]

Crashsite:

You need to take care when comparing potential energy to the potential for an event to occur.

In the case of gravity the energy we use to move two objects further apart is associated with the objects as potential energy. An any instant this is a unique quantity of energy.

When we refer to the potential for any given person to become president we usually talking about "Currently unfulfilled capacity to improve, develop, and achieve impressive feats; Anything that may be possible;.."

As we have said in the past "Apples and Oranges".
--------------------------------------------------------------

"There's no potential energy. That compressed spring. That brick on the upper shelf. That drawn bow with a nocked arrow. They are all in as vigorous motion as when the spring is rebounding and the brick is falling and the arrow is being launched. The motion is simply at a scale that we can't see it so, we teach high school students that the motion is not there....it's just potentially there.

In the case of the brick the potential for energy exists due to the gravitational attraction between the brick and the earth (no motion). The energy is not in the brick as you say.

In the drawn bow the energy is stored in the bow (no motion). One side of it is held in compression while the other is held in extension. Both sides will return to their normal length when the string is released. The movement of the bow will convert the stored/potential energy to kinetic energy.

There maybe some heating in the bow as it is drawn but it is not the storage device. We know this because we can leave the bow drawn and let the heat dissipate. The bow will still fire.

3v0

 

[j.friend]

"There's no potential energy. That compressed spring. That brick on the upper shelf. That drawn bow with a nocked arrow. They are all in as vigorous motion as when the spring is rebounding and the brick is falling and the arrow is being launched. The motion is simply at a scale that we can't see it so, we teach high school students that the motion is not there....it's just potentially there.

"The energy is there in moving molecules. Those molecules are bouncing off each other in an essentially Newtonian way (action and reaction). What's more, they are doing it as vigorously when something is at rest as they are when it is in motion (maybe, even more so)."[/COLOR]

But, kinetic energy is kinetic energy whether it is between adjacent molecules or between masses at absolute zero temperature. Two masses, both at absolute zero, of which neither has any kinetic energy also has no potential for there to be energy either. So, I stand by that part.

ok... lets work with your theory.

i think that what you are saying is that when you put the hypothetical energy into a system the energy is in the object. So when you compress your spring, the only reason why it bounces back is because the particles are vibrating in such a way that they repel each other.

So theoretically you can cool that spring down enough that when not held anymore it will not bounce back as the energy has been taken out of the system.
is this essentially what you are saying.

In another way, the gravitational potential energy of a brick on a shelf, if you cool the brick down enough you will get to a point upon which it will no longer fall to the ground?

I'm just trying to comprehend the ramifications of your theory.

(oh btw poor you, living in america....)

 

[crashsite]

Definition suggestion

In the case of gravity the energy we use to move two objects further apart is associated with the objects as potential energy. An any instant this is a unique quantity of energy.

In high school physics it's enough to say that it takes energy to lift a brink and because, when you drop it, you get the energy back, it has, within the lifted brink, "potential energy".

Then, you examine the brick and find that it's the same old garden variety brick it was before you lift it up and you add gravity. Now, you have a specific force against which you lifted the brick and that interaction constitutes the, "potential energy". W gravity again takes control, the combination of the mass of the brick and gravity make the brick "fall", returning the energy to the system.

You can go a step further and think of the movement of the brick in a gravatational field to be distorting gravity and the, "potential energy" to be in the distortion of gravity. The brick then, doesn't fall. It's pulled back into alignment by the springlike action of the gravitational field snapping back to its most desired configuration.

But, if the gravitational field is distorting and gravity affects everything in the universe, is that distortion being felt (perhaps dynamically) at other places in the universe? Is it even possible for that brick to truly have "potential energy"...at least as defined by high school physics.

It's easy to say, "but that's just major overkill in thinking". But, where do you draw the line? And, who draws the line? You? Me? A famous actor, like Alan Alda, who hosts PBS science shows? Stephen Hawking?

So, one thing we must do is agree on what our definitions of some of these things are going to be (even if they are technically wrong) so that we may have common terms and concepts when we talk about them.

This little exercise shows how easily this sort of thing can get out of hand. Having said that, I really have no problem with the high school physics definition of "potential energy"...with the conditions in the next paraggraph...so long as we're all in agreement that's what we're going to use and that we're not going to step out of bounds with it later.

We need to delineate between gross objects (bricks, speaker cones, masses of air, etc.) and things at the atomic level. We can't ignore the kinetics of molecules (due to heat) just because something "seems" to be inert or passive. We also have to say that, sub-atomic stuff is out. And, we need to stay in the realm of nominal values that we can reasonably expect to encounter when dealing with something like sound.