comparison of weight and voltage

[DrRocket]

Dr.Rocket , this is a question , I don't claim to be advanced to any level I am seeing being discussed . You say voltage is not force . Why then is electro motive force measured in volts .

 

"electromotive force" is somewhat unfortunate terminology. It is not a force. It is a line integral of the E-field. The units of the E-field are volts/meter which when integrated over a path (distance) yields volts.

 

Force on a charged particle comes from the Lorentz force equation:

 

F=q(E + v X B)

Edited June 8, 2011 by DrRocket

[mooeypoo]

Being measured in the same units does not make two things the same.

 

Also, emf is NOT a force on its own, it's the induced current. It's called EMF out of historical definition.

 

In physics, electromotive force, or most commonly emf (seldom capitalized), or (occasionally) electromotance is "that which tends to cause current (actual electrons and ions) to flow."^[1]

http://en.wikipedia.org/wiki/Electromotive_force

 

And in particular, read #Terminology:

The word "force" in "electromotive force" is a misnomer:^[12]

[Electromotive force] has turned out to be an unfortunate choice of words which is still with us 160 years later. In all of physics except electromagnetic induction, the term 'force' is reserved for mechanical action on ponderable matter and is measured in units called newtons. In contrast electromotive force is measured in units of volts and causes charge separation.^[12]

 

I wish people would actually READ the links we give out. I am sorry for this frustrated comment, but really, this whole thing is getting very much tiring.

 

~mooey

[lemur]

Because the fact that wires spark does not prove there is a physical force on the insulator.

What causes the spark? The light is emitted when electrons are excited and de-excited to release the energy as photons, right? So what was the force that had to be overcome to excite the electrons? Their electrostatic attraction to the nucleus, right? So isn't the electrostatic force holding the electrons to the nucleus the force that resists the current? Isn't the reason conductors conduct easily that their conduction-band electrons have to overcome relatively little electrostatic attraction to change levels?

 

The fact that something has gravitational potential is indeed evident if it has weight. However, its weight is not the same thing as its potential, and the amount of weight does not necessarily correlate to the amount of potential energy, given that height also matters.

Yes, I agree with this. Weight is like electromotive force, if I understand that term right. It is not an amount of energy but it does indicate the presence of energy, albeit that the energy is inactive (potential), no?

 

No, it is not a "capacity" to do so in the way energy is. Energy is used up while doing work -- once something does work, it has less energy. However, objects do not "have" force or use it up while doing work. As such, force is not a capacity to do work.

Work is force exerted over a distance. So a motionless object can't be doing work, but it can have the potential to do work if it is exerting force, right? I would describe force as an "intensity" at some given point. F=MA doesn't really make sense considering that acceleration refers to change in speed, which presumes motion. Still, a motionless object can still exert force, right? And the same object can have a capacity to do work, right? Still, I see your point that energy can be measured as an amount of work whereas force can only be measured as the acceleration of a mass at a given point.

 

"But it makes sense to me!" could perhaps be replaced with "So explain in more detail why point 3 is inaccurate, as I don't get the bit about walruses."

 

 

Also knowing what all the terms mean. That seems to be a key issue.

Ok, these are valuable constructive criticisms. I will be more careful to ask people about their criticism, but that also requires that the criticism has content other than "that's not physics, go read a book!" Terms are also not hard to google, and I usually do that already anyway. Thanks for keeping this constructive. I understand that some people lose patience since I do too in various situations. I just can't really do anything until someone gives me something substantive, which you have gone to the trouble of doing.

Edited June 8, 2011 by lemur

[Cap'n Refsmmat]

What causes the spark? The light is emitted when electrons are excited and de-excited to release the energy as photons, right? So what was the force that had to be overcome to excite the electrons? Their electrostatic attraction to the nucleus, right? So isn't the electrostatic force holding the electrons to the nucleus the force that resists the current? Isn't the reason conductors conduct easily that their conduction-band electrons have to overcome relatively little electrostatic attraction to change levels?

