- Electric current
- Current direction
- 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
Current is the flow of charge. We measure current by counting the amount of charge passing through a boundary in one second. Created by Willy McAllister.
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- The video states that water does not conduct. Then why do you have to get out of a pool in a lightning storm? If it's not conductive why be cautious about the storm?(17 votes)
- Water free of any chemicals such as tiny conductors and impurities inside the body of water does not conduct electricity, but water that has tiny conductors and ions such as salt can conduct electricity very efficiently. So I think that what the video states is that pure water does not conduct electricity. Also, to answer your question about why one needs to get out of a pool during a thunderstorm is because pools are filled with lots of chemicals which many of them may conduct electricity.(50 votes)
This is a question I've had since I knew what batteries even were... If the electrons flow from the battery into the conductor, then back through the wiring into the battery, why do batteries loose their charge over time? Wouldn't the electrons flowing back into the battery "recharge" it to an extent? I think the answer has something to do with energy loss during the conversion of energy, but I want to know for sure.(54 votes)
We must ask what caused the charges to separate in the first place.
The answer: chemical energy did work to make them separate. The amount of work is the very definition of voltage:
Voltage = energy required / amount of charge = Joules / Coulomb
As the chemical energy is depleted so follows the voltage.
- When it comes to electrical current, how does the electrical current get all over the world in a mater of seconds, to a point where everyone can use phones, computers(etc.)at the same time when billions of people use it?(16 votes)
- Hello Mathomon,
My hero Admiral Grace Hopper had a good way of explaining this. Ref:
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- Could current be considered a form of flux?(12 votes)
- Yes definitely current in charges is analogous to flux in magnetic fields.Here voltage is called the driving force and in case of flux mmf (Magneto motive force) is called the driving force.(4 votes)
- I have a question based on the conventional current direction and electron flow... why are they opposite in direction? if electron flow direction is the actual direction, the why did scientists knowing that flip it the other way around and make it confusing?>>>>(10 votes)
- The theory of charge and current has deep historical roots. Their definitions do not actually depend on the existence of the electron and proton (seems strange, but there is nothing in the theory of charge that predicts the existence of the electron.) The seemingly "backwards" arrow came about a long time before the discovery of the electron.
Scientists would "fix" this if it was actually a problem. The fix would be rather painful, since we would have to redraw every schematic and rewrite every textbook and republish every scholarly paper. And that would be to fix a problem that does not actually exist.
Yes, the arrow direction is confusing to beginners, but, I promise you if you move ahead and learn about Ohm's Law and solve a few resistor circuits you will get used to the arrow direction and your confusion will melt away.
I wrote this introduction to Charge that might help... https://spinningnumbers.org/a/charge.html, and here's another relevant article... https://spinningnumbers.org/a/conventional-vs-electron-current.html(2 votes)
- What is the meaning of steady current?what are its sources?DC or AC which one is steady current?(7 votes)
- DC would be considered a "stable current" because the current isn't fluctuating. AC on the other hand is basically what it says, Alternating Current.(1 vote)
- if copper is a good conductor because it has one valence electron then why not Na or K used as conductors.(3 votes)
- Sodium (Na) and potassium (K) in pure form are extremely reactive. If you put a drop of water on pure metallic sodium there is a violent reaction with flames. Not something you want to build circuits with.(8 votes)
- So I have been thinking ever since my mom got me a battery charger to literally charge the batterys that were dead when ever I played Wii. I was wondering how does the electrons from an outlet end up back in a battery then when its time to be used still powers what you are using the battery for? Is this the same way people use to charge there electronics or is it a different way when you plug in the charger(4 votes)
- Rechargeable batteries contain chemical reactions that can be forced to run both ways. Non-rechargeable battery chemistry does not have this feature.
In normal operation, electrons come out of the negative terminal and flow through your device (Wii, or phone, or toothbrush).
When it is time to recharge, you connect a recharging device to the battery. It takes AC current out of the wall, converts it to DC, and applies a voltage to the battery that's a little higher than it is used to. Typically 18% higher than the battery's rated voltage.
That extra-high voltage causes the chemistry in the battery to run backwards, so electrons go into the negative terminal.
