**Concept:** Electric Charge

Hey guys. In this video we're going to be talking about electric charge, this is the first video in the series on electricity, an electric charge is a central theme in electricity, let's get to it. First I just want to briefly review atomic structure, this is something we've all seen before and it's not really a big deal, I just want to make sure that we're all on the same page. Remember, that atoms are built up of three things, they're are built up of protons neutrons and electrons. Remember that the protons and the neutrons sit inside the nucleus and the electrons are free to orbit around the nucleus. Alright, this is something we've seen before, this is super basic no big deal. Now, let's get to electric charge, what electric charge is and by the way it's given by the letter Q, what electric charge is, is it's a property of matter, that's very similar to mass on the outset it might not seem like they're all that similar but as I'm about to explain, we can actually draw a lot of similarities between the two. First off mass is a requirement for gravitational force, no mass no force, similarly electric charge is a requirement for the electric force, no charge no force, the actual number that represents mass tells us how strongly that object experiences gravity, the more mass the more gravity, similarly the actual number that represents electric charge tells us how strongly that object experiences electricity, the more charge the more electric force, the key difference between the two is that mass is always positive, right? We haven't seen any negative masses before, no object with negative two kilograms of mass, electric charge on the other hand can be both positive and negative alright? Now, a number that's very important to remember is something called the elementary charge which is represented by lowercase e it's 1.6 times 10 to the negative 19 coulombs, that capital C is a unit of charge coulombs, why is this number so important? Well, protons carry a charge of positive e and electrons carry a charge of negative e, these elementary particles carry the elementary charge.

Now, the charge on an object is going to be a quantity of imbalance, the imbalance between the electrons and the protons within the material, what do I mean by this? Let's take a look at these three atoms that I have, the first hypothetical atom has two protons and two electrons, right? So, there is no imbalance between protons and electrons and the total charge is 0, the second atom has four protons and three electrons. So, there is an imbalance between the protons and electrons, there's one more proton and so the charge is positive e, the charge of that extra proton, this last atom has two protons and three electrons so the imbalance is one electron and the charge is negative e, okay? Notice something important about this, these charges always come in whole multiples of e, okay? They don't come in fractions of e because in order to do so you will need a fraction of a proton or a fraction of an electron and that's just not possible, we call this fact charge quantization, the fact that charge always comes in whole multiples of e. Now, quantization might seem like a really scary word but don't be afraid of it, all it means is the simple fact that charge on object always has to come in whole multiples of e, this will be a word that appears in your textbook so you want to be familiar with it, okay? A sort of quick and dirty way of calculating the charge on any object is using this equation provided for you guys, this is just a number of protons minus the number of electrons times that elementary charge, that will tell us Q the charge on our object, okay? Very easy to use. Now, something, that's important to remember is the fact that most materials are naturally neutral, okay? Normally the number of protons exactly balances the number of electrons so there is no imbalance and therefore there is no charge, okay? Unless the problem specifically tells you that a material is charged do not assume that it is charged, alright? that wraps up our video on electric charge, thanks for watching.

**Example:** Charge of Atom

**Example:** Electrons In Water (Using Density)

Hey guys, let's do another example about electric charge, okay? Water weighs one kilogram per liter has a molecular weight of 18 grams per mole and has 10 electrons per molecule.

Part A how many electrons does two liters of water have? An Part B, what charges, what charge do these electrons represent? So, how many electrons do we find in Part A? what is the charge of those electrons, okay? So, for Part A. First, what we want to do is we want to figure out how to get from liters which is what we're given, we're given two liters of water to number of electrons, this will tell us how to solve the problem, we need to create a sort of map to the solution, okay? Let's start with liters, right? Because that's what's given to us, what can we go to next? Well, we're told that there's a conversion between kilograms and liters that we can say for every liter of water it has a mass of one kilogram. So, we know how to go from liters to kilograms, next we have grams to moles. Now, we don't have kilograms to moles but we know right away that one kilogram is 1,000 grams. So, we can easily go from kilograms to grams and then using the conversion go from grams to moles. Now, our last conversion is electrons per molecule, we don't have our number of molecules yet, we have in moles but we can use Avogadro's number to convert moles to molecules. Now, using our last conversion factor, we can go from molecules to number of electrons. So, this right here is our map, that's going to guide us through this problem, okay? So, let's start doing these conversions, 2 liters of water times 1 kilogram per liter is 2 kilograms. So, our water has a mass of 2 kilograms now right away, we know that that's equivalent to 2,000 grams, okay? So, we've done this step and this step. Now, we need to go from grams to moles, okay? 2,000 grams times 18 grams per one mole is about 111 moles. So, we've done the next step. Now, we need to go from moles to molecules using Avogadro's number 111 moles times 1 mole per 6 times 10 to the 23 molecules is 6. Seven times 10 to the 25 molecules of water, okay? So, we've done this step, the last step is simply to figure out how many electrons are represented by this much water as many molecules of water, we know that it's 10 electrons per molecule. So, it's very simple, we just multiply this number by 10, 6.7 times 10 to the 26 electrons, okay? And we followed our map successfully from liters which was given to us to electrons, okay? Now, part be, what charge does this amount of electrons represent? Well, each electron has a charge e, the elementary charge and we have some number of electrons in which we figured out in part A. So, multiplying these together will tell us our total charge our number is 6.7 times 10 to the 26 and the elementary charge is what? Remember, guys you need to know this 1.6 times 10 to the negative 19 coulombs multiplying those together we get a total charge of 1.07 times 10 to the eighth, cool? Okay.

**Problem:** How many electrons do you have to add to decrease the charge of an object by 16uC?

An object has a positive charge of 5.0 μC. What is the net number of electrons lost/gained?

A. 1.602 x 10^{-19} lost electrons

B. 3.12 x 10^{13} lost electrons

C. 5.0 x 10^{6} lost electrons

D. 5.0 x 10^{6} gained electrons

E. 3.12 x 10^{13} gained electrons

Watch Solution

A metal sphere is insulated electrically and is given a charge. If 50 electrons are added to the sphere in giving a charge, how many Coulombs are added to the sphere? (e= 1.6x10^{-19} C)

A) -50 C

B) -8.0x10^{-18} C

C) -8.0x10^{-20} C

D) -80 C

Watch Solution

The charge on the proton is the same as

A) the charge on the neutron.

B) the charge on the electron.

C) the charge on the atom.

D) the charge on the electron, but of the opposite sign.

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Describe the experiment, which established that atoms contain a small nucleus, which contains almost all of its mass.

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What is the charge on an unknown ion which contains 250 protons, 270 neutrons, and 244 electrons?

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