Resolving Vectors for Electric Charges

[physicsfun_12]

Homework Statement

Hi there, I am having a lot of trouble with this problem. Any help much appriciated.

Thanks inadvance.

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

Homework Equations

E=kQ/r²

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

 

[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.

 

[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 a lot, do you know if the answer I have calculated is correct?

 

[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?

 

[physicsfun_12]

Hello, I used 8.99x10⁹ for k.

Thanks again!

Mike

 

[Fightfish]

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

 

[physicsfun_12]

You are quite right!

I got 6.2284x10⁴ this time!