Volts and amperes explained

I'm trying to understand electricity better, but all I found was those stupid water pipe analogies. Can you explain it to me in terms of actual electricity? What is voltage, the speed of the electrons flowing through the wire? Somehow this makes little sense to me but I don't know. Is amperage then just the "flow through", how many electrons pass through? Where does the energy of the electrons come in, if at all? Thankyou!

• electronics.stackexchange.com/a/136383/11606 Apr 27 '18 at 18:35
• The energy per charge. Apr 27 '18 at 18:39
• Voltage is NOT the energy. It is the Force (Electromotive Force). Apr 27 '18 at 19:50
• Possible duplicate of What exactly is voltage? Apr 27 '18 at 21:25
• Elecrtoboom is usually a comedic what-not-to-do, but he actually covers the history and technicals pretty well in this video. Worth a watch. Apr 28 '18 at 3:45

The voltage of a system is a measure of the electric potential of the system. What this means is that the voltage at a point is a measure of how much potential energy an infinitesimally small charge(small referring to the magnitude of charge, not size) would have if you were to bring it to that point. Voltage is always a relative measurement, meaning that it doesn't make sense to identify how much voltage you have without also describing a reference voltage. (In electronic circuits, this reference voltage is usually your ground.) The SI unit to measure this attribute is volts.

Current is a phenomenon describing the flow of charge, and the amount of current represents the charge flow rate. The SI unit for this flow rate is given in amps.

• That makes sense, ground is therefore always 0 volts? If I may also ask, I'm working on controlling appliances with a micro-controller, everybody tells me to connect everything to the common ground. Why is this needed? Apr 27 '18 at 21:22
• @PeterPoláček Ground is 0V pretty much by definition. You need to connect grounds together (a) because otherwise you could get a confusing situation as different parts of the system have different ideas of what "ground" is, and (b) because electrons need to flow in circuits, so if you don't connect the grounds, you don't provide a continuous circuit. Apr 27 '18 at 23:18
• @PeterPoláček ground isn't zero volts. Instead, if voltage is like altitude, then ground is like "sea level." But there is no such thing as zero altitude (we could measure altitude from the bottom of the deepest trench, or from the center of the Earth, so every spot has infinitely many altitudes at the same time!) In the same way, voltage is always between TWO points, so there is no such thing as a "zero volts" point, and every point has infinitely many voltages at the same time. Circuits are circles, so choose one "sea level" point, and then you can measure the voltage at any other point. Apr 28 '18 at 1:20
• @PeterPoláček - Ground is DEFINED as 0 volts, and every other voltage is measured wrt it. In any circuit you can set any point as ground, and as long as you're consistent it's no problem. Of course, some choices make life a lot easier than others. Apr 28 '18 at 1:22
• @PeterPoláček, zero volts point is chosen by the designer of the circuit .... it is the reference point in the circuit .... it is the place where you attach the black probe of your voltmeter Apr 28 '18 at 1:44

Current is the charge passing a particular point per time. The common unit of Ampere is one Coulomb of charge passing by per second.

EMF (electromotive force) is the push that makes charges flow. You can think of it a pressure that pushes charges and causes current to the extent that the charges can move. A common unit of measure is the volt. One volt can push one ampere of current thru a resistance of one ohm.

Current is easy - it's just rate of change of charge i.e. how many electrons are passing per second.

What is voltage

I can use a mechanical analogy to show what it is and how, in the newtonian mechanical world, it is not very significant: -

First relate the quantities energy (W), mass (m), velocity (v) and momentum (p)

$W = \dfrac{mv^2}{2}$ and $p = mv$

Therefore: -

$W = \dfrac{mp^2}{2m^2} = \dfrac{p^2}{2m}$

Now, if we took the rate of change of work with respect to momentum we get: -

$\dfrac{dW}{dp} = \dfrac{2p}{2m} = \dfrac{p}{m}$

Then, substituting p = mv we get $\dfrac{dW}{dp} = v$

So, in newtonian physics/mechanics, velocity is the the rate of change of work with respect to momentum. Huh?

You can do the same with electrics where: -

Work = energy = $\dfrac{Cv^2}{2}$ and

Momentum $\equiv$ charge = $Cv$.

Does this get you any further to understanding what voltage is? Does it get you any further to understanding what the rate of change of work with respect to momentum is in a mech world?

About the only thing that immediately springs out is that charge is equivalent to momentum. Good luck.

What would voltage look like, if we could see it? The MIT open-source project published some video animations of the physics behind voltage:

Positive and negative charges being pulled apart, creating a pattern of voltage in space.

Pos and neg charges falling together again, where voltage decreases to zero.

(Also lots more)

In those two videos, pretend that the two points are two wires viewed end-wise. In that case the two videos would reveal what happens whenever we connect two parallel wires to a variable power supply, then turn up the voltage starting at zero. When set to significant voltage, the wires have an intense field between them, and can strongly push the charges found inside any resistor.

Voltage is the "dual" of magnetism: fields of voltage appear between the wires connected to a power supply. Note the voltage-field down between capacitor plates, versus the fields of magnetism in the cores of inductors.

This might help. Say you had a reservoir of charges on one side of a resistor (with really high resistance) and ground on the other side which had very few charges.

The voltage difference between them would be pushing hard on the electrons to try and shove them into the ground which had few charges. However, the resistor is difficult to move electrons through as it has a very tight atomic structure; so difficult that it impedes the voltage difference from pushing very many charges through at a time.

Thus, the current (a sort of charges per second) through the resistor would be low despite the voltage difference between the reservoir and the ground being large.

Another interesting thing for you to look into, if a circuit is supposed to be a closed loop then why can the power grid run from the generators to ground? Where is the loop? ^^(If you want the reason spoiled I can edit this answer)^^

• Thanks, that helps! But about the question, I'm not sure.I never realized that if it's a closed loop, it should go the other way as well? Is there just a diode there or something more advanced? Apr 27 '18 at 21:27
• It's quite a beautiful answer. In the universe, the law of conservation always holds; energy is not created nor destroyed. This law is upheld on the small scale, and thus maintains itself on the large scale, too. It's not just that electricity needs to make a closed loop with the generator, it has to maintain a closed loop at every infinitesimal length of wire. As the electron pushes its way down the wire, the balance is maintained by the electrical field around the electron pushing in an equal and opposite direction. The field "trades" energy with the electron. Apr 27 '18 at 21:36
• Oh I see how you meant it... That's really neat Apr 27 '18 at 21:47