Help with electric field

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BHH
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Homework Statement


The picture below shows two stationary charges q1= +2mC and q2= -15 mC.
and I need to find the electric field vector at field point P(0,4m). E= [px, py] .

Also
A little dipole is positions in P(0,4m) and is given with p= [px,py]=[2*10^-8 ,0] Cm.

Find the torque that acts on the dipole because of the electric field and its potential energy

Homework Equations


E=k*q/r^2

The Attempt at a Solution


I have found out that E1=4.5*10[6][/SUP] and E2= 1.7*10^6. I am not sure if the electric field E1 is negative or positive ? I think its negative because the vector goes downwards. And when I find the angle for E2 do I use arctan(8/2)?
 

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Answers and Replies

  • #2
TSny
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I have found out that E1=4.5*10[6][/SUP] and E2= 1.7*10^6.
OK. But I would keep an extra significant figure while doing the calculations. Then round off to the appropriate number of significant figures at the end. Also, don't forget the units.
I am not sure if the electric field E1 is negative or positive ? I think its negative because the vector goes downwards.
Electric field is a vector quantity. It is not good language to say that a vector is negative. However, the components of a vector can be negative or positive. So, if the field points downward (along the negative y axis) then you would say that the y component of the electric field is negative.

And when I find the angle for E2 do I use arctan(8/2)?
This does not look right. Are you trying to find the angle that E2 makes to the horizontal direction or the vertical direction? If you are constructing a right triangle to help find the angle, make sure you have the dimensions of the sides of the triangle correct.
 

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