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Author Topic: What is a volt - on a physical level?  (Read 15585 times)

Offline JP

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Re: What is a volt - on a physical level?
« Reply #25 on: 01/08/2014 18:06:51 »
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.
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #26 on: 01/08/2014 18:15:58 »
Current, voltage, potential all happen between atoms, surely is quantum level.
 

Offline JP

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Re: What is a volt - on a physical level?
« Reply #27 on: 01/08/2014 18:27:31 »
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.
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #28 on: 01/08/2014 18:49:13 »
Did you mean physical level is classic level?
 

Offline JP

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Re: What is a volt - on a physical level?
« Reply #29 on: 01/08/2014 20:44:29 »
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.
 

Offline evan_au

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Re: What is a volt - on a physical level?
« Reply #30 on: 01/08/2014 23:26:20 »
Another way of physically visualising the volt is in a capacitor.

A classic capacitor 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
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #31 on: 02/08/2014 01:19:02 »
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?
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #32 on: 02/08/2014 02:02:48 »
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?

 
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #33 on: 02/08/2014 04:38:59 »
The voltage elephant is pretty spooky, this thread is the proof.
 

Offline PmbPhy

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Re: What is a volt - on a physical level?
« Reply #34 on: 02/08/2014 05:34:54 »
Quote from: evan_au
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
 

Offline evan_au

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Re: What is a volt - on a physical level?
« Reply #35 on: 02/08/2014 07:26:50 »
Quote
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.
« Last Edit: 02/08/2014 07:28:50 by evan_au »
 

Offline PmbPhy

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Re: What is a volt - on a physical level?
« Reply #36 on: 02/08/2014 07:37:22 »
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.
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #37 on: 02/08/2014 08:28:30 »
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.
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #38 on: 02/08/2014 08:36:37 »
Quote
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? 
« Last Edit: 02/08/2014 09:05:17 by jccc »
 

Offline alancalverd

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Re: What is a volt - on a physical level?
« Reply #39 on: 02/08/2014 13:50:21 »
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".
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #40 on: 02/08/2014 15:33:59 »
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?
 

Offline saspinski

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Re: What is a volt - on a physical level?
« Reply #41 on: 02/08/2014 22:12:38 »
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.
 

Offline UltimateTheory

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Re: What is a volt - on a physical level?
« Reply #42 on: 03/08/2014 00:30:34 »
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
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #43 on: 03/08/2014 00:52:05 »
UT, did you mean electrons flow in circles with speed 593044 m/s?  Thanks!
« Last Edit: 03/08/2014 00:53:48 by jccc »
 

Offline UltimateTheory

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Re: What is a volt - on a physical level?
« Reply #44 on: 03/08/2014 01:04:12 »
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.
« Last Edit: 03/08/2014 01:33:25 by UltimateTheory »
 

Offline jccc

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Re: What is a volt - on a physical level?
« Reply #45 on: 03/08/2014 01:33:55 »
If that's true, give 1000 V electron will be flow at 1000x593044 m/s? 2C?

Still pretty confusing, thanks again.
« Last Edit: 03/08/2014 02:07:12 by jccc »
 

Offline UltimateTheory

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Re: What is a volt - on a physical level?
« Reply #46 on: 03/08/2014 01:41:55 »
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...
 

Offline Bored chemist

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Re: What is a volt - on a physical level?
« Reply #47 on: 03/08/2014 10:19:58 »
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)
 

Offline evan_au

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Re: What is a volt - on a physical level?
« Reply #48 on: 03/08/2014 22:40:45 »
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)?
 

Offline alancalverd

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Re: What is a volt - on a physical level?
« Reply #49 on: 03/08/2014 23:18:54 »

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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Re: What is a volt - on a physical level?
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