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Current time:0:00Total duration:5:28

Video transcript

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 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 is a positive current mean so I'm going to move this up a little bit and we're going to 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 item moves when I assign a positive direction to the current the overall positive direction is going to be in this direction that's plus current plus current all right 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 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 minus 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 we're moving in this direction here I would say the current is plus 10 charges per second in this direction and you ask yourself if if it's always electrons carry current why why on earth do we point the arrow in the other way and that actually goes back way back in history to our friend Ben Franklin Bren 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 oops I n and that was in the year 17:47 Ben Franklin is 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 JJ Thompson 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 is often asked by new students is well why don't we switch it around so the current points this way why don't we 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 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 in 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're going and after the change everybody would get there going but the changeover would be just so so costly and the same thing in 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 an electric going in 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 that's the definition of current in a nutshell we put a boundary across and you watch the charges going through in either direction whichever direction they do and you count up and you get Q per second that's current