# Electric field lines questions

1. Aug 29, 2009

### GreyGus

1. The problem statement, all variables and given/known data

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.

2. Relevant equations

3. 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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2. Aug 29, 2009

### GreyGus

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

3. Aug 29, 2009

Is there any picture or a diagram with it?

4. Aug 30, 2009

### GreyGus

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

5. Aug 30, 2009

### Staff: Mentor

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

6. Aug 30, 2009

### GreyGus

Inward for the negative and outward for the positive.

7. Aug 30, 2009

### GreyGus

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

8. Aug 30, 2009

### Staff: Mentor

Yes, my bad. The one you got wrong had to do with specifying the direction of force.

9. Aug 30, 2009

### GreyGus

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.

10. Aug 30, 2009

### Staff: Mentor

That's the one. What's the direction of the field at the point M (not where the arrow head is drawn)?

11. Aug 30, 2009

### GreyGus

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

12. Aug 30, 2009

### Staff: Mentor

One of them is correct.
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.

13. Aug 30, 2009

### GreyGus

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?

14. Aug 30, 2009

### Staff: Mentor

The field at M points East, so which way is the force?
Oops, yes you did. The very first one (force at N).

15. Aug 30, 2009

### GreyGus

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?

16. Aug 30, 2009

### Staff: Mentor

No, the force on a negative charge is opposite to the direction of the field.

17. Aug 30, 2009

### GreyGus

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.

18. Aug 30, 2009