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Class 12 Physics (India)
Course: Class 12 Physics (India) > Unit 3
Lesson 2: Electric current and voltageCurrent direction
The positive sign for current corresponds to the direction a positive charge would move. In metal wires, current is carried by negatively charged electrons, so the positive current arrow points in the opposite direction the electrons move. This has been the sign convention for 270 years, ever since Ben Franklin named electric charges with + and - signs. This convention came about 150 years before the discovery of the electron. Created by Willy McAllister.
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Okay guys,so help me out here.I have learnt that conventional current is the flow of positive charges.But its the negative charges that carries electrical energy,so what actually do the positive charges do in the circuit?What purpose do they serve and do they by chance cause resistance to the negative charges?(6 votes)
The current arrow points in the direction positive charge would flow. If there is no positive charge moving, the arrow still points that way. If only negative charges are moving (like in a metal wire for example) the current arrow still points in the direction positive charges would move IF there was positive charge present. Whenever you see a current arrow, you can be assured that electrons are flowing in the opposite direction. The arrow points in the direction the electrons are coming FROM.(18 votes)
Really couldn't understand why current moves in the opposite direction of the flow of electrons🤔(3 votes)- Your confusion is understandable. This is one of the quirks of electricity. The current in a wire is made of moving electrons. It is our "convention" (habit) to point the current arrow in the opposite direction electrons move. (We point it in the direction a positive charge would move.)
This is explained more in the upcoming video on Conventional Current, and also here: https://spinningnumbers.org/a/conventional-vs-electron-current.html(8 votes)
So when we talk about electricity we talk just about the pure movement of electrons, not ions or anything. So as I understand that singular electron separates from its atom leaving the atom as positively charged ion and when we talk about the movement of positive charge we talk about the movement of positive ions not protons right?(3 votes)- Electricity is about the movement of charge. The charge can be in any form. In wires, the electrons move and the protons sit still. In saltwater, its both types of charged ions that move, Na+ and Cl-. We call that electricity, too. In saltwater, whole atoms are moving around, protons, neutrons, and electrons. When this is going on, it happens that the atoms have either one less or one more electron than protons. I can't off the top of my head think of an example where protons are the main/only current carrier. (Maybe inside a fuel cell?) In biology systems (life) there is lots of proton motion and reacting happening all the time, but we usually don't classify that as electricity, it's called biochemistry.(8 votes)
If negative charges are called electrons and positive charges are called protons, and current is the movements of electrons C=q/s does that mean Current is always negative? Because in math if you have a negative sign in equation, the answer you get is negative. for example this is an equation you are supposed to solve x=t+g and answer you get x=10_15=_5.(2 votes)- I think it makes sense for me since the current is the flow or movement of electron the result become negative which means with your example the answers are correct(0 votes)
- I agree with WILLY, that there were only two theories for electricity at that time period. So did Franklin originally discover the electron or did he propose that theory and stick to it?(3 votes)
Hello Arman,
Benjamin Franklin did not discover the electron. In fact, this discovery had to wait another hundred years for J.J. Thomson.
Know that Franklin was able to observe polarity. Some charges were positive and some were negative. Like all good scientists he made a model to explain the observed phenomenon. This model assumes current flows from positive to negative. It's a good model just like the model of an atom as a miniature solar system.
Like the solar system atom model it was refined over the years. We add complexity as required.
Regards,
APD(6 votes)
Is the current a Vector or a Scalar ?(2 votes)- Hello Loaymohamedabd,
For these introductory studies we assume current is a vector with only two directions.
Later when you learn about AC circuits and concept of a "phasor" you will see that current and voltage become true vectors.
Regards,
APD(7 votes)
Is there a easy version for this? It is too hard for me.(2 votes)- Hello Aqua,
Yes, this is hard even for college sophomores. But don't let that stop you. If you are interested in computers and electronics than you should explore and experiment. A few good places to start are: Instructables.com, Adafruit.com, Sparkfun.com, and Arduino.
While you are at it please look up "Limor Fried."
Regards,
APD(7 votes)
Can we put both ammeter and a voltmeter in a series circuit.(2 votes)- Hello Alhossainzarif,
The rule is: ammeters in series and voltmeters in parallel.
To measure current we want to know the flow "through" the wire. The current flows through the ammeter. Note that an ammeter has a very low resistance. From the circuit's perspective the ammeter isn't even there.
