Electronics > Beginners
Confused about voltage and current
Jimmy:
Forget the water analogy it is only going to confuse you If you have little knowledge of Hydraulics eg resistance is like head pressure
It is electrons flowing Voltage is how many electrons can move at once, Resistance is how much resistance electrons get or how fast they can flow have to flow, Current is how many are flowing.
Yes you get to a point when you cant have any more current at that voltage that is why batt have a short circuit current.
One volt supplies about one trillion trillion electrons
IanB:
--- Quote from: Jimmy on August 20, 2011, 12:20:54 am ---Forget the water analogy it is only going to confuse you If you have little knowledge of Hydraulics eg resistance is like head pressure
It is electrons flowing Voltage is how many electrons can move at once, Resistance is how much resistance electrons get or how fast they can flow have to flow, Current is how many are flowing.
Yes you get to a point when you cant have any more current at that voltage that is why batt have a short circuit current.
One volt supplies about one trillion trillion electrons
--- End quote ---
Huh? ???
amspire:
--- Quote from: IanB on August 20, 2011, 12:38:20 am ---
--- Quote from: Jimmy on August 20, 2011, 12:20:54 am ---Forget the water analogy it is only going to confuse you If you have little knowledge of Hydraulics eg resistance is like head pressure
It is electrons flowing Voltage is how many electrons can move at once, Resistance is how much resistance electrons get or how fast they can flow have to flow, Current is how many are flowing.
Yes you get to a point when you cant have any more current at that voltage that is why batt have a short circuit current.
One volt supplies about one trillion trillion electrons
--- End quote ---
Huh? ???
--- End quote ---
This is all news to me too.
So much for that engineering degree and 40 years of experience.
amspire:
--- Quote from: Jimmy on August 20, 2011, 12:20:54 am ---Forget the water analogy it is only going to confuse you If you have little knowledge of Hydraulics eg resistance is like head pressure
--- End quote ---
Jimmy,
I think I can see what you are trying to say, but the words you have used are the wrong words to cut through the confusion - in fact they make it more confusing.
The trouble with analogies is they they simplify things - often to the point where they are just wrong. kevyk has now had so many analogies that I better not give any more.
--- Quote ---It is electrons flowing Voltage is how many electrons can move at once,
--- End quote ---
I think what you are trying to say is that in a given circuit, then the more volts, the more electron flow. This is true but Voltage itself has nothing directly to do with electrons. Voltage doesn't supply anything. If you have two plates a vacuum with a voltage difference of 1V, then a voltage field will be established between the two plates. If you put one electron between the plates it will be pushed towards the positive plate and it will pick up a certain amount of energy. Put two electrons in, and each electron will pickup exactly the same energy as the single electron, so you now have double the total energy.
So the really important thing here is that Voltage is a measure of force (not energy) and that force will be applied equally to every charged particle that is under the influence of the voltage.
--- Quote ---Resistance is how much resistance electrons get or how fast they can flow have to flow,
--- End quote ---
This just doesn't mean much.
In a wire, current is not electrons that enter the wire and come out the other end. It is a massive sea of free electrons and when current is flowing, a small number (relatively) of electrons are entering one end, and the same small number are leaving the other end. The ones leaving the end are not the same ones that entered the wire.
To us though, it looks like something is going in one end, traveling down the wire and going out the other, so we have this notion called current to quantify this. Positive current actually travels in the opposite direction to the electron drift direction - that is because back when voltage and current was discovered, they didn't know about electrons, so they just picked arbitrary signs.
Here is one way to think of current: One electron enters wire that pushes the one that was already sitting at this end further into the wire. The second electron pushes a third along, the third pushes the fourth and so on, so that finally one electron pops out the far end.
If you think of a wire has this long chain of electrons, then even though each electron has move very slowly - they each only move one step down the wire, the effect of the voltage is almost instantaneous. Almost the speed of light. So to us current seems to be traveling in a wire close to the speed of light, but in fact the average movement of each electron is absolutely tiny - definitely nowhere need the speed of light. Many, many orders of magnitude below the speed of light.
So in a given wire, a voltage will give each electron a very tiny push and the amount speed the electrons have determines the rate that the current flows. So for a given wire with a given voltage across it, you will get a certain current flow. The amount of current flow for a given voltage is measured as resistance. The less resistance, the more current flow.
If you double the voltage, the the portion of the voltage (pressure) applied to each electron doubles and it moves twice as fast, so that electrons start popping our the end twice as fast.
Sorry this is so long winded, but bear with me.
What will happen if you take a wire with double the cross sectional area? If you look back, I said that a given voltage puts equal pressure on each charge particle. So double the area, and you have doubled the number of available electrons. Since the voltage hasn't changed, the force available to each electron is the same as for the thin wire, but there are twice as many electrons. So the result is exactly double the current out.
It is still not a fully accurate description, but that is the best I can do for now.
--- Quote --- Current is how many are flowing.
Yes you get to a point when you cant have any more current at that voltage that is why batt have a short circuit current.
One volt supplies about one trillion trillion electrons
--- End quote ---
Not sure where Jimmy is going here.
In a wire, there are so many free electrons available that you never run out of free electrons. The wire will vaporize with heat well before you get vaguely near any current limit.
Batteries are another story. There are internal resistances then will reduce the voltage at high current, and there is a maximum rate at which ions can move through the electrolyte from one plate to another. That is a discussion about batteries, and not volts, resistance and current.
The good analogies are a lot simpler and easier to remember (like voltage = water pressure, current = water flow, resistance is a measure of how thin the pipe is. The thinner the higher the resistance). I hope this is a few steps closer to understanding what is actually happening electrically.
I know I have said things above that are still attempts at simplifications and are not exactly true. A discussion of voltage fields is probably needed, and I was desperately trying to avoid it.
Richard.
kevyk:
Thank you everyone for your answers! It has given me a better understanding conceptually how it is possible to have huge currents flowing in the thousands of amps while the voltage drop is very low like 1 volt for example. The thing that did it for me was the huge volume of water flowing very gently and slowly in a deep wide river. You can have very slow water travelling down a stream (low voltage) yet have very high currents (movement of massive amounts of water) IF the river were deep and wide.
Makes sense to me! Thanks again for the input!
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