Hi everyone,(adsbygoogle = window.adsbygoogle || []).push({});

I'm reading Feynman's "Lectures on Physics" and I'm stuck on one little part. If any of you out there have the books, I'm on pg. 70 of Vol. II of the 3 volume set.

Can someone tell me how Eq. 6.12 is equal to cos(theta) = z/r?

I'll lay out the problem below.

The section is about the electric field due to a dipole, but I don't think a knowledge of the physics is necessary to clear up the problem I'm having. I understand everything conceptually, but there's a mathematical quirk which I can't explain to myself.

The electric potential (psi) is defined as: [p*cos(theta)]/r^2

where: p = dipole moment (q*d)

cos(theta) = distance along axis of dipole (z) over distance from

center of dipole to point P in question (r)

= z/r

r = magnitude of vector r

q = magnitude of each charge in dipole

d = distance between the two charges of the dipole

Mr. Feynman goes on to "vectorize" p by giving it the magnitude of p (defined above) and direction going from the negative charge to the positive charge.

The part I don't understand is, he redefines cos(theta) = z/r as cos(theta) = vector p (dotted with) direction vector e along length r. How does he arrive at that??

Thanks in advance for all your help.

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# Help with Feynman's math!

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