That still doesn't mean the electrons are pushing on the insulator. The fact that electrons must be excited to make the light in a spark does not mean that the electrons at an insulator are pushing on it with some powerful force until it just breaks.

 

Yes, I agree with this. Weight is like electromotive force, if I understand that term right. It is not an amount of energy but it does indicate the presence of energy, albeit that the energy is inactive (potential), no?

They're still not analogous because the emf is the potential difference across the electric field (in a manner of speaking), and only translates to energy if some charge is placed in the field.

 

Work is force exerted over a distance. So a motionless object can't be doing work, but it can have the potential to do work if it is exerting force, right? I would describe force as an "intensity" at some given point.

It could potentially be doing work, yes, but this is not the same as energy being a potential to do work. An object with a given amount of energy can do a given amount of work that corresponds exactly to how much energy it has, and as it performs the work, that energy is depleted. An object exerting a force can do any amount of work depending on how much energy it has (not how much force), and the force is not "depleted," only its energy.

 

F=MA doesn't really make sense considering that acceleration refers to change in speed, which presumes motion. Still, a motionless object can still exert force, right?

That's because F=ma refers to net force, and more than one force can act on an object simultaneously. If I push on an object, and you're on the other side pushing back with equal force, the object will not move because the net force is exactly 0, and hence a=0. But we are still exerting a force, yes, without moving.

[DrRocket]

Also, emf is NOT a force on its own, it's the induced current. It's called EMF out of historical definition.

 

 

http://en.wikipedia....tromotive_force

 

And in particular, read #Terminology:

 

 

I wish people would actually READ the links we give out. I am sorry for this frustrated comment, but really, this whole thing is getting very much tiring.

 

~mooey

 

emf is not a current, induced or otherwise. emf can cause a current to flow under the proper conditions, but it is not itself a current. It is essentially a synonym for "voltage". A voltage source will cause a current to flow, consistent with the generalized form of Ohm's law (impedance vs pure resistance) but voltage is not current.

 

The terminology is most commonly used in conjunction with Faraday's law in the case of time-varying magnetic fields and magnetic induction, in which case the integral of the E-field around a closed loop is not 0, and in fact is the "emf" in the loop. That is how ordinary electrical generators work.

 

On the other hand, it is not a force either.

 

Thank you for your book suggestions. I don't think they have those at the public library I use but I'll keep my eyes open for similar titles.

 

Any or all of those books should be readily available through interlibrary loan. They are very widely used.

[mooeypoo]

emf is not a current, induced or otherwise. emf can cause a current to flow under the proper conditions, but it is not itself a current. It is essentially a synonym for "voltage". A voltage source will cause a current to flow, consistent with the generalized form of Ohm's law (impedance vs pure resistance) but voltage is not current.

Yes, you are right, I apologize. I meant to rephrase that and forgot to erase after I posted the definition from wikipedia. I think it has a much better explanation though, it should be read.

The terminology is most commonly used in conjunction with Faraday's law in the case of time-varying magnetic fields and magnetic induction, in which case the integral of the E-field around a closed loop is not 0, and in fact is the "emf" in the loop. That is how ordinary electrical generators work.

 

On the other hand, it is not a force either.

Right. But it has "force" in the name, which is just a historical terminology, that, most of all, was my point, and I apologize for mixing things up and causing a bit of a confusion.

 

The wiki terminology is much clearer (hence, please, thread participants, read it).

 

 

~mooey

[swansont]

Can force be measured in volts ? I hope this doesn't sound like the same question .

No. Volts (potential difference) refers to the energy per unit charge. While force and energy are related, via Work, they aren't the same thing. They have different units.

[lemur]

That still doesn't mean the electrons are pushing on the insulator. The fact that electrons must be excited to make the light in a spark does not mean that the electrons at an insulator are pushing on it with some powerful force until it just breaks.

