Hi. It's Mr. Andersen and this AP Physics essentials video 26. It is on the electric
force. On the left of this screen there is an electric field that you can not see. It
is invisible. And to figure out where it is we are going to use a positive test charge.
We are going to put a positive charge there and we are just simply going to let it go.
And I will mark its original location. Watch what happens. Well it is accelerating away.
That means it must be experiencing a net force. And we call that an electric force that is
being created by a charge inside this field. So let's move that test charge somewhere else.
You can see it is accelerating away a little bit more quickly. Now let's put it right here.
So it really accelerated quickly. That means that there must be a charge somewhere around
here and it is a positive charge because it is pushing that positive test charge away.
And so we could represent that field with these field lines. It is a vector electric
field. And if you put any object inside an electric field it is going to experience an
electric force. And that is going to cause an acceleration in the opposite direction.
If it is a positive charge and towards that charge if it is a negative charge. And so
you should be able to understand how an electric field is set up. If we have a positive charge
in the center the field lines are going to move away from that. And by convention if
it is a negative charge on the inside the field lines are all going to point towards
it. But we could have two plates that both have a charge. And in that case the field
lines are going to go from one plate to another. Now it is not exactly likely this. This would
have to be infinitely long plates. It will actually curve on the side. But let's just
assume that they are perpendicular to those plates. And so if we put any object inside
that electric field it is going to experience a force that is a vector force in newtons.
And we call it the electric force. Now what makes that up? Well two things. It is going
to be q multiplied times E. And so what is q? That is going to be the charge of that
object that we are putting inside the electric field. And then what is E? It is the electric
field strength. And that is going to be a vector and we call that, or the units are
going to be in newtons per coulomb. Sometimes you will see that electric field strength
as volts per meter. But those are going to be equivalent. They are exactly the same thing.
And so let's say we put a positive charge in this electric field, we are going to have
a positive q. And so that electric field is going to act on it. It is going to have a
force that is going to go along the field. And so you are going to see acceleration in
that direction. If we take a negative charge that has a negative q, and so it is going
to experience a force towards that electric field. And so it is going to go in the opposite
direction like that. And so you should be able to understand a few electric field maps
in AP Physics 2. First of all if we have a positive point charge all of those field lines
are going to move away from that. And if we have sphere that has symmetrical distribution
of charges, the same thing. If we have got test charge and put it inside that electric
field it is simply going to accelerate away. If we have a negative point charge, you can
see all of the field lines are pointing towards that charge. And same thing, if it is a sphere
that has an equal distribution of those negative charges it is going to be the same exact kind
of a field map. If we add a positive charge now, where is it going to go? It is going
to move right towards that charge. And then if we have plates that have positive on one
side and negative on the other side you can see that the field lines are going to move
from the positive to the negative. And so if I put a charge inside there, where is the
force going to be? It is going to be towards the negative and it is going to accelerate
in that direction. And so here is our equation. Force is equal to q E, where force is that
force experienced by that object. q is going to be the charge of that object measured in
coulombs. And then the electric field strength is going to be the field strength of that
electric field. And that is going to be measured in newtons per coulomb. And you may have to
do some simple problems related to this equation. And so let's say I ask you to calculate the
electric force acting on a proton if the electric field strength is 2.1 times 10 to the 5 newtons
per coulomb. So how are you going to solve that? Well let's write the equation again.
So force is equal to q times E. What are we given? You can see here that we are given
the electric field strength, so we know E. But how are we ever going to figure out force
if we do not know q? Well they are telling us that it is a proton. So what is the charge
of a proton? It is going to be that elementary charge, 1.6 times 10 to negative 19 coulombs.
And so I just multiply that times the electric field strength and I am going to get a force
3.4 times 10 to the negative 14 newtons. That is the force that is being applied to that
particle. And so let's say you are given a question like this. Calculate the electric
field strength if an electron, which we now know that is a negative elementary charge,
experiences a 1.7 times 10 to the negative 8 newton force? Well you could solve this
problem. I am not going to give you the answer, but to solve it we are just going to have
to plug in those two values that we know. And in this we are going to solve for the
electric field strength. And sometimes you will have to solve for that charge of the
objects. And all you would use is that same formula. We are just solving for the unknown.
So did you learn to predict the direction and magnitude of a force exerted on an object
in an electric field? Remember if it is a positive object it is going to move with the
field lines. If it is negative it will move in the opposite direction. And then can you
solve for one if you are given the other two? Either electric field strength, force or the
charge of that object? Well I hope so, and I hope that was helpful.