Magnitude of electric field using point charges

[n77ler]

[SOLVED] Magnitude of electric field using point charges

Homework Statement

Two charges, Q1 = 3.15 μC and Q2 = 5.40 μC, are located at points (0, -2.80 cm) and (0, +2.80 cm), What is the magnitude of the electric field at point P, located at (5.45 cm, 0), due to Q1 alone?


lQ2
l
l
l
l--------------P
l
l
l
l
lQ1

Homework Equations

The Attempt at a Solution

 

[Doc Al]

In order to get help, post your attempted solution and point out where you got stuck.

 

[n77ler]

I used E=( k lq1l ) / r^2

(8.99x10^9) (3.15x10^-6) / (0.0612m)^2
=7.56x10^6 N/C

 

[Doc Al]

I used E=( k lq1l ) / r^2

(8.99x10^9) (3.15x10^-6) / (0.0612m)^2
=7.56x10^6 N/C

Looks good to me.

 

[n77ler]

k got it, I think I was putting in 10^-6 by accident :S lol, thanks!

 

[n77ler]

Ok so there were two other parts to the question, I had to find the xcomponent and ycomponent of total electric field at P. I solved the one for x and got it correct which means I must be using the right angle and stuff to do my calculations. My answer for y was wrong, why is this?

 

[Doc Al]

My answer for y was wrong, why is this?

Beats me. Show what you did.

 

[n77ler]

Workings are on attached file...

 

[Doc Al]

Check your signs. Realize that one field points up, while the other points down.

 

[n77ler]

opps so there should be subtraction to get my final answer...
So the Q1 diagram needs to be a negative answer...

(-3.44x10^6)+ (5.90x10^6)
=2.46x10^6N/C
Thats still not right tho, I don't know what I am going wrong

 

[Doc Al]

opps so there should be subtraction to get my final answer...
So the Q1 diagram needs to be a negative answer...

Q1 is the one on the bottom, right? So where does its field point?

 

[n77ler]

Ok, so the Q1 line points in the positive direction while the Q2 points in the negative direction right? I got the right answer but I just want to make sure my reasoning for sign convention is understood.

 

[Doc Al]

Right. The field from a positive charge always points away from the charge. So the y-component of the Q1 field is + and the Q2 field is -.

 

[n77ler]

Ok thanks once again!