To the Ineffable all,

    There seems to be a bit of confusion about just what voltage is.  I hope to clear that up.  Some sources claim it is the electrical potential (whatever that is) between two points in space or an electrical circuit.  Another definition says it is electromotive force (EMF).  That is a circular definition, because the phrase EMF means a voltage difference, and is not a force anyway. Other descriptions include how it is measured (voltmeter), how it is calculated (resistance formula), but that does not explain what voltage is.  Further attempts to define voltage involve comparing it to hydraulic technology such as the static pressure of a head of water.  But voltage is not pressure.  We know that electrical energy is involved with voltage, but voltage is not energy.  To further confuse folks, we can have a very high voltage associated with a small amount of energy and vice versa.  To understand what voltage is, and is not, we have to go back to basics.

    Electrical charges come in two flavors, positive and negative.  Two or more like charges do not like to get together. They repel one another.  To gather them together, energy has to be applied to the charges.  So now the charges have this accumulated potential energy (P.E.)that was applied to the system to force the charges together.  Now we can build up the same potential energy by bringing many charges somewhat close together, or we can bring few charges very close together.  In both cases, we can have the same P.E. involved, but different voltages.  Why is that?  Well, the reason is that voltage is the energy density of the charge.  In other words, it is the P.E. associated with the charges, divided by the number of charges involved.  In MKS units, that is joules/coulomb.  So voltage is not P.E., but it is proportional the P.E. associated with the charge.  Twice the P.E., twice the voltage. When charge flows, thereby causing current to exist through a resistance, it dissipates its P.E. in the form of heat, and the voltage drops.

    Now let's talk about what voltage is not.  First of all, voltage is NOT a vector quantity.  It is a scalar quantity.  The energy density of a charge has magnitude, but no direction.  The hydraulic analog that some folks like to use for voltage is pressure, and that is a vector quantity.  Voltage can increase and decrease, but so can any magnitude.  That is not a direction.  Sometimes magnitudes and phases of voltages and currents are compared to each other if they have the same shape, and are recurrent at the same frequency.  This is not a vector comparison, it is a PHASOR comparison.  The lines drawn to show this relationship are called phasors and have vector-like properties without being vectors.  Questions?  Ratch 
Posted on 2006-11-08 20:49:53 by Ratch

Interesting definition. So we could also say that voltage is the number of electrons per cubic meter?
Posted on 2006-11-09 04:07:19 by Maverick

...So we could also say that voltage is the number of electrons per cubic meter?

    No, voltage does not relate to a spatial density.  By the way, it is not just negative electrons that have a charge.  Holes, protons, and +ions can also be charge carriers of a positive nature that require energy to group together.    Ratch
Posted on 2006-11-09 07:55:14 by Ratch

So I guess it's not even the density of net charge.
Posted on 2006-11-09 09:33:51 by Maverick

So I guess it's not even the density of net charge.

    You have to define what you mean by "density of net charge".

    Voltage is the energy density of the charge; that being defined as the electrical energy divided by the number of charge carriers.  Spacial or volume density does not enter into the definition.  Ratch
Posted on 2006-11-09 10:28:40 by Ratch