Naked Science Forum
Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: randomplanck on 26/07/2014 20:08:33
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I've asked around about this and am not satisfied with the answers I've been getting so far. I don't mean joule per coulomb, or it's ability to provide EMF, or it's the potential difference or any of those kind of definitions. I'm looking for what it is *physically*. In other words, I can increase gravity by adding mass, I can increase water pressure by squeezing it, but exactly IS this potential difference in charge? I've gotten a wide variety of answers that skirt what I am really getting at. Some people say it's more electrons. Some people say it's more accelerated electrons. Some people give me formulas. It is electrons under some kind of pressure? (Water pressure is always what they tell you it's like). Are you familiar with the TV show COSMOS's "Ship of the Imagination"? It can get very big and very small to show you things in physics. Let's say, I get small and travel inside a battery - what would I look at to tell how many volts the battery provides? Or, let's say I have a one volt button battery. What do I do inside of it to make it a 2 volt button battery? Here is a thread I created at yahoo where I'm not happy with any of the answers - if you care to peruse it. I have comments on most of them. https://answers.yahoo.com/question/index?qid=20140719101319AA3Trma
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I doubt that this will help any, but since you haven't accepted the answers from science (although they were correct), let me step up into a field above science, Ontology.
When you ask science of what something is made, all they can do is give names of components and how they behave. That is their ontological limit. What they "are in reality" is not a science question, much like asking a quantum physicist why something happens, it just isn't his field.
The concept and definition of a potential is, as you were told, "the ability to accomplish something". But it seems that you want more on that. A potential is actually a situation formed by other things. For example, the potential for an apple falling from a tree, is the situation of the apple hanging by a weak stem with nothing under it while in a gravity field. The potential is entirely made of the situation. It is not a substance, but rather what causes substance (action).
In the case of the electric potential, the situation that causes EMR is measured in voltage and is made of the non-homogeneous changing of the surrounding situation (surrounding potentials).
When a potential changes, that changing is what constitutes the substance of which the entire universe is made, commonly referred to as "random EMR". The universe and all substance within is made of the changing of electric potential which in itself is a potential, a situation. That caused-situation then causes the next changing of the situation, the next potential. And that endless process is the entirely of the physical universe, somewhat randomly dispersed.
So the bottom line is that an electric potential is merely a measurement of the unevenness of the situation of surrounding potentials. And to increase a potential is to make the situation more uneven, increasing the changes of potential in one location while decreasing the potential situation in another location; "increasing the gravity beneath the apple, while further weakening the stem".
The universe has no more to offer than that and actually needs no more than that for all physical existence to be what it is.
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While the last two responses were physically correct, an even simpler answer to your question would be:
Voltage is the potential or maybe more understandable, the pressure behind the flow of electrons. So in very simple terms, the voltage is the force or pressure that pushes the electrons thru the wire. Amperage is the current or the volume of electrons that pass thru the wire per second.
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a volt is the measurement of the electrical potential, it defines the strength of the electric field of an object. (the amount of electron difference from neutral) if something is negatively charged that means that there is an an overabundance of electrons on an object from neutral. If something is positively charged that means there is a lack of electrons from neutral.
The electric field defines the electric attraction of electrons from object to object. If you put a positive charge next to a negative charge you might trigger a discharge, that discharge has am "amperage"
an amp is the measurement of movement of electrons (the flow of electrons from negative to positive) the more amps you have the faster the electrons are flowing.
volt defines the amount of electrons and the strength of the electric field
amp defines the movement of electrons and the magnetic field.
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Alas no, SS. The "amount of electrons" is charge, not voltage.
The strength of an electric field is measured in volts per meter, not volts.
An amp is the rate of flow of charge past a point. Nothing to do with the speed of electrons, which is determined by the potential gradient.
Magnetic field is measured in tesla or amps per meter, not amps.
So to return to the original question, the potential at a point is the amount of energy you need to expend to bring a charge up to that point. If you have to expend one joule to move one coulomb of charge from A to B, the potential difference beween A and B is one volt. A coulomb is the charge of 6.24151×1018 protons (not electrons - classical electromagnetic theory deals with positive charges). The utility of this definition is that if you have a capacitor holding one coulomb of charge, say, at 1000V, you can get 1000 joules of work out of it.
And just to make it interesting, the definition of positive and negative still goes back to early history: "if you rub an ebonite rod with cat fur, the rod acquires a positive charge and the fur a negative charge"
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Alas no, SS. The "amount of electrons" is charge, not voltage.
The strength of an electric field is measured in volts per meter, not volts.
An amp is the rate of flow of charge past a point. Nothing to do with the speed of electrons, which is determined by the potential gradient.
Magnetic field is measured in tesla or amps per meter, not amps.
