# Electric field lines questions

## Homework Statement

The electric field from two charges in the plane of the paper is represented by the dashed lines and arrows below.

Select a response for each statement below. (Use 'North' towards top of page, and 'East' to the right)

The force on a (-) test charge at N is directed ....
The magnitude of the E-field at R is .... than at L. s
The sign of the charge on the right is negative.
The force on a (-) test charge at M is directed ....
The force on a (+) test charge at N is directed ....
The force on a (-) test charge at S is zero.
The magnitude of the charge on the left is .... that on the right.

## The Attempt at a Solution

The electric field from two charges in the plane of the paper is represented by the dashed lines and arrows below.

Select a response for each statement below. (Use 'North' towards top of page, and 'East' to the right)

The force on a (-) test charge at N is directed .... South
The magnitude of the E-field at R is .... than at L. s Less than
The sign of the charge on the right is negative. True
The force on a (-) test charge at M is directed .... North
The force on a (+) test charge at N is directed .... North
The force on a (-) test charge at S is zero. False
The magnitude of the charge on the left is .... that on the right. Less than.

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## Answers and Replies

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I would greatly appreciate any help on this one. I don't know which one is wrong, I thought I had all of them.

Is there any picture or a diagram with it?

Yes I attached one to it. It's right under it, it's the attached thumbnails.

Doc Al
Mentor
Imagine a positive charge alone. Which way will the surrounding field point? And for a negative charge?

Inward for the negative and outward for the positive.

I think my answers reflect that logic, don't they?

Doc Al
Mentor
I think my answers reflect that logic, don't they?
Yes, my bad. The one you got wrong had to do with specifying the direction of force.

Could you please tell me which one? Because my answers are based on the fact that for the negative charges the lines point inward so the direction would South instead of north. For the point M I really don't know because it looks more like North east than anything, but North East is not an option so I put North.

Doc Al
Mentor
For the point M I really don't know because it looks more like North east than anything, but North East is not an option so I put North.
That's the one. What's the direction of the field at the point M (not where the arrow head is drawn)?

I've tried everything, North, South, East, West, none is correct. How to know where the field is directed without the graph?

Doc Al
Mentor
I've tried everything, North, South, East, West, none is correct.
One of them is correct.
How to know where the field is directed without the graph?
What do you mean "without the graph"? The graph is what tells you the direction of the field. The arrowheads just tell you which way the lines are meant to go; the field is always tangent to the lines at every point. The direction of the field at M seems clear to me.

Not to me, I have tried all of them but Loncapa is saying they're wrong. I thought it would be East. Did I make a mistake somewhere else?

Doc Al
Mentor
Not to me, I have tried all of them but Loncapa is saying they're wrong. I thought it would be East.
The field at M points East, so which way is the force?
Did I make a mistake somewhere else?
Oops, yes you did. The very first one (force at N).

So the force is pointing at East as well. For the first one if a negative charge is acting on N, the force would point South right?

Doc Al
Mentor
So the force is pointing at East as well. For the first one if a negative charge is acting on N, the force would point South right?
No, the force on a negative charge is opposite to the direction of the field.

OH!!! Now I get it! Thank you very much for your help I appreciate it. I need a different book because my physics do not talk about electric fields and forces to to such detail. Thank you so much.