1. Suppose we are trying to create an equilateral triangle with a proton at each

ID: 1585689 • Letter: 1

Question

of the three points. If the length of each side of the triangle is 3

m, and each

of the protons started from very far away, how much work is done to assemble

this equilateral triangle?

2. A disk with radius R has uniform surface charge density

a. Treat the disk as a series of concentric rings and calculate the electric

potential V at a point on the disk’s axis a distance x from the center of the disk. Assume the potential is zero at infinity.

b. Calculate the Electric field from the potential you found in part (a).

c. Why is it that you cannot recover the potential V for a infinite sheet of uniform surface charge by taking the limit R in your answer found

in part (a)?

Work done is W = U1 - U2

U1 is the Potential energy of three charges seperated by far distance

U2 is the potential energy of the system when protons are at the vertices

then

U1 = 0 J

U2 = (k*q1*q2/r)*3 = (9*10^9*(1.6*10^-19)^2/(3*10^-6))*3 = 2.304*10^-22 J

so work done to assemble is W = 2.304*10^-22 J

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