What is the limit of Volts possible?


by Nile3
Tags: limit, volts
Nile3
Nile3 is offline
#1
Nov21-10, 10:58 PM
P: 42
What is the limit of volts possible per amp? For whatever conductor, a superconductor let's say.
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yungman
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#2
Nov21-10, 11:07 PM
P: 3,843
I cannot think of there is a limitation. The two are totally different.

Limitation of current through the conductor is limited by the heat produced by the voltage drop across the conductor. The amount of current the conductor can take depend on how well the conductor able to dissipate the heat without melting.

The voltage the conductor can carry OFF ground only depend how well the insulation can hold up and how well the conductor be hang away from other ground reference object. Good example is the HV transmission lines, they use towel to keep it way above ground. They carry many many amps of current at many many KV volts. That's how they transmit a lot of power through relative small wires......to keep the current down and so wire don't have to be too big and high voltage to make up the power.

There must be practical limits, but I don't think there is a theoretical limits particular you mention super conductor.
Nile3
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#3
Nov22-10, 10:46 AM
P: 42
Quote Quote by yungman View Post
Good example is the HV transmission lines, they use towel to keep it way above ground.
I think you meant tower.

Okay well, why not use only 1 amp and 10 megavolt instead of 100 amp + 100kvolts? I mean, there must be a point where adding potential difference just doesn't have anymore effect on increasing current transfer.

LURCH
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#4
Nov22-10, 10:55 AM
Sci Advisor
P: 2,507

What is the limit of Volts possible?


I agree there is no limit, based on the following reasoning;

A well-insulated ellectrode can have a potential (voltage). In this case, the ellectrode can be charged up to any arbitrary voltage with no current flow at all. So, ratio of voltage to current can be infinite.
Nile3
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#5
Nov22-10, 10:59 AM
P: 42
Quote Quote by LURCH View Post
I agree there is no limit, based on the following reasoning;

A well-insulated ellectrode can have a potential (voltage). In this case, the ellectrode can be charged up to any arbitrary voltage with no current flow at all. So, ratio of voltage to current can be infinite.
If there's no current how could there be a voltage to current ratio?
yungman
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#6
Nov22-10, 11:07 AM
P: 3,843
Quote Quote by Nile3 View Post
I think you meant tower.

Okay well, why not use only 1 amp and 10 megavolt instead of 100 amp + 100kvolts? I mean, there must be a point where adding potential difference just doesn't have anymore effect on increasing current transfer.
First, you have to think of current is a specification. That is you choose a wire that can carry the specified current without generating enough heat to cause melting or fire. There is really no 1A and 10MV or 100A and 100KV etc. Wire specified for current only, the insulation specify the standoff voltage. But you can change the voltage by how you hang the wire, like using the tower, ceramic standoff etc. In fact I am not sure the HV transmission lines have any insulation, they just use air gap as insulation!!!

No there is no theoretical limit relation between current and voltage. Practically, yes. There is a limit on how much power a generator can generate. Let's just say, if you can generate the power ( I times V ), I can give you the wire(s) to carry it out!!!

As I said, the limitation is on the power capability and the standoff voltage which you can design to overcome. The two are totally separated. No if and buts about this. Wires talk about VA, that is mainly power specifications only, remember VA is power. They have to specify VA at....say 110V or 220 volt. Without the specified voltage, VA don't mean anything.

If you take a regular house hold 3 wire cable. You hook up a 10 KV little power supply with only 100 micro amp capability across two of the wires, there is a good chance the cable will burn because of the insulation break down and punch through.
rcgldr
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#7
Nov22-10, 04:02 PM
HW Helper
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In the real world, the voltage limit is when the voltage ionizes the air which then acts as a conductor. For a spherical conductor in air:

Voltmax = radius (meters) x 3 x 106 (newtons / coulomb)
yungman
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#8
Nov22-10, 04:28 PM
P: 3,843
Quote Quote by rcgldr View Post
In the real world, the voltage limit is when the voltage ionizes the air which then acts as a conductor. For a spherical conductor in air:

Voltmax = radius (meters) x 3 x 106 (newtons / coulomb)
And if there is any moisture, the number will drop.

