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## Class 10 Physics (India)

### Course: Class 10 Physics (India)>Unit 3

Lesson 1: Electric current & circuit

# Intro to current (& Amperes)

Let's explore what electric current is, how we calculate it, and it's unit (Amperes) and it's direction. Created by Mahesh Shenoy.

## Video transcript

let's explore what electric current is and see how we can calculate it so we'll start with the word current current is a measure of how much something flows per second so let's write it down it's a measure of how much how much something it can be anything then we'll take examples something flows per second so for example if we were to calculate how much water flows per second in a pipe at some point that would be water current if we were to find how much air flows per second say in a tunnel at some point that would be air current but for electric current we calculate how much charge flows per second how much charge flows per second through say a wire at any point so let's take some examples so imagine we have a metallic wire over here and the speciality of metallic wires is they have a lot of electrons inside them which are free to move around lots and lots of electrons free to move around and so if we push these electrons then these electrons can flow and we'll see in future videos how we can push the electrons but once the electrons start flowing we have charge flowing because we have because electrons have are charged particles and so when electrons are flowing we have a current and so now let's say you stand over here and somehow calculate how much charge is flowing through this point through this point let's say you concentrate on this point of the wire and you calculate how much charge is flowing through this point and let's say you measure and you find out that there are 10 coulombs of charge 10 coulombs of charge falls through that point in five seconds in five seconds so what's going to be the current at that point well the current which we usually represent as I if we need to calculate how much charge force per second so how many coulombs are flowing per second so right now we have 10 coulombs per 5 seconds so that amounts to five goes two times we get two coulombs per second and we can now say that is the current at that point two coulombs per second which means every second two coulombs are passing by let's take another example let's say again you take a different wire different current and now you find out three coulombs flows in six seconds six seconds can you calculate what the current is going to be over here pause the video and see if you can try this yourself first all right now the current I is going to be we have three coulombs for six seconds and 3/6 is one or two and so you get half so the current over here is half columns for a second and that's how you calculate current so let's generalize this let's see if we can come up with a general formula to calculate the current in general if we find let's say Q amount of charge flows in T amount of seconds then we can now write current in general is going to be the amount of charge Q divided by the time for which you waited divided by T and if you look carefully that's what we did in both the cases we will divide by divided the charge by time and that's how you calculate the current in general and so the unit for current is coulombs per second and coulombs per second is often called amperes amperes which we write capital a in short and this is named after this French scientist Andre ampere who did a lot of work on this in in electricity so over here we could say this current is two amperes amperes just means coulombs per second so we're here also we could say this is 0.5 amperes we'll just take one more example so that we are super clear with this so let's go down a little bit let's say we have a current somebody says we have a current of 3 amperes what does it mean well it means there are three coulombs of charge passing every second now can you calculate how much charge passes in five seconds so if you wait for five seconds how many charge would have passed through that point pause the video and see if you can try this yourself all right let's see we already know that three coulombs are passing every second since we want to calculate in five seconds how much it Cal how many charges passes we just multiply this by five five seconds second second cancels and we get fifteen coulombs fifteen coulombs or you could have also used this formula is the same thing notice we are multiplying the current with time so if you multiply the current with time you get the charge one last thing we can talk about is the direction of the electric current so let's say all the electrons over here for example are flowing to the right and I ask you what direction is the electric current and you may say intuitively it's to the right because all the electrons are flowing to the right but it turns out it's the opposite the direction of the electric current we chose is the opposite direction of the flow of electrons and you may be wondering why did we do that well the main reason for this is because electrons are negatively charged particles you see when we were when we assign the direction of the electric current we had no idea about electrons and protons and everything it was way before we discovered them we just thought there is some kind of positive charge that is moving in a wire and that's what's giving us current and therefore we chose the convention for the current as the direction of flow of positive charges and because electrons turn out to be negatively charged particles we have no option now we have to choose the convention to be in the opposite direction of electrons so if the flow if the flow is of positive charges the current will be in the same direction if the flow is of negative charges the current will be in the opposite direction since electrons are negatively charged particle the current direction we choose is in the opposite direction so to quickly summarize what we learned electric current is how much charge flows per second through any wire at a given point we saw how to calculate the electric current we saw its units are coulombs per second which is also called amperes and finally we saw that the direction of the current that we choose is the direction in which a positive charge moves now because in all the wires which are metals it's always the electrons that are going to move and electrons are negatively charged particles almost in all the electric circuits and wires that we'll be dealing with the direction of the flow of the direction of the current is always going to be in the opposite direction of the flow of the electrons