Perhaps if I go at this from another angle. Someone told me to make more 2 volts, wire 2 1 volt batteries together. Someone else told me, in a galvanic cell, it is the speed of the chemical reaction that increases voltage. Or, use a bigger magnetic field at your generator - answers that I all assume are correct. Maybe I have to involve a battery designer. Someone told me it is the total kinetic energy of the electrons. My question, is how to I increase that energy? I would think a battery designer or generator designer would know. The ontology answer above was interesting, but then said energy is analogous to a situation, such an apple hanging from a tree. Well..*what* is the *situation* with voltage? The strength of an electric field seems to be volts per meter. How do I increase an electric field? There are many ways? They reduce to one basic concept? They reduce to a handful of concepts?
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Let's try to break this down a little bit.
Electrons are negatively charged and atomic nuclei are positively charged, therefore they are attracted to each other.
The simplest atom contains one electron and one proton. When the atom is in its lowest lying energy state the electric field attracting the electron and proton is about 13.6 volts. That is it takes 13.6 electron-volts (eV) of energy to remove the electron from the influence of the proton (it sounds like you are already familiar with the notion that charge times voltage is energy, so one electron-volt is one volt times the charge of an electron). Energy, in the form light or heat, can excite the hydrogen atom, such that the electron is farther away from the proton (or has more kinetic energy, however you want to think of it). Starting from an atom in this state, a smaller voltage is required to remove the electron.
To consider other elements we can think about how much energy is required to remove an electron from a neutral atom (it's easier to think about energy and then divide by the electron's charge). This is called the ionization energy, and these are known and reported for almost all of the elements (for instance, see https://dept.astro.lsa.umich.edu/~cowley/ionen.htm).
For instance to remove an electron from a lithium atom takes 5.4 eV. Adding an electron to a lithium ion releases 5.4 eV. Removing an electron from a silver atom take 7.6 eV (and adding one to a silver ion releases 7.6 eV).
Now, one step more complicated. Rather than dealing with single atoms or ions releasing or capturing free electrons, we can think about the reaction of one ion with one neutral atom. The positively charged ion can steal an electron from the neutral atom if there is a large enough voltage. A silver ion will steal an electron from a lithium atom to produce a silver atom and a lithium ion, releasing 2.2 eV (the potential or voltage between the two is 2.2 V). This requires that the atom and ion come close enough to each other to transfer the electron (very close, we can call it "in contact")
Note that the potential between two atoms of the same element is 0. This allows electrons to move from one atom to another without coming into contact, as long as they are in contact with atoms in between that can carry the electron from one atom to the ion (wires). Imagine a silver wire connecting a piece of lithium metal (neutral) to a soltion containing silver ions. A silver ion can bump into the silver wire and steal an electron from a silver atom in the wire. This silver atom steal an electron from the next atom in the wire, which steals from the next, etc. etc. etc. until the silver wire can steal an electron from the lithium. This is what goes on in a battery.
A battery is just two materials with different ionization energies. One which attracts electrons and doesn't have them, and one that attracts less strongly but does have them. The difference in their ionization potentials is the difference in the potential that the battery can create.
Of course, this is somewhat of a simplification, but I hope it helps your understanding.
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Physically it's just pressure resulting from electrons repelling each other and trying to stay apart - pressure difference across an electronic component is measured in volts. Here's a page which provides a clear introduction to the idea without diving straight into jargon about potential: http://www.magicschoolbook.com/science/p-science11.html (http://www.magicschoolbook.com/science/p-science11.html)
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let's say I have a one volt button battery.
A pedantic clarification: The common 1.2 volt size AA chemical power source is technically called a "cell (http://en.wikipedia.org/wiki/Electrochemical_cell)".
The chemical reaction whose voltage was described by chiralSPO occurs in one cell.
When we see the common 9 volt or 12 volt chemical power source, this is properly called a "battery", which is two or more cells connected together in series. This produces a voltage which is a multiple of the voltage produced by a single cell.
As mentioned by RandomPlank, you can take two 1V cells in series to make a 2V battery (but this does not make a 2V cell).
Some early cells looked like open tubes (like a mortar), and when you had many of them connected together to produce a higher voltage, I imagine that it looked like an artillery battery. I presume this is where the English term "battery" came from.
Another form of early battery looked like a stack of metal plates with liquid-soaked sheets between them (each pair of metal plates represents one cell). This is possibly the source of another old term for battery, a "pile".
But since all these messy construction details are today hidden from sight behind a glossy label, common usage calls both cells and batteries "a battery".
let's say I have a one volt button [cell]. What do I do inside of it to make it a 2 volt button [cell]?
An answer to this part of the question was embedded in the answer from chiralSPO:
Make your button cell out of silver and lithium, and a single cell will produce around 2.2V instead of the more common 1.2V of the carbon-zinc cell.
