
Title:
08ps02 Frantic Field Lines Solution

Description:

To see which one of these diagrams has the field lines drawn correctly,

let's just look through each one and see if we can find any flaws with them.

These field lines look pretty good. They all point from positive to negative charges.

But something odd is happening right here.

We notice here that two field lines are actually intersecting.

What would that mean if we had a particle right here in this electric field?

Well according to this field line, the particle should be pushed this way assuming it's positive.

And according to this field line, it should be pushed this way.

Now we know that at a given location an electric field can only push on a particle

in one direction so this case what's pushing on it in two different directions is just nonsense.

We know that this diagram has to be wrong.

And in fact anytime we have a field line diagram like this

where two of the lines intersect we've done something wrong.

So we'll keep that in mind when we're looking at the other diagrams.

Now this one looks a little better but it doesn't look like there are any lines

that intersect each other so we don't have any contradictions of the field pushing

in two different directions at once.

But there are still some odd things happening in this diagram though.

In particular, imagine a particle right here.

If we imagine this particle has a positive charge the field line say

that this particle should be pushed in this direction by the electric field.

Well that doesn't seem right.

This positive charge will be right next to this giant negative charge,

which would really pull it in, and since this positive charge over here is relatively small,

we would expect this force to be more like that, pointing towards the negative charge.

Because this negative charge is so big,

we'd expect these lines showing how positive charges will move

to bend more towards the negative charge like this.

This means this one can't be the right answer either which of course leaves only this answer.

And let's just do a check through to make sure everything makes sense here.

I don't see any lines intersecting each other so we don't have the problem from this one.

And it looks like all the field lines up here bend in towards the giant negative charge

like we'd expect them to.

Everything looks in order in this electric field line diagram.

If you were able to identify this one as the correct diagram, great work.