An electric field is…
Well it’s kind of like…
What the heck is an electric field?!
Let’s start first with another question.
So let’s say we’ve got some charge, let’s make it a positive charge.
We’ll call it Beverly.
So we’ve got our positive charge Beverly, and all of a sudden another charge comes into
the vicinity of Beverly.
It’s a positive charge as well, and let’s call this charge Ethel.
So we’ve got two positive charges, Beverly and Ethel, so what’s going to happen?
Well because of our prior knowledge of charges we know they are going to repel.
So this seems pretty simple, straight forward even, but let me ask you this.
What is causing this repelling force?
Is it a contact force?
Well no because there’s no actual contact…
Is it a gravitational force?
Well electric charges don’t have mass so that’s not it.
How about a psychological force?
Well electric charge doesn’t have consciousness so that’s not it.
Well then what?!
It’s the electric field!!
See I did know!!
The electric field.
It’s pretty much responsible for all of the influence that electric charge has over
one another.
So the repelling force experienced by Beverly and Ethel is due to their electric fields.
And if Ethel were a negative charge, the attractive force would be due to both of their electric
fields as well.
The electric field is responsible for all of the influence that charge has over one
another.
But the electric field of a charge does not have uniform strength, it doesn’t have the
same strength everywhere.
Say we placed Ethel here, quite close to Beverly.
She would experience a stronger repelling force than if we placed her say here, relatively
far from Beverly.
So how can we accurately represent this varying change in strength?
Some kind of lines of the field?
That’s right!
Field lines!
So field lines were an idea that were introduced to help us visualize what was going on with
an electric field.
They’re actually pretty useful, and they look something like this.
So by convention positive charges have field lines with arrows pointing out and negative
charges have field lines with arrows pointing in.
I’m going to talk about the arrows a bit more in a sec, but right now I just want to
explain how electric field lines help us visualize or interpret the electric field strength of
a charge.
So where the lines are close together, or we can say where the density of the lines
is high, that’s where the electric field is strong.
Where the density of the lines is low or where the lines aren’t so close together, that’s
where the electric field is weak.
So let’s say we draw a circle around our charge Beverly, quite close to her, here the
electric field lines are pretty close together right they have high density, so here the
electric field is strong.
Now if we draw circle around Beverly again a bit further away, the lines aren’t so
close to each other they have lower density so here the electric field is weaker.
So that’s a really nice representation of the varying strength of electric field.
Where the density of the lines is high, the electric field is strong.
Where the density of the lines is low, the electric field is weak!
So coming back to those arrows I mentioned that positive charges have field lines with
arrows pointing out and negative charges have field lines with arrows pointing in.
This is just convention and it’s meant to demonstrate what would happen to a positive
charge if it were placed in an electric field.
So let’s bring back our other positive charge Ethel and we’ll call her a test charge.
So if we place test charge Ethel in the electric field of Beverly, Beverly is a positive charge
so her electric field lines point outward.
And this is to demonstrate that when you place the positive test charge Ethel in Beverly’s
electric field she will follow the trajectory of the arrows pointing outward.
And this makes sense because they’re both positive charges so they’re going to be
repelled.
So if Ethel just follows the direction of the arrow, this is repelling because she’s
being pushed away.
So now if we replaced Beverly with a negative charge, this new negative charge would have
electric field lines with arrows pointing in.
So if we placed test charge Ethel in the electric field of this new negative charge, she would
still be following the direction of these arrows because she would be drawn toward this
negative charge because opposite charges attract.
So the electric field line arrows indicate the direction that a positive test charge
would move if placed in the electric field.
So the formal definition of an electric field at a particular point is E is equal to F divided
by q.
So let me break this down.
E is the electric field, and we can see that it’s a vector field as it has direction.
Q is the magnitude of the test charge placed in the electric field.
So in this case it was Ethel but it an be any magnitude or any test charge.
F is the force experienced by the test charge, due to the electric field.
So to sum up, this just means the electric field magnitude at a point is equal to the
force experienced by a test charge, divided by the magnitude of that test charge.
So when I first saw this definition I thought it was kind of weird because it seems like
the electric field strength E is dependent on the charge magnitude q, where it shouldn’t
be because at a particular point the electric field should be constant.
But then I remembered that the force experience by a charge is directly proportional to the
magnitude of that charge, so this means that as q increases F will also increase, leaving
the E the electric field strength constant.
So this definition is pretty cool because if we rearrange it in the form F is equal
to E q, we can place any charge we want in the electric field and calculate the force
that’s experienced by it.
Pretty cool huh!
One more thing, because the electric field definition is force divided by charge, the
units for electric field is simply Newtons per Coulomb.
Okay guys that’s the end of the video as always I hope it helped.
If you have any questions or requests leave me a comment below and thanks for watching.
Bye!