He is asking about hypathatical question, so hypathatically, you can put insulation around conductor and in general design, say using teflon etc, you get min of 400V per mil ( very conservative ) and you can put as thick as you want and it will stand up to as high a voltage as you want. Just need a real workable number and go from there.

Bare transmission line on tower is just a cheap way to transport power using air insulation, you can use insulated lines, it's just money!!!

That's why the wire is not the gating factor, it's the other factor that usually are the limiting factor. Far as the wire go, only thing to worry is the current which can generate heat due to resistance of the wire that cause voltage drop and IV is the power. OP mentioned about super conductor, then theoratically there is no limitation because resistance is zero.
Nile3
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#9
Nov25-10, 01:43 PM
P: 42
Well I must not understand how electricity works properly then. I was certain that there ought to be a limit in voltage per amp for any conductor in a vaccum...

Let's say you have planet A which is completely positive and planet B which is negatively charged. A multiple kilometer long line of pure diamond is attached between them. The line is only 5 mm in diameter. How many volt for each amps can the current be made of at the maximum?
cjameshuff
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#10
Nov25-10, 02:15 PM
P: 210
Quote Quote by Nile3 View Post
Well I must not understand how electricity works properly then. I was certain that there ought to be a limit in voltage per amp for any conductor in a vaccum...
There is a limit in current carrying capacity before the conductor melts due to resistive heating, and for a non-superconductor, the voltage dropped across the conductor is proportional to the current through it. It's not very useful to consider voltage to be the limiting factor though.


Quote Quote by Nile3 View Post
Let's say you have planet A which is completely positive and planet B which is negatively charged. A multiple kilometer long line of pure diamond is attached between them. The line is only 5 mm in diameter. How many volt for each amps can the current be made of at the maximum?
Diamond's not a conductor, so amps will be zero and no ratio between them can be defined. The diameter and even existence of the line is irrelevant, the voltage is determined entirely by the charge of the planets and distance between them. The voltage can be arbitrarily high, limited only by the material and proximity of the charged objects to each other.
Nile3
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#11
Nov25-10, 02:31 PM
P: 42
Very clear explanation, thank you. I made a mistake with thinking about diamond, reason why is it's a good conductor for heat not electricity :p

Just a last question, if the planets charge are very high, why would distance affect their voltage? I mean electricity cannot propagate through vaccum, so there's no difference if they are 1 km or 100 appart correct?
cjameshuff
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#12
Nov25-10, 05:06 PM
P: 210
Quote Quote by Nile3 View Post
Just a last question, if the planets charge are very high, why would distance affect their voltage? I mean electricity cannot propagate through vaccum, so there's no difference if they are 1 km or 100 appart correct?
Electrical fields and charged particles have no difficulty whatsoever traveling through vacuum. The reason vacuum acts as an insulator for most purposes is that it requires work to force electrons off the surface of a conductor, how much work depends on the material and its temperature...this is why most vacuum tubes use heated cathodes coated with alkaline earth oxides. Propagation of fields through the vacuum doesn't have this limitation...a charged body in vacuum will be surrounded by an electrostatic field.

Voltage (at least when changing magnetic fields aren't involved) is potential energy difference per charge. If the two planets are moved further apart while having the same charge, an electron or ion released from one has further to fall through the electrical field between them before it meets the other. The potential energy is greater, the charges are the same, so the voltage is greater. Condenser microphones rely on this...a constant charge is maintained on the microphone diaphragm, and changes in distance between it and the other plate cause changes in voltage.
gerben
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#13
Nov25-10, 06:09 PM
P: 540
Upper limit:
Ohm's law, R=V/I, shows that the number of "volts per amp" can be arbitrarily high if you can get the resistance between two spots at different electrical potential arbitrarily high.

Lower limit:
In a superconductor you have R=0 so Ohm's law, R=V/I, shows the lower limit is zero.


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