Same polarity but different direction of current flow. How annoying! - Page 1
Forum section: electronics / general

author: Circlotron
author_rank: Super Contributor
timestamp: September 21, 2020, 01:12:51 am
content: Say we have a capacitor and we ground one end and begin sucking electrons out of the other end via a resistor. This end of the capacitor is positive polarity. Then we connect the resistor across each end of the capacitor and begin to discharge it. Now electrons are flowing back into the positive end of the capacitor. Same polarity, opposite direction of current flow! Does this situation have a name?Edit. positive and -> positive end

author: BravoV
author_rank: Super Contributor
timestamp: September 21, 2020, 01:51:04 am
content: AC ?

author: S. Petrukhin
author_rank: Super Contributor
timestamp: September 21, 2020, 01:53:42 am
content: Quote from: Circlotron on September 21, 2020, 01:12:51 amWe do not suck out electrons, we push them there more through the minus, and then they flow out of the minus. A capacitor stores energy in an electrostatic field. This inductance uses an electromagnetic field and it then returns as self-induction with reverse polarity.

author: greenpossum
author_rank: Frequent Contributor
timestamp: September 21, 2020, 01:56:43 am
content: Sourcing and sinking

author: newbrain
author_rank: Super Contributor
timestamp: September 22, 2020, 10:25:31 am
content: Quote from: greenpossum on September 21, 2020, 01:56:43 amTo expand a little bit (conventional current flow in the following, not the topic here):Current flows from a higher potential to a lower one.In your example, the plate of the capacitor that is not connected to ground will be at a higher "positive polarity" with respect to the groundwhen discharging, but at a lower polarity (negative) with respect to the voltage sourcewhen changing- otherwise, current could not flow in to "suck electrons" out.Potential are always relative.Reference points are paramount.Edit: armature is not used in English AFAICS - Italian mother tongue shows...

author: newbrain
author_rank: Super Contributor
timestamp: September 22, 2020, 10:30:14 am
content: Quote from: S. Petrukhin on September 21, 2020, 01:53:42 amActually, unless your source is based on nuclear beta decay, the total number of electrons is unchanged in the circuit.Every electron you push into the negative plate has to come from somewhere, namely, the positive plate.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: September 23, 2020, 04:02:22 am
content: Quote from: newbrain on September 22, 2020, 10:25:31 amThe direction of current in the circuit from high potential to low, from + to - is assumed conditionally, it is not related to the physical process. The current can be carried, again conditionally, by electrons and holes (lack of electrons) . In general, no one yet knows the physics of current, as the principle of gravity, the attraction of particles.

author: ebastler
author_rank: Super Contributor
timestamp: September 23, 2020, 05:38:33 am
content: Quote from: Circlotron on September 21, 2020, 01:12:51 am"Confusion"?

author: tkamiya
author_rank: Super Contributor
timestamp: September 23, 2020, 05:54:05 am
content: Positive and negative, and direction of flow of current was defined before involvement of electron was well understood. When we did understood electrons, we found out that electrons actually flow from negative to positive. So what you are observing is correct. To rectify this situation somewhat, explanation that "hole" (location of absence of electron) moves the same way current moves.BUT, there is nothing flowing from positive to negative. Hole is just an illusion created to explain away the mistake. This whole thing is really screwed up. It's one of the first thing taught in electrical engineering, and it confuses a lot of students.An only way around this is, strictly think in one or the other, either current or electrons. Try not to think of them in one argument like you are trying to do. My suggestion is to follow the popular convention of current flowing positive to negative, and forget the physical action of electrons. You can think separately if the situation warrants otherwise. You'll get used to it eventually though.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: September 23, 2020, 08:24:31 am
content: Quote from: tkamiya on September 23, 2020, 05:54:05 amSir, I'm 50 years old, I don't need to get used to anything.

author: tkamiya
author_rank: Super Contributor
timestamp: September 23, 2020, 02:12:33 pm
content: But, but, but.... SIR! I'm older than you and I got used to it.

