Phy 122 – Assignment 2
A. 1. The proton. One way to see this is to remember that field lines point from + to -,
and to also remember that they point “downhill;” that is, toward lower potential. So, + is
at the higher potential.
2. a.
b.
V=
PE = -10 J = - 10 V
Q
1C
( PE is negative because potential energy was lost.
increase and negative for a decrease.)
anything is positive for an
PE = Q V = (1.602 x 10-19 C)(-10 V) = -1.60 x 10-18 J; that is, 1.60 x 10-18 J lost.
(The way the question is phrased, the answer would be a positive number. It says
“how much is lost?” not “what is PE?” However, that’s subtle enough that I
wouldn’t take off credit for giving a negative answer in this case.)
3. a. C = C0 = (3.4)(4.9 pF) = 16.7 pF (Look up
= 3.4 for nylon on formula handout.)
6
b. From the same table, breakdown happens if E reaches 14 x 10 V/m
E = V/d
6
-4
V = Ed = (14 x 10 V/m)(1.16 x 10 m) = 1624 V
Rounding to three digits, 1.62 kV
B. 1. Potential = potential energy per unit charge.
2. a. Q1 = Q2 . While they are being charged, charge flowing out of C1’s
negative plate has no place to go except into C2’s positive plate.
b. V1 < V2 . From C = Q/V, there is an inverse proportion between C
and V if Q is the same on both. So, the bigger C has the smaller V.
3.
C. 1. No effect. Capacitance is a fixed property of the capacitor, determined by how it is
made. (In C = q/V, q is what changes in response to changing V, not C.)
2.
-14
3. E = F/q = (1.64 x 10 N)/(1.602 x 10-19 C) = 1.024 x 105 N/C (q is actually negative,

but that only matters if you are keeping track of E ’s direction.)
V/d
V = E d = (1.024 x 105V/m)(.012 m) = 1228 V, which rounds to 1.23 kV
D. 1.
2. The 5 μF and the 4 μF are in parallel with each other:
Ceq = C1 + C2 = 5 + 4 = 9 μF
Putting 9 μF where that group used to be, the right-hand part of the circuit is equivalent
to this:
Series: 1/ Ceq = 1/3 + 1/9 + 1/9
1/ Ceq = 3/9 + 1/9 + 1/9
1/ Ceq = 5/9
Ceq = 9/5 = 1.80 μF
Putting 1.80 μF where that group used to be, we have:
Parallel: 10 + 1.8 = 11.8 μF
E. 1. Stays the same. Potential is a property of the field, and the field did not change. (If
you were looking at V = PE/q, PE goes up by the same factor as q.)
2.