How can voltage actually exist/change if electricity can't actually speed up or slow down?

Question

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I am new to hardware/electronics and am coming from a software background. I'm ashamed to say, but I've never fully understood the difference between volts and amps. If you Google "volts vs amps" you'll get a plethora of articles/links/blogs/videos that all try to explain the difference, but IMHO they actually do a really lousy job at it, and here's why:

Most analogies to volts/amps come in one of several very-similar forms:

• Water flowing through a stream; or
• Cars traveling on a highway during rush hour
• etc.

Let's take the "flowing water" analogy; it is typically explained that:

• Volts: the pressure of the water traveling through the stream; and
• Amps: how much water is traveling through the stream

The problem here (I believe) is that electricity is always traveling at a constant speed (speed of light), regardless of how much power you're generating, or regardless of the gauge size of the wire you're using. Unlike the speed of water traveling through a stream, the speed of electricity is constant and cannot be sped up or slowed down (I understand in the context of theoretical physics this is probably not true, but we're talking consumer electronics here).

With flowing water, if we increase the size of the stream (its width or depth), then not only will more water pass through any given point (volume), but the speed of the water will also increase. This is given to us by the Hagen-Poiseuille equation.

So I've always had this mental hang up with trying to understand exactly what volts are, because the analogies that are always used just don't stack up.

It makes sense that amps can be changed by changing the gauge of the wire the current is traveling through; this is the same as increasing the volume of the stream; but...

To me, it doesn't make sense that volts should ever be capable of changing, because volts are essentially the speed/pressure of the water passing through the stream, but electricity can't actually speed up or slow down! This is where the analogies break down for me, and is the root of my confusion.

Can someone please help me understand how electricity can have different levels of "pressure" or "potential", which is what volts are said to be, in a way that makes sense?

Again, if electricity travels at a constant speed, and we make the wire bigger, I understand that we will get more volume/amps. But I don't understand how this will create more "pressure" since the bigger wire will not actually make the electricity speed up.

Answer 1

I've only ever taken those water analogies with a grain of salt. Such analogies are really useful when you first begin to dive into theory, but, at least to me, the actual nature of the electric phenomena is what makes most sense.

To start, I am sure you have read somewhere that voltage is actually a measure of potential difference between two points, namely ground and some other node (although any two points are still valid, ground is nothing more than an arbitrary reference point in a circuit). Taking it a step further and knowing that:

$$Voltage (V) = \dfrac{Joules}{Coulomb}$$

What we have is an electric field through which we intend to move a charge. If we want to move charge against the field of some charge with the same polarity then we must expend some degree of energy to do so. For instance, if we move one 1 C of charge agaist a field and end up expiring 1 Joule of energy, we say that the potential difference, i.e. voltage, is 1 Volt!

In example, moving B to A would require some measure of energy in Joules. Depending on the magnitude of the charge you are moving you can calculate the related voltage that move represents. Since A is shown as +Q, then moving from A to B actually releleases energy and B to A requires input (i.e. batteries, for example).

Now simply take this and naturally extend its the application of voltage in free-space to the application of circuits. The only difference being that the electric field is largely contained within the wires we use to connect devices and circuits. As such, we now note that when a voltage exists across a component that what we really have is a difference in electric field intensity which is characterized by the necessary energy expenditure to move charge across it, voltage. Actually, we initially put energy into the system by chemically generating the voltage to supply a circuit, and then expend that stored energy in the form of EMF, heat, and light depending on your application.

This naturally leads into current, since electric current is nothing more than a derivative of an electric field. Namely, with an electric field exists across a component then a measure of disorder exists. A natural system will always try to minimize this disorder (i.e. entropy).

As far as your other question goes regarding the "speed of electricity", note that the propagation of the voltage appears instantaneous but the actual speed of electroncs within a wire is really quite slow. See Drift Velocity.

Answer 2

The problem here (I believe) is that electricity is always traveling at a constant speed (speed of light),

I believe this is your key misunderstanding, and if this is addressed the answers to your question will be much more clear.

Electromagnetic waves always travel at the speed of light (for the medium they're travelling in).

But current is not measuring the motion of electromagnetic waves, it is measuring the flow of charge carriers.

And charge carriers are not massless objects, and they travel much slower than the speed of light. They react to forces the same way as any other massive object does, according to

\$ \vec{a} = \vec{F}/m\$,

where the force is produced by the interaction of the electric field with the charge:

\$ \vec{a} = \vec{E}q/m\$.

In steady state conditions, the accelleration due to the field will be balanced by some retarding force, such as due to collisions with the material the current is flowing through. And these motions are all just a small bias on the general random motion of the carriers in most cases. However a stronger field will tend to produce a faster-moving charge carriers, and a weaker field will tend to produce slower-moving charge carriers.