There are many combinations of elements that could produce a 2V cell, but you need to consider things like cost, toxicity, corrosiveness, flammability, storage life, internal resistance, self-discharge, tendency to spill and other practical problems (some battery chemistries have a tendency to produce hydrogen gas and explode).
Some common cell types are listed here (http://en.wikipedia.org/wiki/Battery_(electricity)#Battery_chemistry), with the voltages they produce.
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I've asked around about this and am not satisfied with the answers I've been getting so far.
I’ve decided to go back to square one and look at the other forum where you discussed this. You started that thread off with the question What exactly creates voltage in a physics sense? If it is EMF - what is EMF made of?
In that thread you asked about voltage but in this thread you’re asking about What is a volt clarifying by saying I don't mean joule per coulomb, or it's ability to provide EMF, or it's the potential difference or any of those kind of definitions. This is a serious problem since those are two different questions. The volt is merely the unit of electric potential. Voltage is the measure of difference in electric potential in units of volts. An EMF, also measured in volts, is a source of power.
I'm looking for what it is *physically*. In other words, I can increase gravity by adding mass, I can increase water pressure by squeezing it, but exactly IS this potential difference in charge?
Definition of Volt: The unit of electric potential V where V is defined such that E = -grad V
See - http://en.wikipedia.org/wiki/Volt
Definition of Voltage: A difference in electric potential between two points
See - http://en.wikipedia.org/wiki/Voltage
Definition of Electromotive Force (EMF): The voltage developed by a source of energy such as a battery of a motional EMF. Physically an EMF is the non-electrostatic power delivered to a circuit per unit current.
See - http://en.wikipedia.org/wiki/Electromotive_force
I believe that this is the best that can be done to answer all of your questions. If not then let me know and I’ll try to do better.
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how to I increase that energy?
Another pedantic clarification: In the context, I think you mean "How do I increase that voltage?", which is described below for electrical generators.
However, when you have a doubled voltage, this will cause the current through an external resistive circuit to double, by Ohm's law (http://en.wikipedia.org/wiki/Ohm%27s_law).
The power in the load will increase by a factor of 4, since the voltage has doubled, and the current has doubled.
This means that the energy that originally would be delivered into the load in 4 seconds will now be delivered in 1 second (assuming something does not get overloaded and blow a fuse).
use a bigger magnetic field at your generator. How do I increase that [voltage]?
Inside an electrical generator, there are wires moving in a magnetic field. These wires tend to accumulate an excess of electrons at one end of the wire, which leaves a deficit of electrons at the other end of the wire. This creates a voltage which can be used to to produce useful work like turning on a light if the circuit is completed by a wire outside that magnetic field.
For an electron in a wire, you can write F=v x B
- Where F is the Force, moving the electron towards one end of the wire (which then produces a voltage)
- v is the velocity of the particle relative to the magnetic field
- B is the strength of the magnetic field
- x represents vector multiplication; this is similar to the multiplication we learnt in school, only it takes into account the direction and angle of the velocity and magnetic field
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So it is quite correct to say that you could double the voltage by:
- Doubling the strength of the magnetic field B
- Doubling the velocity of the particle relative to the magnetic field (eg by pouring more steam into the turbine, making it spin twice as fast)
- Changing the angle of the wire relative to the magnetic field, so it is better aligned for maximum voltage. (In AC generators, this angle changes cyclically, producing the alternating voltage which eventually appears at our wall sockets).
See http://en.wikipedia.org/wiki/Magnetic_field#Definition
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Physically it's just pressure resulting from electrons repelling each other and trying to stay apart - pressure difference across an electronic component is measured in volts. Here's a page which provides a clear introduction to the idea without diving straight into jargon about potential: http://www.magicschoolbook.com/science/p-science11.html (http://www.magicschoolbook.com/science/p-science11.html)
OK - First off, I really appreciate all these very interesting answers from all you erudite folks. This rather simple explanation, is I think what I was going for. From the article above - "You now understand what that means: it's just a flow of electrons from a high pressure area (with more electrons in it than the average) to a low pressure area (with fewer electrons than the average). If this pressure difference is small, then you have a small voltage and the electrons will drift gently across from the high-pressure zone to the low-pressure zone, but if the difference is great, you have a high voltage and the electrons will rip across into the low-pressure zone at high speed. "
Is that a legit way of thinking about it? An area with more electrons than average? So the pressure analogy to water, is close to reality? And a followup, let's say I have the same number of electrons, but they are accelerated, such as in a Hydrogen Flouride laser? That's more voltage - faster electons = more kinetic energy? Is this correct? As to what I am really asking, is a more rigorous definition of the above. What is this "pressure".. charge flux? I'm not sure.. but getting closer. Sorry if I am beating a dead horse.
