 ## Subtitles section Play video

• - [Instructor] Okay, so we know that electrical charges

• create electric fields in the region around them

• but people get confused by electric field problems

• so you got to get good at at least two things here

• if you wanna proficient at dealing with electrical field.

• You should get good at determining the direction

• of the electric field that's created by a charge.

• If you've got some charge

• and you wanna know which way does that charge

• create an electric field,

• you should get really good at that.

• And if you know the direction of the field,

• you should get good at finding the direction

• of the electric force exerted on a charge.

• If there's some charge floating around

• in an electric field,

• you should be able to say,

• oh, okay, I can determine the electric force.

• If you get good at these two things,

• these problems are gonna be way easier

• and the whole process is gonna make a lot more sense.

• Let's figure out how to do this.

• How do you do these things?

• We'll do the first one first.

• Let's try to tackle this one.

• Let's try to figure out how do you determine

• the direction of the electric field

• that's created by a charge.

• Let's say we didn't know, this is what the electric field

• look like around a positive charge.

• I just gave this to you

• but how do we know that this is what the electric field's

• supposed to look like?

• What we can do is this.

• We can say that we know the definition of electric field

• is that it's the amount of electrical force

• exerted per charge.

• In other words, if you took some test charge,

• think of this Q as the test charge

• and we usually just make this a positive test charge

• so this is easier to think about.

• If you took some positive test charge into some region

• let's do that, let's put some positive test charge in here.

• We take this test charge, we move it around.

• All we have to do to figure out

• the direction of the electric field,

• since this Q would be positive,

• we can just figure out what direction

• is the electric force on that positive test charge.

• In other words, the direction of the electric field E

• is gonna be the same direction as the electric force

• on a positive test charge.

• Because if you know about vector equations,

• look at this electric fields vector,

• this electric forces vector.

• This electric field is just gonna adopt

• the same direction as the electric force

• as long as this Q is positive.

• If this Q were negative it would flip the sign

• of this electric force

• and then the E would point the opposite direction.

• But if we keep our test charge positive

• then we know, okay, the electric field's

• just gonna point the same direction

• as the electrical force on that positive test charge.

• Here's what I mean.

• We take our positive test charge.

• We move it around.

• If I wanna know the electric field at this spot right here,

• which way does the electrical force

• point on that test charge?

• The electric force would point to the right

• since it's being repelled

• by this other positive charge over here.

• I know that the electric force points to the right,

• these charges repel each other.

• And since the electric force points to the right,

• that means the electric field in this region

• also points to the right.

• It might not have the same magnitude.

• The electric force might be 20 newtons

• and the electric field might be 10 newtons per coulomb

• but they have the same direction.

• And I can move this charge somewhere else,

• let's say I move it over here.

• Which way would the electric force point?

• Well, these positive charges are still repelling.

• I'd still have an electric force to the right.

• That electric force would be smaller

• but it would still point to the right

• and that means the electric field

• also still points to the right,

• it would be smaller as well

• but it would still point to the right.

• And if we wanna determine the electric field elsewhere,

• we can move our positive test charge,

• I'll move it over to here.

• I'll ask which way is the electric force

• on this positive test charge?

• That would be in this direction

• since these positive charges are repelling each other,

• they're pushing each other away

• so this positive always gets pushed away

• from this other positive charge.

• And so, that also means that the electric field

• is pointing in that direction as well.

• We keep doing this.

• I can move this somewhere else.

• I can move this positive charge down here.

• The charges repel so the electric force

• would point downward.

• And that means the electric field would also point down.

• If you keep doing this,

• if you keep mapping what's the direction

• of the electric force on a positive test charge?

• Eventually, you realize oh, it's always just gonna point

• radially out away from this other positive charge.

• And so we know the electric field

• from a positive charge is just gonna point

• radially outward, that's why we drew it like this.

• Because this positive charge would push

• some positive test charge radially away from it

• since it would be repelling it.

• Positive charges create electric fields

• that point radially away from them.

• Now what if the charge creating the field

• were a negative charge?

• So, let's try to figure that one out,

• let me get rid of this.

• Let's say the charge creating the electric field

• were negative, a big negative charge,

• how do we determine the electric field

• direction around this negative charge?

• We're gonna do the same thing,

• we're gonna take our positive test charge

• and we're gonna keep our test charge positive,

• that way we know that the direction

• of the electric force on this positive test charge

• is gonna be the same direction

• as the electric field in that region.

• In other words, the positivity of this test charge

• will just make it so that the electric field

• and electric force point in the same direction.

• And if we do that, I'll move this around.

• We'll just put it at this point here,

• we'll move this test charge here.

• Which way is the force on that test charge?

• This time it's getting attracted to this negative charge.

• Opposite charges attract

• so the electric force would point this way

• and since it's a positive test charge

• and it preserve the direction in this equation,

• that means the electric field

• also points in that leftward direction.

• And we can keep mapping the field

• we'll move the test charge over to here.

• The electric force this time is gonna point up

• because this positive test charges

• is attracted to this negative charge.

• And if the electric force points up,

• that means the electric field also points up in that region.

• And you'd realize the electric force

• is always gonna pull a positive test charge

• toward this negative creating the field around it.

• And because of that, the electric field

• created by a negative charge points radially inward

• toward that negative charge.

• This is different.

• Positive charge created a field that pointed