# E-field with charges

1. Mar 10, 2008

### n77ler

[SOLVED] E-field with charges

I've tried this 8 times and have 2 tries left, Looking for some hints

1. The problem statement, all variables and given/known data
The figure shows the E-field in the plane of two point charges. (Give ALL correct answers, i.e., B, AC, BCD...)
A) Q1 is positive.
B) A - charge placed at a' would move left.
C) Q1 and Q2 have the same sign.
D) A + charge at a' would move left.
E) The magnitude of E is the same at a' and b'.
F) |Q1| is larger than|Q2|

2. Relevant equations

3. The attempt at a solution

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2. Mar 10, 2008

### n77ler

noone? :(

3. Mar 10, 2008

### Hootenanny

Staff Emeritus
Sorry n77ler, I didn't notice your post! Okay, how about you state whether each statement is true or false and give your reasons and we'll take it from there?

I can't promise that I'll get back to you tonight as it's pretty late here, but I'm sure someone else will jump in. If not I'll check in again tomorrow

4. Mar 10, 2008

### n77ler

OK so for A I think it is true because direction of field is supposed to travel away from a positive charge, and Q1 has arrows pointing away from it.

B: a negative charge here, I think it would move left? But im not sure about this one

C: False because arrows are going in opposite directions towards each point Q2 is negative and Q1 is positive i think.

D: I think this is true but im not positive about it

E: I think false. I dont think they can be the same magnitude of E because the radius is different for each one isnt it?

F: True. If e field lines are more dense around a point it means that it has a higher charge or maghnitude of e

5. Mar 10, 2008

### Hootenanny

Staff Emeritus
Correct
In which direction do electric field lines point? Think about how the electric field vector is defined. (see you answer below)
Correct.
Correct
It is indeed false, but for the wrong reasons. What does the density of the electric field lines tell you about the strength of the electric field (see your point below).
Correct.

6. Mar 10, 2008

### n77ler

Electric fields point negative don't they?

7. Mar 10, 2008

### Hootenanny

Staff Emeritus
They point from positive charges to negative charges or in the direction in which a positive test charge would move.

8. Mar 10, 2008

### n77ler

ok so the negative charge would move to the right making it false

9. Mar 10, 2008

### n77ler

10. Mar 10, 2008

### Hootenanny

Staff Emeritus
Yes, I would go with that

11. Mar 10, 2008

### n77ler

good stuff, now I have the right answer and understand :) thanks!

12. Mar 10, 2008

### Hootenanny

Staff Emeritus
No worries