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Actually folks - I think I am good. The recent posts by evan_au and PmbPhy clear it up pretty much. It really is "pressure", correct? In other words, electrons trying to stay apart with a lot of PUSH force, or more kinetic energy. Does that about sum it up?
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If you're thinking about this from the perspective of building circuits the pressure analogy is quite reasonable. On smaller scales we have to add some caveats, but it is still an ok descriptor.
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If you're thinking about this from the perspective of building circuits the pressure analogy is quite reasonable. On smaller scales we have to add some caveats, but it is still an ok descriptor.
Those caveats might be what I've been trying to get at. Any way you can elaborate? If just a bit?
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To be more precise, voltage is not directly analogous to a pressure, since pressure depends on force per unit area, whereas voltage is in units of energy/charge and does not depend on area. This is why we can say a battery is measured at 12 V independent of the thickness of the wires we hook up to it. Whereas if we were measuring pressure, the pressure would increase as we used thinner and thinner wires. If we fix the diameter of all wires, we could make the analogy, since both energy and pressure depend on force, but there's a more straightforward explanation for voltage that gets at its ties to energy:
The value of voltage at any point isn't useful, but the difference in voltage between two points is useful. It tells you how much energy is gained (or lost) by an electron in traveling between those two points*. Why do we care about energy? Because it's the currency we use to power light bulbs, motors, etc. So if we know the energy gained by an electron in traveling from on terminal of a battery to another, we can design a circuit to use this energy to do something useful. We also, of course, need to know the number of electrons flowing, which is generally characterized by current, since the total energy depends on the energy per electron times the number of electrons.
So what is the closest analogy to water? Imagine we have a dam in which water flows from a high point to a low point. Obviously it gains speed as it flows downhill, and this gain in speed is a gain in energy. We can use this energy to power a turbine (and the water obviously flows more slowly after turning the turbine since we've extracted some energy). The amount of energy gain per unit water (gram of water or per water molecule) would be akin to voltage. Obviously if we send it down a pipe, we can change the pressure at any point by changing the pipe diameter, but the energy gain per unit water does NOT depend on pipe diameter.
* Strictly speaking, as others have said, voltage difference between two points is the work done per unit charge in moving between those two points. Work in the sense of physics means a change in (mechanical) energy, so work tells you how much energy you must put in or that you gain from doing something. So what voltage difference tells you is the energy per unit charge that you have to put in to move a charge between two points or how much you gain from a charge moving between two points.
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It isn't pressure, but pressure difference. You can remove lots of electrons from a circuit and it will still function fine using the ones that are left, the same pressure differences being generated as before but with a lower pressure at any given point. For this reason, you can't tell what the voltage is just by measuring the electron pressure at one point in a circuit, but you must measure it at two points and see what the difference is (in terms of proportionality, not absolute value).
I'm not convinced that the pressure is higher in thinner wires than fat ones, JP. Are you sure you've got that right? If you picture the -ve end of a battery being connected to a wire, the battery will pump electrons out into the wire until it can't force any more into it, at which point the electron pressure pushing back into it will equal the force from the battery and everything will stop (there's no circuit at this point). If you take that wire away and replace it with a thicker (or thinner) one, the same will happen with electrons being pushed into it until the pressure reaches a level that stops the battery pumping any more in. That pressure should be same regardless of the thickness of the wire (though to make it so you may need to attach another wire of the same thickness and length to the other end of the battery each time to enable it to pick up enough electrons there, bringing the pressure in that wire down to a lower pressure in the process, the same low pressure each time).
It's a bit more complicated once the circuit is complete and the electrons are flowing, but you couldn't have a thick wire running into a thin wire (thinking in terms of the direction the electrons are flowing) with the pressure going up when it gets into the thin bit as that would stop any electrons flowing any further and block the circuit. I don't think you were suggesting that the pressure could go up in that way, but you appear to be saying that if the complete wiring of the circuit is thinner there will be a higher pressure in it, but the battery only knows how much pressure it is pushing electrons against and not how fat the wire is, so it should always be stopped at the same pressure (well, ignoring the effects of running down).
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It isn't pressure, but pressure difference. You can remove lots of electrons from a circuit and it will still function fine using the ones that are left, the same pressure differences being generated as before but with a lower pressure at any given point. For this reason, you can't tell what the voltage is just by measuring the electron pressure at one point in a circuit, but you must measure it at two points and see what the difference is (in terms of proportionality, not absolute value).
It can't be simply a pressure because the units are wrong. Pressure is force/area and potential (voltage) is force*length/charge. It's a subtle difference, but an important one to really understand what potential is describing. Pressure can definitely be a useful analogy, but it can also be confusing, particularly when one starts using potential to talk about things other than circuits (such as using it to prescribe an electric field over a region of space).