Voltage is measured "across" a component. The voltmeter has a high resistance. Once again, from the circuit's perspective the voltmeter is invisible.
Please don't take these rule of thumb to the extremes. A real ammeter has some non zero resistance and the real voltmeter has some non infinite resistance. Installation of either instrument can disturb the circuit under test...
Regards,
APD(6 votes)
- Respected sir/madam
why we take current direction opposite to flow of electrons rather than hols(+ve charge) direction in real time and what is the necessity of electron current ?.(2 votes)- Hello Kchandra,
This is simply the convention we use today. We could have used either direction but the positive to negative convention remains the most popular. Unfortunately, one of the first lessons in electricity introduces this inconsistency. Just go with the flow and accept positive to negative :)
For what it's worth, nearly all test equipment is built assuming positive to negative flow.
Regards,
APD(4 votes)
- what is the direction of current?pls give some examples.did not understand properly (Video).pls reply as soon as possible.(1 vote)
- Current direction always confuses starting engineers. We point the current arrow in the direction positive charge moves (or would move if it was present). The electrons flow in the opposite direction of the arrow.
(Conventional current is not a "kind" of current. It is a convention for direction. It's kind of like how weather vanes point INTO the wind, not away from the wind.)(4 votes)
Video transcript
- [Voiceover] In the last
video, we talked about the meaning of current, and current is defined to
be the movement of charge, amount of charge per second. We looked at a copper wire where electrons are carrying the current, and we looked also at a salt solution where both positive and negative ions are carrying the current
across this imaginary boundary where we keep track of how
many charges are moving. So now, next we need to talk about how do we define a positive current? What does a positive current mean? So, I'm gonna move this up a little bit. And we're gonna get to something
that causes some confusion. But, we'll try to clear it up here. The positive sign, this is
a convention that we use. The positive sign for current is the direction that
positive charges move. So, that's the direction... this is the direction a
positive sodium ion moves. This is the direction a
negative chlorine ion moves, When I assign a positive
direction to the current, the overall positive direction is gonna be in this direction. That's plus current. Plus current. Alright, and the same thing
actually holds for a wire. If I draw, let's go back
and draw my wire again. If I draw my copper wire, and inside there we know that
all these little electrons, all the electrons are
moving in that direction. So q- is moving in that direction. And when I assign a direction
for the arrow for a current, the current arrow points this way. Plus current. So for instance, if I had
10 electrons per second that were moving in this direction here, I would say the current is
plus 10 charges per second in this direction. And ask yourself: If it's always electrons
carrying a current, why on Earth do we point
the arrow in the other way? And that actually goes
back, way back in history, to out friend Ben Franklin. Ben Franklin is the person who said, who basically made a decision that said the charge on
an electron is minus. Now, when he made that decision, nobody knew there was such
a thing as an electron. So Ben Franklin. And that was in the year 1747. Ben Franklin's a person
who assigned plus and minus as being the two types
of charge that existed. And when he did that he had no idea. Nobody had any idea that
there was such a thing as an electron. Electron was discovered by a
physicist named J. J. Thomson, In the year 1897. That was when the electron was discovered. And believe it or not, if
we do the subtraction here, look at this, it's 150 years. For 150 years of electric research, we were able to figure out
a lot of really good things, assigning current going in the opposite direction
of electrons in metal. And it's all okay. It's all okay. One of the questions that's
often asked by new students, is why don't we switch it around so the current points this way? Why don't we assign an electron, why don't we make current go this way? So, why didn't we just switch it around? Well, we had a 150 years of experience, and now we've actually had another 120, since the discovery of the electron. And we've managed to get by with this. We have not chosen to do it. One of the reasons we
don't do it is because it would basically, it
would be a big mess. Imagine if for instance,
say in the United States we decided, Oh, driving on
the right side of the street is the wrong side of the street. We want to all drive on the left. Imagine the chaos that would cause. Now, before we made the change, everybody managed to
get where we were going, and after the change, everybody
would get there going, but the change over would
be just so, so costly. And the same thing with electricity. We can do perfectly well talking about current going the direction of a positive charge, just like we did here
with the positive sodium going in this direction. This is actually how electricity
is conducted in your body. So, it's not uncommon to have
positive charge moving around. That's the definition of
current in a nutshell. We put a boundary across, and you watch the charges going through, in either direction, which
ever direction they do, and you count up and you get q per second. That's current.