The power circuit is designed so you can operate your device at the same time you are charging. If it takes 0.5 amp to charge the battery and 1 amp to run the device, then there is 1.5 amp being pulled out of the wall.(5 votes)
- 8:08How is current actually sent through a salt water solution? In the solution there are Na+ ions that are drawn towards the negative wire lead (attached to the negative battery terminal), and Cl- ions drawn towards the positive wire lead (attached to the positive battery terminal). If all the Na+ ions congregate to the negative end, and all the Cl- ions congregate to the positive end, then there are no ions in between and current flow stops. So how does this solution produce consistent current flow.(4 votes)
- Once an ion deposits its charge, its forced to the back; crowded out to the back where it picks up another charge. They don't congregate, they pick up, deposit and then move to the back.(3 votes)
- How would you know how many orbiting electrons would be on each line? For copper, there would be a certain amount of electrons on the outside, but what about Zinc, for example? How many electrons would be orbiting on the lines around the Zinc nucleus? Also, speaking of lines, how many lines would there be? Is there a specific amount of electrons allowed on a line before a new one is created, or is a line even created? Is there an infinite amount of electrons on each line?(2 votes)
- You ask a number of deeply interesting questions about atomic structure. Dig into this section of Khan Academy Chemistry: https://www.khanacademy.org/science/chemistry/electronic-structure-of-atoms(5 votes)
- [Voiceover] All right, now we're gonna talk about the idea of an electric current. The story about currents starts with the idea of charge. We've learned that we have two kinds of charges, positive and negative charge. We'll just make up two little charges like that. And we know if they're the opposite sign, that there'll be a force of attraction between them. And if they have two like signs, here's two charges that are both positive, and these charges are gonna repel each other. So this is the basic electrostatics idea, and the same thing for two minus charges. They also repel. So like charges repel, and unlike charges attract. That's one idea. We have the idea of charge. And now we need a place to get some charge. One of the places we like to get charge from is copper, copper wires. A copper atom looks like this. Copper atom has a nucleus with some protons in it, and it also has electrons flying around the outside, electrons in orbits around the outside. So we'll draw the electrons like this. There'll be orbits around this nucleus. Pretty good circles. And there'll be electrons in these. Little minus signs. There's electrons stacked up in this. And even farther out, there's electrons. So there's kind of a interesting looking copper atom. Copper, the symbol for copper is Cu, and its atomic number is 29. That means there's 29 protons inside here, and there's 29 electrons outside. It turns out, just as a coincidence for copper, that the last orbital out here has just one electron in it, that guy right there. And that's the one that is the easiest to pull away from copper and have it go participate in conduction, in electric current. If I have a chunk of copper, every copper atom will have the opportunity to contribute one, this one lonely electron out here. If we look at another element, like for instance silver, silver has this same kind of electron configuration, where there's just one out here. And that's why silver and copper are such good, good conductors. Now we're gonna build, let's build a copper wire. Here's sort of a copper wire. It's just made of solid copper. It's all full of copper atoms. And I'm gonna put a voltage across this. There's our little battery. This is the minus sign, this is the plus side. And we'll hook up a battery to this. What's going on in here? Inside this copper is a whole bunch of electrons that are associated with atoms. It's a neutral piece of metal. There's the same number of protons as there is electrons. But these electrons are a little bit loose. So if I put a plus over there, that's this situation right here, where a plus is attracting a minus. So an electron is gonna sort of wander over this way and go like that. And that's gonna leave a net positive charge in this region. So these electrons are all gonna start moving in this direction. And down at the end, here, an electron is gonna come out of this battery, travel in here, and it's gonna go in there and make up the difference. So if I had a net positive charge here from the electrons leaving and going to the left, this battery would fill those in. And I'm gonna get a net movement of charge, of negative charge, around in this direction, like this. The question is, how do I measure that? How do I measure or give a number to that amount of stuff that's going on? So we wanna quantify that, we wanna assign a number to the amount of current happening here. What we do is, in our heads, we put a boundary across here. So just make that up in your head. And it cuts all the way through the copper. And what we know, we're gonna stand right here. We're gonna keep our eye right on this boundary down in here. As we watch, what we're gonna do is, we're gonna count the number of electrons that move by here, and we're gonna have a stop watch and we're gonna time that. So we're gonna get, basically, this is charge, it's negative charge, and it's moving to the side. What we're gonna do is, at one little spot right here, we're just gonna count the number that go by in one second. So we're gonna get charge per second. It's gonna be a negative charge moving by. That's what we call current. It's the same as water flowing by in a river. That's the same idea. Now I'm gonna set up a different situation that also produces a current. And this time, we're gonna do it with water, water and salt. Let's build a tube of salt, of salt water, like this. We're gonna pretend this is some tube that's all full of water. I'm also gonna put a battery here. Let's put another battery. And we'll stick the wire into there. We'll stick the wire into there. This is the plus side of the battery, and this is the minus side of the battery. Water is H20, and this does not conduct. There's no free electrons available here. But what I'm gonna do is, I'm gonna put some table salt in it. This is ordinary salt that you put on your food. It's made of sodium, that's the symbol for sodium, and chloride, Cl is chloride. Sodium chloride is table salt. If we sprinkle some table salt into water, what happens is, these dissolve and we get a net plus charge here and a net minus charge on the chlorine. So out here is floating around Na's with plus signs and Cl's nearby, really close nearly, with minuses. Let's keep it even. Now, when I dip my battery wires into this water, what's gonna happen is, this plus charge, this plus charge over here from the battery is gonna attract the minus Cl's. So the Cl's gonna move that way a little bit. And over here, the same thing is happening. There's a minus sign here. There's a minus from the battery. That's going to attract this, and it's also gonna repel Cl minuses. So what we get is a net motion of positive charge, plus q going this way, and we get minus q going this way. How do we measure that current? How do we measure that current? Well, we do it the same way as we did up there with copper. We put a boundary through here in our heads. We stand here and we watch the charges moving by. What we're gonna get is some sodiums, Na's, moving this way, and chlorines moving this way. Just like we showed here. Na moving this way. So there's gonna be plus charges moving through the boundary and minus charges moving through the boundary in the opposite direction. If I take the total sum of that. For example, if I see one Na go this way and one Cl go this way, that's equivalent to two charges moving through the boundary. Hope that makes sense. It's equivalent to two charges, one going this way and one going this way. Because they have opposite signs, they add together and make two charges. In this case, current is equal, again, to charge per second.