I'm not convinced that the pressure is higher in thinner wires than fat ones, JP. Are you sure you've got that right? If you picture the -ve end of a battery being connected to a wire, the battery will pump electrons out into the wire until it can't force any more into it, at which point the electron pressure pushing back into it will equal the force from the battery and everything will stop (there's no circuit at this point). If you take that wire away and replace it with a thicker (or thinner) one, the same will happen with electrons being pushed into it until the pressure reaches a level that stops the battery pumping any more in. That pressure should be same regardless of the thickness of the wire (though to make it so you may need to attach another wire of the same thickness and length to the other end of the battery each time to enable it to pick up enough electrons there, bringing the pressure in that wire down to a lower pressure in the process, the same low pressure each time).
You're right in this one. Having thought about it a bit more, I was thinking of a constant force, not a constant voltage. The fact that voltage scales per unit charge and charge distributes over the wire keeps pressure constant.
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Could be current is partial wave partial particle? Particles cannot move at C speed anywhere.
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If you're thinking about this from the perspective of building circuits the pressure analogy is quite reasonable. On smaller scales we have to add some caveats, but it is still an ok descriptor.
Those caveats might be what I've been trying to get at. Any way you can elaborate? If just a bit?
Ultimately everything boils down to the attraction between negatively charged electrons and positively charged atomic nuclei. When atoms are isolated the electronic structure is fairly straightforward, where electrons populate different energy levels (orbitals) depending on their kinetic energy and angular momentum. All of these orbitals are centered at the nucleus, and are essentially isotropic (spherically distributed).
When atoms come together to form materials, the orbitals of each individual atom mix with each other to various extents, and form the electronic structure of the material. This determines whether a material is a good conductor, semiconductor, insulator etc.
On a large scale, people consider wires to be more or less equal, and measure voltages as the difference of potential between any two components of a circuit. But if we look closely at the electronic structure of any of the materials in a circuit differences become apparent. For instance, if one took two electrically neutral semiconductors, one n-type and one p-type, and brought them into electrical contact, there would actually be a brief spike in current between the two as electrons move from the n-type to the p-type, resulting in a slightly negatively charged p-type semiconductor and slightly positively charged n-type semiconductor. Apparently there was a voltage difference between these two neutral materials. This is because of the differences in electronic energy levels inherent to each material.
Let me restate this: A voltage can exist between 2 materials that are both electrically neutral. And this has to do with the ability of different elements to attract electrons. There are also inherent differences between say, copper wires and aluminum wires, even though they are both similarly great conductors of electricity.
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It can't be simply a pressure because the units are wrong. Pressure is force/area and potential (voltage) is force*length/charge. It's a subtle difference, but an important one to really understand what potential is describing. Pressure can definitely be a useful analogy, but it can also be confusing, particularly when one starts using potential to talk about things other than circuits (such as using it to prescribe an electric field over a region of space).
Yes, you're right about it only being an analogy because it isn't technically pressure and the maths of it is different. If you think about what's going on in cause-and-effect terms, when you stick a whole lot of electrons into a capacitor it gets harder and harder as you put more in, so that feels like pressure, but it isn't the same kind of pressure as you have with a gas. In a gas you have molecules bouncing off each other and the pressure changes with temperature as the molecules move at different speeds, but that's quite different from stationary electrons repelling each other, and the degree to which they repel each other as you reduce the number of electrons in a given amount of space will, I imagine, change by the inverse square law as they sit further apart, whereas with molecules of a gas they will apply a force proportional to the number of molecules in that space. (Please correct me if I've got that wrong.) It seems fair to use the word "pressure" to describe both things, but if it's being used in a technical sense within physics it should not be used to describe voltage. This is one of those cases where a word is more than one word - there are two words "pressure" with one of them having a very precise meaning while the other in ordinary language is much more adaptable.
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In a gas you have molecules bouncing off each other and the pressure changes with temperature as the molecules move at different speeds, but that's quite different from stationary electrons repelling each other,
Where is stationary electron? If stationary electrons repelling each other, why don't they attracting nucleus? Isn't electrons orbiting and can't be located? Uncertainty principle?
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Someone else told me, in a galvanic cell, it is the speed of the chemical reaction that increases voltage.
alas, not true. The voltage of a galvanic cell depends on the difference in electronegativity between the plates. Hence all the headlines about "run your radio from a potato/lemon/wet diaper" are nonsense: the energy comes from the copper and zinc plates, not the potato!
Or, use a bigger magnetic field at your generator - answers that I all assume are correct.
Kind of, but not quite. You can increase the strength B of the primary magnet, or increase the number of turns n of the pickup coil, or the speed of rotation. The induced votage depends on n dB/dt
Someone told me it is the total kinetic energy of the electrons.