Answer 3

It's not about speed, but about quantity. Cars are better than water for demonstrating this:

It's not about how fast the cars move, but how many of them move past a specific point in a given time.

The resistance defines how many lanes the road has. Lower resistance is like a wider road with more lanes. It allows more cars to pass the same point at once.

The voltage defines how tightly packed the cars are. 10 cars spaced 1m apart pass in a shorter space of time than 10 cars spaced 10m apart.

The result of combining the two is the cars per second (say), which is the current.

Now, if you have a 3 lane road full of cars spaced 1m apart, and you narrow it to a 1 lane road, those cars are going to have a hard time fitting into the single lane. Some will end up spilling off onto the verge or crashing into each other. That's the heat dissipation you get in a resistor. The excess cars are bursting out of the sides, some of them on fire ;)

And as a result, less cars will be coming out of the other side of the narrow bit. The difference in the number of cars each side of the 1-lane "resistor" is the voltage drop across it.

Answer 4

I think your speed-of-light assumption is a bogus one. I also hate analogies and I think they are doing more to confuse you at this point than to help you (especially the car analogy). I'll try to explain this in terms of the actual physics (electron interactions).

The electrons in any conductive medium are always moving around randomly, repelling one-another and finding new atoms around which to orbit. Remember that there are many electrons, they are all tightly packed, and they all repel one another. The current is the net flow of these electrons in a particular direction. Voltage, a.k.a. potential difference, is the difference in electric field potential. Electrons will always prefer to move in the direction of lower potential (positive charge) or away from negative charge (other electrons). So when the voltage is applied, they will continue moving around randomly, but they will tend towards the direction of lower potential giving off energy (often in the form of heat) along the way.

The problem with the cars on the highway analogy is that cars are organized and directed in the proper direction by an intelligent driver. Just remember that electrons are stupid and move around randomly. They are influenced primarily by two things: their repulsion of other electrons and their attraction to protons. So when you introduce an excess of protons in one location, the nearest electrons will move closer. As they move out of the way, others will fill in the space they left behind because the first electrons are no longer occupying that space, repelling the others away.

Answer 5

Your speed of light assumption is incorrect. Current flow is not at light speed - if it were, then you could make an electron beam just by bending a wire and letting the electrons fly off because they can't make the curve.

There are two parts to current flow that you need to separate. One is the propagation rate of a voltage change, and the other is the speed of the electrons in the wire.

The propagation rate of a voltage change is near to light speed, and depends on the conductor and the insulator (dielectric) around it.

The drift velocity of electrons, on the other hand, is SLOW. Wikipedia gives an example of a current flow of 3Amperes in a wire with a 1mm diameter. The drift speed in that case is about one meter per hour - if you had a 1mm wire one meter long and pushed 3 Amperes through it, then it would take an hour for an electron put in at one end to come out the other side. There will, of course, be electrons coming out the whole time - just not the ones you put in.

Current flow CAN speed up or slow down, the drift speed of electrons also depends on the current flow in Amperes. If you put a higher voltage on your wire, the electrons WILL move faster. A higher voltage pushes the electrons harder, which causes a higher current flow, and a higher drift speed.

What doesn't change with a higher voltage is the speed with which a voltage change propagates through a wire. That is, as I've said, determined by the material in the wire and the insulator around it. For this discussion, as close to light speed as doesn't matter. If you apply a voltage to one end of a wire and leave the otehr end free, then the same voltage will appear instantly on the free end.

The nearly instantaneous speed of the voltage propagation is what makes electrical distribution practical. You "put pressure" on one end of the wire and almost immediately electrons come out the other end -but NOT the ones you just put in. Depending on distance and current flow, they may take hours or days to move that far. Wikipedia Driftvelocity

Answer 6

I know my answer may not be as eloquent as the others but the way that I understand voltage/amperage is that in physics you have what are called "potentials". This can be in the form of energy, gravity, etc. What this in essence means is that some object or particle has more energy because it is at a higher potential. This can be in the form of an object being held up higher than another identical object, hence it has greater potential energy. An object may also be at a greater temperature, hence it too has greater potential energy. In the context of voltage this potential energy refers to the amount of charge a particle has in reference to some point(ground or zero charge).

Amperage is the flow of electrons through a given area. The speed doesn't necessarily change, but the area for which it flows can (think of a bigger wire gauge).

Now in reference to your water analogy. The reason water flows is because of newton's third law. For every action there is an equal and opposite reaction. Every single water molecule has to push on the next molecule in order for there to be acceleration. In the case of voltage we move out of newtonian mechanics and into the world of electromagnetism where objects are acted on not in a linear fashion but more in a "field"-type fashion where the potential can act on any particle at any time regardless of it's neighbor's behaviors. This is why this "speed" of electrons you speak of is consistent and doesn't increase as in the water analogy. I hope this clarifies things a bit!