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Electrical engineering
Course: Electrical engineering > Unit 1
Lesson 1: Getting started- Electric current
- Current direction
- Voltage
- Conventional current direction
- Preparing to study electrical engineering on Khan Academy
- Basic electrical quantities: current, voltage, power
- Numbers in electrical engineering
- Defining the standard electrical units
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Conventional current direction
By convention, we define positive direction of current to be in the direction a positive charge would move. Electrons (with their negative charge) move in the opposite direction of the positive current arrow. Created by Willy McAllister.
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Sal says (2:26-2:39) that the energetic electrons that are pumped out of the battery go back into the positive current, how then does a battery run out of energy?(10 votes)
Hello Michael,,
True, the electrons go in a circle. But there is work being done to make them move. For everything that moves a small amount of the battery's energy is given away.
These videos may help:
https://www.youtube.com/watch?v=XdgKqf4h0tQ
https://www.youtube.com/watch?v=d1AokT4-pC0
Regards,
APD(9 votes)
So the mountain analogy had electrons flowing from the negative terminal (the top of the mountain) to the positive terminal (the bottom). Circuit design was said to be all the stuff the electron bumps into on the way down. My question is are circuits designed with electron current in mind or conventional current? In my mind, I see a circuit designed with conventional current in mind working the opposite based on the electrons flowing in the opposite way designed by the circuit. Any clarification to why that is not so would be appreciated!(5 votes)
There's an unfortunate dirty trick teachers play on beginning EE's. The idea of current is introduced by telling you about electrons and how they flow with cool mountain analogies. Then in the next breath I tell you the current arrow points the other way (by convention) (I ask you to flip the mountain onto its peak). I wish this didn't have to happen, but no teacher has figured out how to avoid doing this right at a fragile time in your learning. Conventional current is not a different kind of current, it's just a goofy way to point the current arrow. We point it into the electron current flow instead of the other way. It's the same as if I gave you a paper map and told you to hold it with South at the top. You can still read the map fine, and you can give anyone directions as long as you use cardinal directions (turn East and walk for an hour, then turn North). There's nothing "wrong" about holding your map this way.
Almost every circuit (all circuits in KA's EE subject area) are designed with conventional current in mind. It has zero impact on how the circuit works. The electrons just happen to flow in the opposite direction of the current arrows. As you get deeper into the material here this will become more and more apparent and hidden residual stress will melt away. I promise.
When you get to studying solid state electronics, (the how's and why's of what's going on deep inside a transistor) you will once again track the movement of electrons. You will have no trouble flipping between conventional vs. electron current because you will be so deep into EE it won't be a problem.
For now, my advice is to press ahead and start working with conventional current. After you do some activities with battery/resistor circuits and Ohm's Law it will become natural, and you will see how there's just one "kind" of current.(20 votes)
Okay, I've never fully understood the difference between current and voltage. From his explanation, current is like counting how many electrons pass through at once. Is voltage how hard they're being pushed/pulled through? Wouldn't that just proportionally increase the amount of current anyway?
Also, if the direction of current doesn't matter, does it matter where on a circuit you put specific parts, like resistors and whatnot?(5 votes)- You got almost everything right. Current is charges flowing. To measure current and give it a number, you stand at a point and count how many charges go by in a second. If 1 coulomb of charge goes by, that's defined to be 1 ampere. Voltage is how hard the charges are being pushed/pulled. You can think of it as electrical pressure, or as the slope of a mountainside that charge is rolling down. Increasing voltage proportionally increases current. That is the idea captured in Ohm's Law.
The direction of current does matter. The idea of 'conventional current' has kind of a quirky definition, it's the direction positive charge would move. It happens that electrons move in the opposite direction of the conventional current arrow. That doesn't mean we don't care which way current flows, it just means our definition of positive current is a little unexpected. It's something all new EE's take a little time to get used to.(12 votes)
What is the difference between a positive and a negative charge? I mean, why can't we reassign electron lack as negative charge and excess electrons as a positive charge?(4 votes)- The naming of the charges as + and - is an arbitrary human choice. There is no sense that an electron is aware of its name, and knows nothing of "negative-ness".
There are two kinds of charge, and on (and only one) rule: "Like charges repel, unlike charges attract."
A long time ago when scientists were struggling to figure out electricity they came up with words to describe what they thought was going on. They could have come up with the system you describe, but they didn't. We still use those words today.
It is a coincidence that we use arithmetic signs (+ and -) as the names for charge. In some ways that is unfortunate, because it gives the impression charges are doing arithmetic or being "positive" or "negative". We could have gotten along just fine with other names. In fact, the original names for charge were "vitreous" and "resinous" (glass-like and resin-like). Whatever their names they follow the one rule.(6 votes)
In the functioning of the battery , chemical reactions release electrons which want to move to the cathode but the electrolyte prevents them from doing so. Hence, it move along the circuit .
So is it safe to assume that all electrons moving in the circuit are from the battery.