In a vacuum tube (like an x-ray tube) the kinetic energy of the electrons hitting the anode equals the charge multiplied by the anode voltage. But the anode voltage is there even if you switch off the electron flow.
My question, is how to I increase that energy? I would think a battery designer or generator designer would know.
see above. More cells in series, different cell electrodes, faster or bigger generator.
The strength of an electric field seems to be volts per meter. How do I increase an electric field? There are many ways? They reduce to one basic concept? They reduce to a handful of concepts?
Increase the voltage or decrease the distance between the electrodes. One concept!
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In a gas you have molecules bouncing off each other and the pressure changes with temperature as the molecules move at different speeds, but that's quite different from stationary electrons repelling each other,
Where is stationary electron? If stationary electrons repelling each other, why don't they attracting nucleus? Isn't electrons orbiting and can't be located? Uncertainty principle?
Please edit your post to tie the quotes to the right people. (Edit: It's a bug in the system which just did the same with this post. The cure is to add an extra /quote in square brackets before you type your reply.)
"Relatively stationary electrons" was the intended meaning, though it may be wrong too. I'm guessing that if you chuck lots of electrons into a capacitor, they find an arrangement where they are collectively as far apart from each other as they can get and they just stay more or less in those positions. The real story may be more interesting though.
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In theory, the distance between electron and electron should be greater than the distance between electron and nucleus. Based on the charges they carry.
The real story is truly interesting. Seems so far we are still guessing?
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No, we're not guessing.
We're discussing classical electrostatics here, which doesn't include quantum effects. So long as you don't need precision on the atomic scale, and for most circuits (and for all classical circuits) you don't. A stationary electron is one which we pretend is perfectly still and located at a point because including corrections for the uncertainty principle would change our answer by such a tiny amount that it wouldn't matter to the problem we're considering.
Of course, if the problem is to model an atom, this quantum effect does matter, which is why we need a non-classical model of the electron in that case.
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Current, voltage, potential all happen between atoms, surely is quantum level.
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Current, voltage, potential all happen between atoms, surely is quantum level.
Sure, but it also happens on the classical level, and that's the topic of this thread.
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Did you mean physical level is classic level?
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Did you mean physical level is classic level?
Classical physics is any model where we can ignore quantum effects--typically because objects or distances are big enough that properly accounting for quantum effects won't change the answer. The discussion here has been about classical physics.
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Another way of physically visualising the volt is in a capacitor.
A classic capacitor (http://en.wikipedia.org/wiki/Capacitor#Overview) has two flat conductive plates (electrodes), with a dielectric (insulator) in-between. The insulator may be vacuum, air, plastic, glass etc.
The positive electrode has a deficit of electrons, and the negative electrode has a surplus. The proximity of the two plates means that the electrons on the negative electrode are attracted to the positive charge on the nearby plate, and the electrons tend to cluster on the nearest side. This means that you have to do less work to pump even more electrons into the capacitor.
The Voltage is represented as V=Q/C.
- Where V is the voltage
- Q is the charge on the capacitor, in Coulombs (1 Coulomb represents 6.24x1018 electrons)
- C is the capacitance, in Farads
To double the voltage, you could:
- Double the number of electrons on the capacitor
- Replace the dielectric by another one with half of the permittivity (but you can't get a permittivity lower than a vacuum)
- Double the separation of the plates
- Halve the area of the plates (but you can't do this by just cutting the capacitor in half, as each half will end up with only half of the electrons)
See: http://en.wikipedia.org/wiki/Capacitance#Capacitors
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Did you mean physical level is classic level?
Classical physics is any model where we can ignore quantum effects--typically because objects or distances are big enough that properly accounting for quantum effects won't change the answer. The discussion here has been about classical physics.
Classical physics? Same charges repel, opposite charges attract?
Voltage is caused by electron pressure difference. Electrons always flow to the lowest pressure position. Where is that position?
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When electrons flowing in conductors, its speed is very small, far smaller than the speed of current. This suggests current is EM field movement, electrons pushed by the field movement gaining momentum.
So voltage might relate to EM field strength?
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The voltage elephant is pretty spooky, this thread is the proof.
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A classic capacitor has two flat conductive plates (electrodes), with a dielectric (insulator) in-between. The insulator may be vacuum, air, plastic, glass etc.
There are many different kinds of capacitors, one not really deserving the name classic over the other. The cylindrical capacitor is another example of a capacitor and is more popular. It can be looked at as a flat capacitor whose plates each form a cylinder. See http://hyperphysics.phy-astr.gsu.edu/hbase/electric/capcyl.html
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When electrons flowing in conductors, its speed is very small, far smaller than the speed of current.
When you run a very high current (almost enough to melt a copper wire), the electrons are moving very slowly - only 1 or 2 mm per second.
