ghostbusters911
Compute the potential difference between an initial point O(x=0,y=0) and P(x=9.00 cm,y=4.00 cm) if the electric field in the region is E = (8.50 N/C)(.300 ihat + .500 jhat).

equations: V=Ed

distance between point O and point P = 0.09^2+0.04^2= 0.0097, sqrt = 0.0985m

magnitude of E is 8.5(.3) = 2.55 ihat, 8.5(.5) = 4.25 jhat
2.55^2 + 8.55^2 = 24.565, sqrt = 4.956 V/m

V=Ed------->0.0985m(4.956 V/m) = .488 V
which is wrong, is there a mistake?
Kino
Hint: without calculating, what is the PD between the origin and any point along the line through the origin parallel to (0.5ihat-0.3jhat)? What does that suggest about your formula for V?
johanndoesmath
am i right in assumig that you would take the dot product of the magnitude of the distance vector with the magnitude of the electric field?
Kino
Since you're not the OP, answer in white below. Highlight to reveal.

Drop the 'magnitude's and you're spot on. Basically you need the magnitude of E times the magnitude of the distance parallel to the electric field. A bit of trig will show you that that is V=|E||x|cos(theta), where theta is the angle between the vectors. That is more compactly written V=E.x
johanndoesmath
i dont mean to steal this persons thread, but out of my own intellectual curiousity--why is the value negative?
Kino
It isn't. At least, I get a positive value. Post your working if you like - we'll see where we disagree.
PhysOrg scientific forums are totally dedicated to science, physics, and technology. Besides topical forums such as nanotechnology, quantum physics, silicon and III-V technology, applied physics, materials, space and others, you can also join our news and publications discussions. We also provide an off-topic forum category. If you need specific help on a scientific problem or have a question related to physics or technology, visit the PhysOrg Forums. Here you’ll find experts from various fields online every day.