Confused about voltage and current - Page 1
Forum section: electronics / beginners

author: kevyk
author_rank: Contributor
timestamp: August 19, 2011, 04:28:48 pm
content: A basic electronics question here:If current can be likened to molecules of water passing a fixed point and voltage can be likened to the difference in pressure between two ends of a water hose, is it possible to have a huge current (say 10000 amps) flowing while the difference between the two ends is very small (say 1 volt)? This question is of course assuming the wire/hose can handle the capacity without failure and offers negligible resistance itself.If so, I don't understand how this can be because the voltage potential (difference between the two ends) is so little, how would it be possible to have such a high water /current flow?Make sense?

author: Neilm
author_rank: Super Contributor
timestamp: August 19, 2011, 05:03:45 pm
content: Quote from: kevyk on August 19, 2011, 04:28:48 pmThe simple answer is yes. I have seen equipment that will output 2kA at 3V. It does this by the application of Ohms law - R=V/I so the output would have to be less than 1.5m ohms. This would be done by a very high tech piece of equipment - VERY thick wires. Seehttp://www.q80united.com/admincp/files/datasheets/PCITS2000-2.pdfThe more current you want to pass, the lower you make the resistance for any given voltage.10kA would be found in power stations, where they use thick bus bars to carry the current. The voltage over these has to be low or they would overheat and waste power.YoursNeil

author: Unknown
author_rank: Unknown
timestamp: August 19, 2011, 05:13:54 pm
content: Quote from: kevyk on August 19, 2011, 04:28:48 pmWhat you defined so far is the electric charge. To define current you need to sayQuoteSo 100 electrons passing a fixed point is just some charge changing place. OTOH 1000 electrons passing a fixed point in1 seconddenotes an amount of flow i.e.current.This was the boring bit. The fun bit is that it is perfectly possible to have 10000 amps flowing through a conductor. In fact the superconducting magnets of CERN carry 13000A. The ATLAS experiment carries 21000 A.http://www.ted.com/talks/brian_cox_what_went_wrong_at_the_lhc.htmlhttp://public.web.cern.ch/press/pressreleases/releases2006/PR17.06E.html

author: kevyk
author_rank: Contributor
timestamp: August 19, 2011, 05:31:45 pm
content: Thanks. This is more of an understanding question than a practical one.So it sounds like it is possible to have huge currents flowing while the voltage is very very low.But how can this be if the voltage (potential difference) between the two ends is so small, how is it possible even with a 0 ohm resistance (water hose with no resistance) to have a flow rate so large? What would be pushing the water/electrons so 'fast' (high current) if the potential difference is so low (1 volt)?

author: Mechatrommer
author_rank: Super Contributor
timestamp: August 19, 2011, 06:03:34 pm
content: well i like this kind of imaginative question. i thougt of roof analogy, but i think waterfall is better one. if you look from above (upstream), the river (before waterfall) they move slowly, but the river has large width, but since the water move slowly, you think it only small amount of water? wrong. the amount is the same when you look from below the waterfall, you'll see massive volume of water falling down fast, they are both the same amount (upstream and downstream)... V=IR... so upstream V is very low, but R is low as well. but downstream V is very high, but R is high as well (bottleneck where the water start falling), so in the end, they are the same I.... massive. the massiveness will depend how much volume of water in the system (start from mountain upstream down to the waterfall), ie how much "energy" in the system. you can have hi potential but low energy system (like electrostatic in your body or van de graff) but you also can have lo potential but hi energy system, like welding machine, you can touch without feeling anything but harmfull to lo resistance conductor.

