# Resolving Vectors for Electric Charges

1. Jan 14, 2010

### physicsfun_12

1. The problem statement, all variables and given/known data
Hi there, I am having alot of trouble with this problem. Any help much appriciated.

Two point charges, each of 4x10-6C are placed 1m away at points A and B respectively. Calculate the electric field strength at a point P, which is a distance of 1m from both A and B in the cases when:

i) both charges are positive
ii) one is positive and one is negative

2. Relevant equations
E=kQ/r2

3. The attempt at a solution
Ok well I know that I need to work out a resultant, however I am not sure whether to use sin or cos, and even then how to use them to get the correct answer!

Thanks again for any input

Mike

2. Jan 14, 2010

### Fightfish

When you separate a single vector into two perpendicular components, the three lines form a triangle, yes? Then you can easily apply your trigonometric relations easily with reference to the diagram. E sin theta would give the component opposite to the angle, while E cos theta gives the component adjacent to the angle used.

3. Jan 14, 2010

### physicsfun_12

Oh I see, so since the x components would cancel, you would just double the y component which is cos30*the elctric field strength calculated

i.e. 6.28104NC-1

Thanks alot, do you know if the answer I have calculated is correct?

4. Jan 14, 2010

### Fightfish

That's for part i) I presume?
My answer appears to differ from yours from the 3rd significant digit; what value are you using for k?

5. Jan 14, 2010

### physicsfun_12

Hello, I used 8.99x109 for k.

Thanks again!

Mike

6. Jan 14, 2010

### Fightfish

Hmm...press your calculator again, you appear to have read some digits wrongly in the answer you provided above.

7. Jan 14, 2010

### physicsfun_12

You are quite right!

I got 6.2284x104 this time!