What is a volt - on a physical level?

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Offline jccc

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Re: What is a volt - on a physical level?
« Reply #50 on: 04/08/2014 15:52:09 »

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.



Offline evan_au

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Re: What is a volt - on a physical level?
« Reply #51 on: 24/08/2014 12:51:27 »
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 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.

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.