Main content

## Class 10 Physics (India)

### Unit 3: Lesson 1

Electric current & circuit# Unit of charge (Coulombs)

Let's explore what the unit of electric charge (Coulombs is) . Created by Mahesh Shenoy.

## Video transcript

- [Instructor] In a
previous video we discussed what Electric Charge is. Basically it's a property that
protons and electrons possess because of which they can attract and repel other protons and electrons. We call this "The Electric Force." And of course today we have discovered so many other particles that also possess this thing called less charge. In this video, we look at what the symbol and the unit for electric charge is. So the symbol for the
electric charge is q. And we measure it in a
unit called Coulombs, Coulombs. Denoted by a capital C. So just like how we
measure mass in kilograms and we measure height in say, meters, we measure charge in Coulombs. And the word Coulumb is
named after the scientist the French Scientist, Charles Coulumb, who did a lot of work in investigating the force between these charged particles. And to give you a sense
of how big a Coulumb is, turns out that the charge on an electron or a proton is a very, very
small fraction of a Coulumb. So if you were to look at
the charge on an electron which we usually represent as e, it turns out to be so small. Let's see, we write it as 0.00, and there are so many zeros
I can't write them all zero one six and some
other numbers, Coulombs. And the number of zeroes over here are 18. There are 18 zeroes in between. Just goes to show you how small the charge on the
electron or the proton is. So the electron has this much
charge, but it's negative. Proton also has the same
value of the charge, but it's positive. So they have the same values, but one has positive charge and the other one has negative charge. And of course, since it is so small we often like to represent this
in the scientific notation. So we will shift this
decimal all the way to here. Now, when you write it
as 1.6 x 10 to the power, now let's see how many, how many times we have shifted it. To shift from here to
here, we shift 18 times and then one extra shift,
19 shifts to the right and so we'll write it as
negative 19 over here. So, so many Coulombs is the charge on an electron or a proton. Nelson's Coulomb is a
brand new unit for us. It would be nice if we
could get some sense for how big it is. Just like how we have some
sense for what a kilogram is or how big one meter is, it would be great to
have some sense for what or how big a Coulumb would be. So to do that, here's how
I like to think about it. Now that we know that the charge
on an electron or a proton is such a small number, for electrons it would be negative, for protons it would be positive. We could ask ourselves,
how many, say electrons would make up a total
of one Coulomb charge? You get it. So it's gonna be a huge number because charge on each electron is so tiny we need a lot of electrons
to make up one Coulomb. But exactly how much, is the question So let's think about this. Let's go down a little bit. And so here's the question. How many electrons together make up a negative one Coulomb worth of charge? So, great idea to pause the video first and see if you can try
to do this yourself. Using this information try to figure this out yourself first. All right, let's do this. So to do this, we know one electron, so let's write that down. We know one electron is worth a charge of 1.6 x 10 to the minus 19 Coulombs. And since it's an electron
we're dealing with, it would be negative. But I want one Coulomb
worth of charge, not this. So to make this right
hand side one Coulomb, I will divide the right-hand side. Let's divide the right-hand side by 1.6 x 10 to the minus 19. So that this will cancel and
become negative one Coulomb. So here you have to divide
left-hand side as well by 1.6 times 10 to the power minus 19. So on the left-hand side,
we have one divided by 1.6. Let's bring in my calculator
and see what that gives me. So it'd be one divided by 1.6. That will be 0.625. So that will be on the
left-hand side, 0.625 that's one divide by 1.6, and this will become 10 x
10 to the power plus 19, plus 19. And that so many electrons is equivalent to now a charge of negative one Coulomb. So to get one Coulumb worth of charge, we need so many electrons, so many and of course we can also
write it in proper note to be 6.25 x 10, to
the power 18 electrons. We shifted one zero by borrowing one 10. So anyways, these many electrons such a huge number 10 to the 18 electrons, these many electrons together make up a charge of negative one Coulomb. And so when I went to ask myself, how big is a Coulomb? I like to think of it as one Coulumb is equal than to so many electrons. You take so many electrons together, that itself represents what a Coulumb is. That's how I like to think
about what one Coulumb is. So to summarize what we
learned in this video, is the unit for electric
charge is Coulombs. We saw what the charge on the
electrons and the protons are. And we also saw that one
Coulomb, how big a Coulumb is. It's equal into so many
electrons or so many protons. Because the protons and electrons have the same value of charge. But one is positive, the
other one is negative.