Homework Help: Determine electric field at point P, Electric fields question

1. Apr 9, 2013

riken9

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

Heres the probelm: http://imgur.com/TbzJxVa

2. Relevant equations

e = kq/r^2

3. The attempt at a solution
q-p
Ex = (9*10^9)(4)/(0.80)^2 = 5.625*10^10

Q-p
E1 = (9*10^9)(6)/(1)^2 = 5.4*10^10

Ex = E1sin45 = -3.82*10^10
Ey = E1cos45 = 3.82*10^10

P
p = SQRT((5.625*10^10)+(-3.82*10^10))^2+(3.82*10^10)^2) = 4.42 *10^10

The answer is actually 35 N/C, i don't know what am doing wrong. Help please!

2. Apr 9, 2013

climb515c

Looks at your charges, 4 and 6. They should be 4*10^-9 and 6*10^-9. Also double check your angle.

3. Apr 9, 2013

riken9

Why should they be *10^-9, sorry i was taught very poorly how to do these questions so i have limited knowledge on this topic.

4. Apr 9, 2013

climb515c

In the problem it states the charges are -6nC and 4nC. The "n" in nC is nano, which means 10^-9.

5. Apr 9, 2013

riken9

I found the angle to be 37 degrees, now my answer is around 49 Nc; which is still not close 35 Nc. Any more tips of what might of gone wrong?

6. Apr 9, 2013

climb515c

The angle is right.

It must be a small error in the algebra somewhere. If you keep getting it wrong, post your work here.

7. Apr 9, 2013

riken9

Heres what i am doing:

q-p
Ex = (9*10^9)(4*10^-9)/(0.80)^2 = 56.25

Q-p
E1 = (9*10^9)(6*10^-9)/(1)^2 = 54

Ex = E1sin37 = 32.5
Ey = E1cos37 = 43.1

P
p = SQRT((56.25)+(-32.5))^2+(43.1)^2) = 49.22

The 32.5 is negative because am thinking the - charge on Q makes the electric field go in ward, so its positive in the y axis and negative in the x axis.

8. Apr 9, 2013

climb515c

Your cos and sin are wrong, you have them mixed up. Cos is for stuff dealing with "x" and sine is for stuff dealing with "y" (for this problem).

try it as Ex = E1cos37, and Ey=E1sin37.

All your reasoning is correct, just a simple mistake.

9. Apr 9, 2013

riken9

Thank you so much! To think, the only reason my answers were wrong cause of my stupid mistakes. Might need to be be careful, and not rush these questions.