
#1
Dec1111, 11:17 AM

P: 2

I was given this equation as the lines of electric fields of a dipole(two opposite charges separated by a finite distance)
e=(1/r^3)*((3cos^2(theta)1)^2 +sin^2(2theta))^0.5 and I was asked to plot it. I guess it must be something like this: but when I try to plot it in wolframalpha.com in polar coords.I dont get the output I expect. The question is is it the right equation? 



#2
Dec1111, 01:19 PM

P: 767

Hmm, I also graphed it with wolfram, and it appears to not follow the characteristics of a dipole. It more appears to follow the characteristics of the electric field for like charges, rather than unlike.




#3
Dec1111, 04:17 PM

HW Helper
P: 6,189

Welcome to PF, silverfox!
I checked what the equation is for an electric dipole and found this: http://en.wikipedia.org/wiki/Dipole#...lectric_dipole If I work this out in polar coordinates, I get a slightly different formula than the one you have for what appears to be the magnitude of the electric field. (You can use that [itex]\mathbf{p} = qd\cos\theta \mathbf{\hat r}  qd\sin\theta \hat{\textbf{θ}}[/itex].) Can it be that you or someone else made a calculation mistake? 



#4
Dec2111, 09:16 AM

P: 2

plotting electric field lines of a dipole
I worked a bit more on the problem but I couldn't find an equation myself nor could plot the ones you said or I found on wikipedia...
I was told that if E(r, theta) is the first equation I wrote then E(r, theta, t) would be the same thing times sin(wt) but I dont get it, How does time affect the electric field lines? And I also thought that p is a border between + and  charges in a dipole which is equal to qd and is a constant value am I wrong? 



#5
Dec2111, 10:06 AM

HW Helper
P: 6,189

In the link I gave you can find an equation for E containing only p and r as variables.
If you substitute the p I gave in my post, you get E(r,θ). The formula you gave in the OP looks like E(r,θ), but it is not quite right. It does not have a time dependency. To make it time dependent, you would need to make the 2 charges time dependent. p is the vector dipole moment, which is constant. It is given by p=qd, where q and +q are the charges, and d is the constant vector from the negative charge to the positive charge. However, a constant vector is dependent on θ in polar coordinates, since the unit vectors change with θ. 


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