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2011, 06:09:49 pm
content: Quote from: kevyk on August 19, 2011, 05:31:45 pmOf course it's possibleThe point you are missing is what numbers apply to huge, low, very low, etc? Is huge one microamp, one milliamp, one amp, one thousand amps, one million amps? OK, so you think one thousand amps is huge, but why? How do you know it's huge, really? Likewise, why do you think one volt is very low? Why isn't one volt huge? Do youknowhow big a volt is? If you say yes, how do you know?QuoteWell to go with your water analogy, think of a big wide, deep river flowing slowly along. Draw a pair of lines across that river one foot wide and ask how much water flows between those two lines every second? Millions of gallons? Yet what is the pressure difference between each of those lines one foot apart? Do you think you could even measure it?So to address your question, the answer is not how can it be, but how could it not be? What is there to stop the flow happening?

author: ejeffrey
author_rank: Super Contributor
timestamp: August 19, 2011, 08:45:13 pm
content: Quote from: kevyk on August 19, 2011, 05:31:45 pmSure. I have a superconducting magnet at work that has 100 amps flowing with essentially zero voltage drop. The magnets used at CERN to curve the particle beams into a circle have something like 8000 amps flowing and zero volts dropped.By zero I mean like zero. Immeasurably small. The current keeps flowing indefinitely when you connect the ends together and shut off the power supply. Zero.

author: Zero999
author_rank: Super Contributor
timestamp: August 19, 2011, 08:56:52 pm
content: Quote from: kevyk on August 19, 2011, 04:28:48 pmLook up Ohm's law.http://www.datasheetcatalog.org/datasheet/stmicroelectronics/1458.pdf

author: IanB
author_rank: Super Contributor
timestamp: August 19, 2011, 08:58:22 pm
content: Oh no! Not Ohm's law!

author: BBQdChips
author_rank: Contributor
timestamp: August 19, 2011, 09:09:51 pm
content: Quote from: kevyk on August 19, 2011, 04:28:48 pmIf the resistance is low, then the flow "can" be high. I think you are jumping to a conclusion or maybe making an assumption that when this current flows, somehow the voltage remains. That is typically not true. Well, it's always not true to a certain extent.Another analogy for you is, to get the same amount of flow from 2 hoses, one will require more pressure. That's because it's smaller. If your desired flow rate is 50 gallons (190L)/ second, and you are using a 1/4 inch drinking straw (6mm), then you better be able to get some pressure behind that. Now, if you are using a pipe that is 6 feet in diameter (2m), like something that drains a dam, well, you don't need very much pressure. Then, 500 gallons(1900L)/ second is easy.I went to discuss some manufacturing issues with a vendor last week. This vendor draws wire for us (makes wire smaller). During this process, it goes through an electrical annealing process that changes some mechanical properties. The wire is roughly .062 inches (1.5mm) in diameter and it runs at a speed of about 130 feet per second (40m/sec), give or take.They anneal that wire at 24V DC, and do it over a span of about 3 feet (1m). It goes from room temp, up to 900F (480C) and back to 130F (very hot room) in a 1 meter space. That's pretty fast. How? Well, they use the 24V at 5600A... Our 17 mile rolls of wire (27km) is electrically softened in 8 minutes, give or take. Not many volts, but a LOT of amps...Fyi, my definition: I call 5600 amps a lot of amps... Especially across a 1/16" wire.

author: Jimmy
author_rank: Regular Contributor
timestamp: August 20, 2011, 12:20:54 am
content: Forget the water analogy it is only going to confuse you If you have little knowledge of Hydraulics eg resistance is like head pressureIt 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

author: IanB
author_rank: Super Contributor
timestamp: August 20, 2011, 12:38:20 am
content: Quote from: Jimmy on August 20, 2011, 12:20:54 amHuh?

author: amspire
author_rank: Super Contributor
timestamp: August 20, 2011, 01:33:56 am
content: Quote from: IanB on August 20, 2011, 12:38:20 amThis is all news to me too.So much for that engineering degree and 40 years of experience.

author: amspire
author_rank: Super Contributor
timestamp: August 20, 2011, 04:08:03 am
content: Quote from: Jimmy on August 20, 2011, 12:20:54 amJimmy,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.QuoteI 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.QuoteThis 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.QuoteNot 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.

author: kevyk
author_rank: Contributor
timestamp: August 20, 2011, 05:15:30 am
content: 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!