If yes , then why do conductors need to have free electrons?(2 votes)- Good question. It made me think. A wire (or any material) starts out neutral (same number of electrons and protons). And even when there is a current flowing, the wire has to stay pretty much neutral. If you imagine a current where you are putting a whole bunch of extra electrons into a wire you have a situation where there is a huge collection of negative charge in one location, which creates a gigantic repulsion between individual charges. That repulsion is enough to stop any battery from forcing more current into the wire.
It is actually pretty fascinating that an electric current flows in a conductor while the conductor remains essentially neutral the whole time. One electron comes in at one end, many other electrons in the wire "move over" one position (like musical chairs), and finally one electron pops out the other end. You get movement of charge (current) while the number of electrons stays the same everywhere.(7 votes)
Why is Conventional Current represented by an "i"?(3 votes)- The symbol for current is "I" or "i". It comes from the French word for intensity, as in "intensité du courant". This is the term André-Marie Ampère used to describe current.
Another reason is that "C" is already taken by Coulomb (the unit of charge).(5 votes)
- can i recharge my battries with convontional current?(3 votes)
- Ha! Yes you can. The electrons don't know we call them negative. They are still happy to recharge your battery.(3 votes)
- Charge is moving slowly, so why does a bulb glows so fast
I mean what does it have to do with the electric field ?
I would appreciate your answer.(2 votes)- The motion of electrons in a metal wire (the "drift current") is remarkably slow, something like a quarter millimeter per second. The reason this enough to make a bulb glow is there are SO MANY electrons moving.
Imagine the widest river you have ever seen, the Mississippi, or the Thames, or the Amazon. They are moving pretty slowly at their widest part. But if you draw a line straight across the river and count how many water molecules pass through the line in a second, you get an enormous number. Put that many electrons through a light bulb and you get a bright light.(6 votes)
Why do most circuits have a component called Ground (GND) if the battery and copper wire exchange electrons perfectly fine?(3 votes)- GND is not an actual component (even though the ground symbol looks like it might be one). GND is a designation we make to indicate which node in a circuit we choose to be the node with v = 0.
In electrical devices that have a power cord, the ground node is also connected via the power cord to eventually the actual ground outside. This is a safety feature.
In electrical devices that don't have a power cord (like a mobile phone or a flashlight), we still assign one node to be the reference node, and call it ground. If you ever use a circuit simulator program to model a circuit, it will require you to assign a ground node.(4 votes)
What is the opposite of conventional current?(1 vote)
That'd be electron current flow which goes from negative to positive.(4 votes)
2:26Video transcript
- [Voiceover] When we
start to study electricity, we need to get an idea of what is Current and what is Voltage and in two earlier videos, I talked about the idea
of current and voltage, current and voltage and,
and what they meant. And when we talked about current, it's easiest to describe current
when we talk about wires. Let's say we have a copper wire. We talked about a copper
wire and inside it was, there was electrons in it, and they have a negative charge, we know they have a negative charge, and if we put a voltage on them, those electrons would move
in some direction like that. So if I put a plus voltage over here and a minus voltage over here, the electrons are repelled by the minus voltage and they're attracted to the positive voltage. That is called an Electron Current. So talking about current in terms of what's actually
happening inside a wire makes some sense, it's easier
to understand current and that these electrons are moving around. And whenever we talk about this, we'll talk about it specifically that there's an electron
current going on here. Now at the same time,
what I said in that video, and I'll say again, is the convention for
describing current is this. This is called the
Conventional Current Direction. The convention we've had
for hundreds of years is that current is the direction that a positive charge would move if there was a positive charge there. So, whenever we talk
about current from now on, it'll always be conventional current, and in fact, we don't even need to mention conventional any more, it's just current. Current is the direction that
positive charges would move. If we ever talk about electron
current then we'll use the word, electron current. Now, as a reminder, when
we talked about voltage, uh, this was built up by analogy and the analogy was to electrons
rolling down a mountaintop, so here's our mountain, remember this? And I built a battery or another
voltage source like this, and we said, that what
a battery does is it pumps out energetic electrons, and they go down a hill. Roll downhill and go back into the, back into the positive
terminal of the battery. And when we design
circuits, what we do, is we, we put stuff in the
way of this electron on its path, and this is where
we build our circuits. So the, the electron current is going in this direction
here down the hill. The conventional current direction or the current direction is this way. So now, I'm gonna redraw
my circuit and my battery. I'm gonna flip the battery around it until the positive terminal is on the top, and I'll put my circuit over on the side over here like this. There's my circuit that I just built. Let's connect those circuits up like that. This is the plus side of the battery, this is the minus side. The plus side goes with the long bar and the minus side goes
with the short bar there. And the current direction here, the conventional current direction, or just plain current direction, is in that direction. Out of the positive and
back into the negative. From now on, this is
what we mean by current. And we know that the
electrons are in here. They're heading around this
way, like that, but that's okay. This is the, this is the nomenclature for conventional current
or just plain current.