1 year ago with 287 notes

**Electric Field Due to a Uniformly Charged Disk **

Perhaps one of the most difficult concepts in general physics is uniformly charged surfaces. Really, I can’t think of anything more rigorous than E&M and complex integration.

Usually with these problem sets we are given the task of finding the E field at a distance and/or the E field when two charges are in the system.

So when given a uniformly charged disk and asked to find the E field at a distant point here’s how we do it:

First we need to divide the disk into flat rings so we can calculate the E field at our point P by adding up all the rings. Each ring will be out at a distance *y* and will have a radial width of *dy*.

Next we’ll set up the E field equation E=kQ/r^{2}, but write it as dE=kdQ/√(x^{2} + y^{2}).

Now we come to our first tricky part of E field integration; the dQ. When we do these problems we must remember our charge densities - linear, surface, volume; lambda, sigma, and rho respectively.

- Linear - lambda, 1 dimension : dQ = λdx
- Surface - sigma, 2 dimensions : dQ = σdA
- Volume - rho, 3 dimensions : dQ = ρdV

Here, we use sigma and use the relationship dQ = σdA = σ2πydy. All that means is that our charge, dq, is equal to the surface charge density, σ, times the area of the charged concentric ring.

Now that we have our charge set we can look at the x and y components of the E field. If you look at dE_{y} you’ll see that as you go around the ring there is another y component that cancels each other out. Therefore we only focus on dE_{x} since the vector sum of dE_{y }will add to 0.

Our dE_{x }= kdQ cosθ /(x^{2} + y^{2}). We use cosθ because on our free-body diagram dE_{x} is the adjacent-hypotenuse component of dE. Further substitute the dE_{x }equation by using x/√(x^{2} + y^{2}) for cosθ and 1/4πε for k.

Only thing left to worry about is your trig identities of integration. Take the integral of dE_{x} from 0 to R and you will find the E field at point P.

- centripetalinertia reblogged this from anndruyan and added:
Not doing too well in this class but I’m glad I was able to understand this.

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Cuanto tiempo tratando de entender estas simples ecuaciones que describen el movimiento de partículas electromagnéticas...

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This may be relevant to my life right now.

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