However, when you turn on a light switch, the light turns on "immediately" - the electrons do not propagate from the switch to the light globe at 1 or 2 mm per second before it turns on. The wire is already full of electrons, but the movement of 1 electron can affect nearby electrons, and so the voltage propagates down the wire at a velocity which is roughly 2/3 of the speed of light (the exact speed depends on the geometry of the wire).
So the voltage and current (aggregate behaviours of many electrons in the wire) propagates down the wire much faster than the speed of the individual electrons.
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When electrons flowing in conductors, its speed is very small, far smaller than the speed of current.
There's no such thing as speed of current. There's only speed of charge carriers. That's all.
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When electrons flowing in conductors, its speed is very small, far smaller than the speed of current.
There's no such thing as speed of current. There's only speed of charge carriers. That's all.
What is the charge carriers in a live wire? Electrons? What's electrons speed in the wire? 1mm per second?
That's not all, experts are still working hard while we are talking easy.
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When electrons flowing in conductors, its speed is very small, far smaller than the speed of current.
When you run a very high current (almost enough to melt a copper wire), the electrons are moving very slowly - only 1 or 2 mm per second.
However, when you turn on a light switch, the light turns on "immediately" - the electrons do not propagate from the switch to the light globe at 1 or 2 mm per second before it turns on. The wire is already full of electrons, but the movement of 1 electron can affect nearby electrons, and so the voltage propagates down the wire at a velocity which is roughly 2/3 of the speed of light (the exact speed depends on the geometry of the wire).
So the voltage and current (aggregate behaviours of many electrons in the wire) propagates down the wire much faster than the speed of the individual electrons.
Alas! What's new? What's your point?
Is volt electron pressure? Or pressure difference? Or EM field strength?
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The voltage elephant is pretty spooky, this thread is the proof.
On the contrary, it's very straightforward as long as you don't look for misleading analogies. And don't restrict your thinking to electrons: many common materials have positive charge carriers, and proton accelerators behave in exactly the same way as electron accelerators with the signs reversed. Just think "unit charge" and "energy".
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The voltage elephant is pretty spooky, this thread is the proof.
On the contrary, it's very straightforward as long as you don't look for misleading analogies. And don't restrict your thinking to electrons: many common materials have positive charge carriers, and proton accelerators behave in exactly the same way as electron accelerators with the signs reversed. Just think "unit charge" and "energy".
Unit charge is electron and proton, what is energy? Charge potential? 1/2mv^2? How's energy or charge relate to volt? Look 2 pages of discussing, what's the correct analogy?
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If 2 points of a circuit are joined by a high resistance wire, we can measure the power by the heat generated in the wire in a given time, and the current by the deflection of a magnetic needle.
Potential measured in volts, is the limit of the ratio power / current when the current tends to zero.
Normally we are taught that power = P = V x I. I think it is more correct to take V by definition as the limit of the ratio between P and I, when I -> 0. Both P and I have physical meaning.
This ratio, in my opinion, has some practical applications, (thickness of wires for example) but not exactly a physical meaning.
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This ratio, in my opinion, has some practical applications, (thickness of wires for example) but not exactly a physical meaning.
Wrong. If you multiply power P by time, and I by time on both sides of equation you will get:
E = Q*U
Energy (carried by all electrons) is Charge multiplied by Voltage.
You can clearly see it even from dimension analyze. Power is in Watt which is Joules/seconds, multiplied by seconds will give Joules. 1 kWh = 1000 Wh = 1000 W * 3600 s = 3.6 MJ = 3,600,000 J (energy unit used by power stations on bills).
Q=I*t
And Q/e - is quantity of electrons.
e = -1.602e-19 C (charge of single electron)
1 C / 1.602e-19 C = 6.242*10^18 electrons
Each of them carrying Kinetic Energy = e*U
Electron in circuit with U=1 V has E.K.= 1 eV or 1.602e-19 J
Which is also:
E.K=1/2*me*v^2
reverse it, and you will receive
v=sqrt(E.K.*2/me) = sqrt( 1.602e-19 * 2 / 9.11*10^-31 ) = 593044 m/s
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UT, did you mean electrons flow in circles with speed 593044 m/s? Thanks!
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UT, did you mean electrons flow in circles with speed 593044 m/s? Thanks!
No.
Electrons flow from negative electrode of battery to positive electrode.. It's not circle (not in traditional meaning of this word). They flow from place where is abundance of them to where is shortage of them.
593044 m/s is velocity of electrons in electronic circuit that has U=1 V. With different voltage they will have different velocity..
Typical battery NiMh has ~1.25 V.
Connect two batteries in series = ~2.5 V.
Connect quad batteries in series = ~5 V.
