Its from an example in the book, and it doesnt seem to make sense,(adsbygoogle = window.adsbygoogle || []).push({});

A rod of length l has a uniform positive charge per unit length (lambda) and a total charge Q. Calculate the electric field at a point P that is located along the long axis of the rod and a distance a from one end.

So then the example takes a small part of the rod, dx which has charge of dq, and is distance x from point P.

dq = (lambda)dx and dE = ke dq/x^2 or ke lambda dx / x^2

fine so far.

Now the example says we must sum up the contributions of all the segments.

it becomes an integral E = (integral) from a to l+a of ke lambda dx/x^2

the example breaks the dq component out into ke lambda [ - 1/x ] from a to l+a Im somewhat confused now.

then it goes on, ke lambda(1/a - 1/l+a) = keQ/a(l+a) ??? what??!!

okay it kind of makes sense, the total charge divided by length, but the last part there is a divide by zero error to my thought process, multiplying an item with example values: Lambda(1/a - 1/b) or Q/l(1/a - 1/b) might give (Q/l * 1/a) - (Q/l * 1/b) if doing the same thing for the actual values, should give Q/la - Q/la + l^2 ?? no?

reducing it down to Q/a(a+l) ? the book doesnt explain how it arrived at this.

Can anyone give an example of calculating the Field from a charged rod? apparently its the same in a ring from the x axis but using vectors, but this concept seems tough, thanks for any explanations

**Physics Forums | Science Articles, Homework Help, Discussion**

Dismiss Notice

Join Physics Forums Today!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

The friendliest, high quality science and math community on the planet! Everyone who loves science is here!

# Finding Electric Field on rod or ring?

**Physics Forums | Science Articles, Homework Help, Discussion**