Sample Exam I Problems
PHYSICS4B
Problem #1; Point Chana
(a) A point charge is placed at each comer of a square with side length a. The magnitude and ¾harg~ are indicated in the figure, assume alt charges are fixed. Three of the . c
&1gn of~ point
1s positive as sho~. What is the magnitude and direction of the net electric : O t
negative and one
to the four charges m tenns of q, a and k (= 1/4ffé? Provide the angle when characterizing: ~ due
===
~ – – – -a – – – $ 2q
-2q Ci:.:
I
1
a,
I
I
,a
I
-q
0- —–0 -q
a
(b) How mum work is required to create this charge distribution?
Q Problem #2: Motion of a Charged Particle in an Electric Field
An electron of mass m and charge, -e, moves in circular motion a distance r around a fixed positively charged
nucleus with charge +Q = 4e and mass M, as shown. Treat +Q as a point charge. The electron orbits at a radius
of r :=;. 0.10 Din from the center of the nucleus. Find the energy needed to separate the two charges, ie. the energy
to ioniz.e,éve answer.in eV. ¥glect the gravitational interaction between the charges. Hint: Use Newton,_s laws
ofmotion and conservation of energy. Draw a free body diagram in your solution.
lDcJcc_tron=9.ll x I0-31 kg; e= l.6x I0-19 C,k=9x I09Nm2/C2 , leV= 1.6x 10-19J
Problem #3: Continuous Charge Distribution
Find the electric field a distance b above the end of charged rod oflength L with constant linear charge density )
clv
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gcoblem #4: Gauss’ Law
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