mr.p: Good morning.
Today we are going to review the electrostatics topics
covered on the AP Physics I exam.
Bo: Hey guys.
Billy: Hey Bo. Bobby: Hi Bo.
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mr.p: Let's start with the concept of the elementary charge.
1.6 times 10 to the negative 19 Coulombs.
It is called the elementary charge,
because it is the smallest charge ever to be isolated.
Two examples of particles with this magnitude charge
are the proton and the electron.
They both have the same magnitude charge,
the elementary charge;
however, the electron has a negative charge,
and the proton has a positive charge.
The electron is a fundamental particle; the proton is not.
The proton and the neutron are made up of quarks,
more specifically up and down quarks.
A proton is composed of two up quarks and one down quark,
and a neutron is composed of one up quark
and two down quarks.
Because the charge on an up quark
is equal to positive 2/3 times the elementary charge,
and the charge on the down quark
is equal to negative 1/3 times the elementary charge,
the net charge on a proton is equal to the positive
of the elementary charge,
and the net charge on the neutron is zero.
[Bo] Hold up, Mr. P. Mr.p: Yes, Bo?
Bo: I thought you said nothing could have a charge
less than the elementary charge.
Billy: Yeah, a quark has a charge less than
1.6 times 10 to the negative 19 Coulombs.
That doesn't make any sense.
Bobby: Actually, what he said is that no isolated charge
can have a charge smaller than the elementary charge.
Right?
mr.p: Correct, Bobby.
A single quark has never been isolated.
We've never found a quark by itself.
They're always found in groups,
like they are in the proton and the neutron.
Okay, next let's talk about the Law of Charges.
Bobby, what is the Law of Charges?
Bobby: Umm.
Billy: Unlike charges attract.
Bobby: And like charges repel. That's right.
mr.p: Therefore, if we have two like charges,
two positive charges, for example,
they will repel one another.
And if we have two charges that are unlike,
positive and negative, or negative and positive,
they will attract one another.
And if we have two negative charges,
they are again like charges,
and they will repel one another.
The magnitude of this force
can be found using Coulomb's Law.
Coulomb's Law gives us the electrostatic force
that exists between two charges.
k is called Coulomb's constant,
and has a value of 8.99 times 10 to the ninth
Newtons times meters squared divided by Coulombs squared.
q1 and q2 are the charges of the two particles, and r...
Well, actually, let's bring up a similar equation,
so that we can talk about the value of r.
Newton's Universal Law of Gravitation,
what I like to the Big G equation.
Notice the similarities.
We have Coulomb's constant instead of
the universal gravitational constant,
and we have charges instead of masses;
however, we have the same r,
which means, class, r is not?
All: The radius.
mr.p: r is?
All: The distance between
centers of charge of the two charges.
mr.p: This can be confusing, because sometimes r is?
All: The radius.
Bo: Again.
mr.p: Correct, r is defined as the distance
between the centers of charge of the two charges.
Now, Coulomb's Law functions very much
like Newton's Universal Law of Gravitation,
but it is very important to underscore
the difference in magnitudes between these two constants.
Coulomb's constant is times 10 to the ninth.
Universal gravitational constant is
times 10 to the negative 11th,
which means that Coulomb's constant is
approximately times 10 to the 20 greater
than the universal gravitational constant.
Which means, typically, the electrostatic force
is much, much, much greater
than the gravitational force.
Okay, now let's talk about conservation of charge.
Charge is conserved in an isolated system.
For example...
Let's say we have two spheres that are
electrically isolated from everything else.
The net charge on the first sphere
is positive four Coulombs.
The net charge on the second sphere
is negative two Coulombs.
We're going to touch those two spheres together,
and then separate them, and the question is,
what are the final net charges on each of the two spheres?
Billy?
Billy: Well, the system is isolated,
so the total net charge will be conserved.
Therefore, the total net charge is
positive four plus negative two Coulombs,
or positive two Coulombs.
That's the total net charge.
I'm not sure what the net charge
on each sphere will be though.
Bo: Remember, like charges repel, so it will balance out
with half the total charge on each sphere.
Billy: Oh, so then each sphere will have a net charge
of positive one Coulombs after we pull them apart.
Bobby: Does that mean that each sphere will have a total
of 1.6 times 10 to the 19 protons on them?
mr.p: Actually, no, Bobby.
Each sphere doesn't end with 1.6 times 10 to the negative 19
protons on it. Let's figure that out.
The net charge on one of the spheres
is going to be equal to excess number of charges
on that sphere, multiplied by the elementary charge.
Which means, the excess number of charges
on one of the spheres is going to be equal to
the net charge on that sphere,
divided by the elementary charge.
And because the net charge on each of the spheres
is positive one Coulombs,
we can divide that by the elementary charge,
with more sig figs, 1.6022 times 10 to the negative 19
Coulombs per proton, and that gives us approximately
6.24 times 10 to the 18th protons.
Bobby: Oh, right, sorry.
Each sphere will have a total
of 6.24 times 10 to the 18 protons.
mr.p: Actually, no. Each sphere will have an excess
of 6.24 times 10 to the 18th protons.
Each sphere has a heck of a lot more
protons and electrons than this.
It's just that each sphere has
6.24 times 10 to the 18th more protons
than it does electrons.
Bobby: Right, that is not just the sphere's electric charge.
It is the net electric charge.
Billy: Wait, there was only one sig fig on positive one Coulombs,
so our answer should only have one significant digit.
Bo: Yeah.
mr.p: We have reached the end of my review
of electrostatics for the AP Physics I exam.
My next review lesson is about electricity.
You are more than welcome to enjoy that video.
You may also visit my website,
flippingphysics.com where you will find
all of my AP Physics I review lessons,
organized with lecture notes.
Thank you very much for learning with me today.
I enjoyed learning with you.