author: helius
author_rank: Super Contributor
timestamp: September 24, 2020, 07:31:25 am
content: Quote from: tkamiya on September 23, 2020, 05:54:05 amThe hole is a real phenomena that only exists in p-type semiconductors. The wires in basic electric circuits definitely do not conduct by means of holes!Quote from: tkamiya on September 23, 2020, 05:54:05 amBatteries and wires are human inventions, not fundamental physical objects. It is undesirable to define the latter for the convenience of the former.In general, the conception that "electrons are moving like little corpuscles in a tube" is just naïve and misleading. Electrons do not even have a definite location and thus the idea that they are "flowing from negative to positive" is wrong. This wrong intuition gives rise to a host of other wrong ideas, like the idea that electrons "start out" from the negative terminal and must travel around the circuit before entering the positive terminal, so they must flow very quickly because current is equal in both branches from the moment the circuit is completed. Visualizing electrons flowing is the sign somebody doesn't understand electronics.

author: Berni
author_rank: Super Contributor
timestamp: September 24, 2020, 08:09:04 am
content: So what about charging a battery? In that case you are also pushing current into the battery positive terminal.The reason the positive terminal is called the positive terminal because it is at a higher electrical potential than the negative terminal. But the flow of electrons is dictated by a difference of potentials, so as soon as you connect two different voltage potentials that starts pulling electrons along and you get current.So when you are changing a battery then the charger is creating an even higher voltage potential than the positive battery terminal. So since current flows from a higher potential to a lower ones means that the current starts flowing towards the battery + terminal.Then when you connect a load to the battery you connect it between the positive and negative terminal. So in that case the batteries positive terminal is higher potential than its negative terminal so current flows out of the positive terminal, to the load and back down to the low voltage negative terminal.You can imagine wires being water pipes, voltage is water pressure, current is water flow. It always flows from high pressure to low pressure.

author: BravoV
author_rank: Super Contributor
timestamp: September 24, 2020, 08:27:40 am
content: Quote from: Berni on September 24, 2020, 08:09:04 amI always used this example when I explained this to friends or youngsters asking this, similar but slightly different example, as at some audiences, like to ask what if the pipe break or burst ? And where the water goes ?I used wire = blood vein , current = blood "flow" (not the blood it self, but the flow, and the blood must ALWAYS be there in the vein) and voltage = blood pressure, also to spice things up, heart is the battery/power source.This example, hopefully will suppress the pipe burst question, and my case it worked all the times.Of course, should they ask about the pipe/vein burst or blood spillage, all I need to answer is .. ITS BAD !

author: ChristofferB
author_rank: Frequent Contributor
timestamp: September 24, 2020, 09:16:39 am
content: Keeping in the analogy, the answer could be to compare a burst pipe to a short to ground. A high flow (low impedance) path to the lowest elevation/potential

author: TimFox
author_rank: Super Contributor
timestamp: September 24, 2020, 02:28:56 pm
content: In the case of a transmission line, such as a small-diameter coaxial cable, it is useful to measure at a plane (perpendicular to the cable), where the voltage is defined locally between center and shield, and the current is defined as flowing in either direction along the wire through the plane. This avoids any discrepancies at high frequencies due to non-local measurements. The product of the voltage and current determines which way the (AC) power is flowing.

author: S. Petrukhin
author_rank: Super Contributor
timestamp: September 24, 2020, 03:01:34 pm
content: Quote from: helius on September 24, 2020, 07:31:25 amThere is a well-known physical experience: if a coil with a wire is spun around its axis, and abruptly stopped, an electric current will appear on the outputs. This suggests that there is some substance in the wire that has mass and inertia. People think of it as electrons.

author: tkamiya
author_rank: Super Contributor
timestamp: September 24, 2020, 04:08:07 pm
content: Quote from: helius on September 24, 2020, 07:31:25 amIt depends on how accurate the explanations has to be. If someone is asking which direction current flows and I were to answer in terms of electrons, taking so far as to get quantum mechanics involved will surely lose the audience. I'd settle for better understanding, rather than being exact and lose the conversation altogether. Yes, I understand the concept you are talking about and can explain it in that term. I actually took a course involving this exact manner in high school. The term used then was "like flowing this direction" when talking about transfer of free electrons, and it's successive transfers. Electrons not having defined location but is a probability is an idea of quantum mechanics. I'm not sure if we have to involve that in even semi-technical conversation.

author: ebastler
author_rank: Super Contributor
timestamp: September 24, 2020, 04:10:26 pm
content: Quote from: Berni on September 24, 2020, 08:09:04 amThank you for the second post in a long thread that is actually on topic, i.e. addressing the OP's confusion.(newbrain was the first to explain it, in reply #4.)