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I know power is measured in the terms of watts, current is measured in amps, so is there an X for voltage?

Passerby
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skyler
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    Yes, "voltage". – Kaz Mar 29 '13 at 01:41
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    The unit of voltage is "volt". – Alfred Centauri Mar 29 '13 at 01:50
  • ... current is measured in amps and voltage is measured in volts. X for volts is voltage. Question doesn't really make any sense. – Leon Heller Mar 29 '13 at 04:59
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    In the title you ask for the quantity of volts, but in the question you ask for the unit of voltage. This question answers itself, _and_ could be easily [looked](http://bit.ly/10XSYwq) [up](http://bit.ly/10XSZAF). –  Mar 29 '13 at 06:48
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    In some languages and in older English use you may see that voltage is often used to measure electrical tension. One term that still sees some use in English is high-tension lines for high voltage lines. – AndrejaKo Mar 29 '13 at 15:32
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    Voltage measures potential. I can't believe all the long answers below that don't mention that. – Imbue Mar 29 '13 at 18:42

5 Answers5

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Voltage is the difference in energy between two points in an electric field, expressed per unit of charge. A Volt is a Joule per Coulomb: \$V = J/C\$. The voltage between two points tells us how much energy each electron will gain or lose when it moves between those two points.

The separation of opposite charges stores energy. If we separate charges such that there is one volt between them, that represents less energy than if we separate the same charges such that there are two volts between them. And of course we store more energy by separating more charges at the same voltage.

This is why power is related to both current and voltage.

Kaz
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  • power is measured in watts
  • current is measured in amps
  • electric potential is measured in volts
RedGrittyBrick
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\$1V=\dfrac{1J}{C}\$

\$1A=\dfrac{1C}{s}\$

\$\therefore P=IV=\dfrac{C}{s}\dfrac{J}{C}=\dfrac{J}{s}=W\$

Any of the different formulas for power can be derived using these relationships.

Matt Young
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Victorian textbooks had an excellent take on this, introducing electricity for engineers more used to hydraulics or steam...

  1. Power is the equivalent of hydraulic power;
  2. Current is the equivalent of current;
  3. and Voltage is the equivalent of pressure.

In fact up until about the 1920s you will see textbooks talking about "electrical pressure, measured in Volts".

It is a really good analogy in that the concepts translate perfectly, and really make understanding the difference between voltage and current, and how circuits work.

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    The biggest weaknesses in the model are that (1) empty space is something of an obstacle for electrons, not for fluids; (2) a typical engine will map pressure to torque and current to rotational speed, while a typical electric motor will map current to torque and pressure to rotational speed. A free-wheeling engine will consume substantial current at low pressure, while a free-wheeling motor will consume minimal current even at high pressure. I've tried, unsuccessfully, to imagine a mechanical engine that works like the electric motor. – supercat Apr 16 '13 at 18:21
  • The first can be handled by realising that a channel for fluid maps onto a conductor for electric current and there is no reason to expect the same "material" to do both jobs. The second is more tricky; understanding back-emf (electric) and perhaps early cutoff (steam!) may help. But you are right : describing the concepts as mapping "perfectlY is too strong. –  Apr 16 '13 at 20:51
  • The fact that the behavior of motors and engines is, in a sense, "90 degrees apart" [motors convert current to torque and voltage to rotational speed, while engines convert pressure to torque and flow to rotational speed] means the mapping isn't perfect, but is in itself "interesting". A motor driven by electric rather than magnetic fields would translate voltage into torque. Pondering the possibility makes me wonder if it would be possible to construct a remotely-efficient device that acted as a two-way convolution, such that... – supercat Apr 17 '13 at 14:58
  • ...the voltage on one pair of terminals would translate into current on the other and vice versa [in the same sense that an ideal motor continuously maps voltage to speed and current to torque, and a real motor behaves much like an ideal motor in series with some resistance and inductance on the electrical side, and in parallel with bearingf friction on the mechanical side]. – supercat Apr 17 '13 at 14:59
  • Isn't the "vice-versa" case essentially a motorised Wimshurst machine? –  Apr 17 '13 at 15:17
  • Conceptually, perhaps, but could such a setup manage even 10% efficiency and/or be even remotely responsive? If one takes a motor which is driven at a certain voltage and increases the mechanical load on it, the current drawn will change almost instantly regardless of the mass attached to the motor [the motor will have inductance, but it won't delay things much]. – supercat Apr 17 '13 at 15:46
  • I am not going to disagree with that! –  Apr 17 '13 at 16:46
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Voltage is an electric potential energy difference. Think of it like gravity but for the force between electrons rather than the attractive force between two objects with mass.

Google defines it as

The SI unit of electromotive force, the difference of potential that would carry one ampere of current against one ohm resistance.

NickHalden
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