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If that's true, give 1000 V electron will be flow at 1000x593044 m/s? 2C?
Still pretty confusing, thanks again.
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If that's true, give 1000 V electron will be flow at 1000x593044 m/s? 2C?
No.
v=sqrt(e*U*2/me) = sqrt( 1.602e-19*2*1000 / 9.11*10^-31 ) = 18,753,704 m/s
But the faster it's moving we need to switch to using relativistic equations, instead of using E.K.=1/2*m*v^2...
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Nope, the drift velocity of electrons in typical wires is of the order of half a millimetre per second.
http://en.wikipedia.org/wiki/Drift_velocity
That's superimposed on their thermal velocity which at room temp is about 1700 m/ sec
So, for a longish wire - say 15 metres of 1 mm diameter the resistance will be 0.33 ohms and the current at 1 volt will be about 3 Amps.
That's the same as the worked example on that wiki page and
your calculated velocity for the electrons will be wrong by about a factor of a thousand million (give or take a few zeros)
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Is UT calculating the velocity of an electron in a vacuum, while BC is calculating the velocity in a copper wire (where there are lots of copper atoms, impurities and crystal boundaries to slow down the electrons)?
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Unit charge is electron and proton, what is energy? Charge potential? 1/2mv^2? How's energy or charge relate to volt? Look 2 pages of discussing, what's the correct analogy?
There is no useful analogy. The relationship between energy (measured in joules) and charge (measured in coulombs) is 1 joule per coulomb = 1 volt. That's it. Energy is the ability to do work: one joule will raise the temperature of a gram of water by about 0.24 degrees. One coulomb is the charge of about 6.2 x 10^18 protons.
Measurement of charge is quite difficult but current (charge flowing past a point per unit time) is easy so we generally measure amperes (1 amp = 1 coulomb per second). Now consider an electric kettle running at 240 volts, 5 amps. 240 x 5 = 1200 joules per second so it will heat 1 gram of water at 0.24 x 1200 = 288 degrees/second, or more realistically 1 liter (1 kg) at 0.288 deg/s, say 4 minutes to boiling.
It's important not to get too hooked on to electrons when dealing with current electricity: the current in aluminium and many semiconductors is due to the movement of positively charged "holes" (see the Hall effect for an explanation of how we know this) and in liquids or gases, by the movement of ions with both positive and negative charges. But the definition remains the same: 1 volt = 1 joule per coulomb.
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Unit charge is electron and proton, what is energy? Charge potential? 1/2mv^2? How's energy or charge relate to volt? Look 2 pages of discussing, what's the correct analogy?
There is no useful analogy. The relationship between energy (measured in joules) and charge (measured in coulombs) is 1 joule per coulomb = 1 volt. That's it. Energy is the ability to do work: one joule will raise the temperature of a gram of water by about 0.24 degrees. One coulomb is the charge of about 6.2 x 10^18 protons.
Measurement of charge is quite difficult but current (charge flowing past a point per unit time) is easy so we generally measure amperes (1 amp = 1 coulomb per second). Now consider an electric kettle running at 240 volts, 5 amps. 240 x 5 = 1200 joules per second so it will heat 1 gram of water at 0.24 x 1200 = 288 degrees/second, or more realistically 1 liter (1 kg) at 0.288 deg/s, say 4 minutes to boiling.
It's important not to get too hooked on to electrons when dealing with current electricity: the current in aluminium and many semiconductors is due to the movement of positively charged "holes" (see the Hall effect for an explanation of how we know this) and in liquids or gases, by the movement of ions with both positive and negative charges. But the definition remains the same: 1 volt = 1 joule per coulomb.
Thanks!
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A classic capacitor has two flat conductive plates (electrodes)...
There are many different kinds of capacitors, one not really deserving the name classic over the other.
The international symbol of a fixed capacitor (http://en.wikipedia.org/wiki/Capacitor) shows two flat parallel plates, with a gap between them.
There are many physical implementations of modern capacitors, providing increased capacitance in a smaller space by using multiple parallel flat plates and using various dielectrics like glass, mica, organic plastics or oxide layers, or by rolling the plates and dielectric into a spiral/cylinder.
One material that was undoubtedly tried as a capacitor dielectric at some time was quartz - it has great insulating properties, but it is piezoelectric, which produces some bizarre results if you try to use it in a capacitor. Although most modern capacitors have a dielectric, the symbol which shows capacitor plates with material between the plates is the international symbol for a quartz oscillator (http://en.wikipedia.org/wiki/Crystal_oscillator#Modeling).
So I suggest that the "classic capacitor" comemorated in the international symbol for a fixed capacitor is the air-insulated parallel-plate capacitor, like the historical model shown in the photograph here (http://en.wikipedia.org/wiki/Capacitor#Overview).