Yes, I've been chastised enough times by Swansont for thinking classical mechanics into quantum matters. However, the electrons are still attracted to the nucleus by electrostatic force, which means that moving them away from the nucleus requires counter-force, right? It just so happens that the amount of energy/work required to get them to move is specific to their quantum conditions, right?

 

They're still not analogous because the emf is the potential difference across the electric field (in a manner of speaking), and only translates to energy if some charge is placed in the field.

But isn't a potential difference a force impetus in the same sense as mass and distance create a gravitational force impetus?

 

It could potentially be doing work, yes, but this is not the same as energy being a potential to do work. An object with a given amount of energy can do a given amount of work that corresponds exactly to how much energy it has, and as it performs the work, that energy is depleted. An object exerting a force can do any amount of work depending on how much energy it has (not how much force), and the force is not "depleted," only its energy.

My whole point with this was to address questioning as to how potential energy can be empirically observed. It cannot be empirically observed as an amount of energy, because that would require selecting an arbitrary frame. Still, it can be observed to be present in the form of force, so it's not as if a bowling ball on a table in a space station can be observed to have potential energy the same as the ball on the table on Earth or the moon. The force indicates the presence of potential energy, though it is not a quantity of energy.

 

That's because F=ma refers to net force, and more than one force can act on an object simultaneously. If I push on an object, and you're on the other side pushing back with equal force, the object will not move because the net force is exactly 0, and hence a=0. But we are still exerting a force, yes, without moving.

Idk, I think the presumption that an object has weight (i.e. not mass but weight) regardless of whether it's sitting still on the ground or falling suggests that force can be observed even when net force is zero due to the ground pushing up against the object. This seems like an abstract issue to me, though, since anyone with a truck parked on their foot will tell you that an object sitting still on the ground can be exerting force. I suppose technically, though, the reason the person feels pain is that the weight on their foot is doing work in the form of pushing nerve-cells into unfamiliar shapes.

 

 

[mooeypoo]

Idk, I think the presumption that an object has weight (i.e. not mass but weight) regardless of whether it's sitting still on the ground or falling suggests that force can be observed even when net force is zero due to the ground pushing up against the object.

Sorry, I might be missing something here, but what is weight without mass?

 

 

 

[lemur]

Sorry, I might be missing something here, but what is weight without mass?

I'm not saying anything about the relationship between mass and weight except that weight represents the force of a mass due to gravity, i.e. the same object with the same mass weighs less on the moon than on Earth although it has the same mass because the acceleration of gravity is different.

[Cap'n Refsmmat]

Yes, I've been chastised enough times by Swansont for thinking classical mechanics into quantum matters. However, the electrons are still attracted to the nucleus by electrostatic force, which means that moving them away from the nucleus requires counter-force, right? It just so happens that the amount of energy/work required to get them to move is specific to their quantum conditions, right?

Something like that.

 

But isn't a potential difference a force impetus in the same sense as mass and distance create a gravitational force impetus?

Not necessarily.

 

Suppose I have a very large spherical shell of mass. From a long distance away, it has a gravitational force on me which I can feel, creating a gravitational potential. If we've defined gravitational potential to be 0 at an infinite distance away (as is conventional), then as I approach the shell the gravitational potential becomes more and more negative. Now suppose I enter the shell. Inside, the gravitational potential will be constant and negative, because of how the gravitational force from all around cancels out. There will be a large negative potential, but no gravitational force whatsoever.

 

So if I measure the potential difference between inside and outside the shell, I'll see a significant value, but it's not an impetus to cause a force.

 

Mass creates a gravitational force, and electric fields create an "electric force" of sorts. The gravitational potential and the electric potential difference are only ways of measuring the work they may do.

 

(Now, if you represent the potential as a scalar field, you can use the gradient to find the direction of force on the object, but it's not the potential causing it -- potential is just a way of measuring it...)

 

My whole point with this was to address questioning as to how potential energy can be empirically observed. It cannot be empirically observed as an amount of energy, because that would require selecting an arbitrary frame.

Why can't we select an arbitrary frame?