Physics 6B – 2nd Midterm NAME:______________________
Dr. Nassar (1-hour exam -- 3x5 card allowed)
Complete the following problems.
1. Three point charges q1 = 2 µC, q2 = 1 µC, q3 = 2 µC are located at the corners of an
equilateral triangle (a = 1.0 m), as shown below. Another charge q4 = - 1 µC is placed at
the base of the triangle. Calculate the net electric force (magnitude and direction) on q2.
66
92
13 2
2 10 1 10
9 10 1.8 10
1
FF N
−−
−
××
==×=×
66
92
42
1 10 1 10
9 10 1.2 10
0.87
FN
−−
−
× −×
=×=×
-------------------------------------------
0
x
F∑=
14
2 22
2 cos30
2(1.8 10 )(0.86) 1.2 10 1.9 10
y
FF F
N
− −−
∑= −=
× −× =×
-------------------------------------------
22
1.9 10 2 10F NN
−−
=××
in the positive y-direction (or 90o)
2. Find the equivalent resistance of the circuit below and the current through
the resistor indicated below.
Equivalent resistance = 1.42 R (or 17/12 R)
Total Current: I = ε/1.42R = 0.71 (ε/R)
0.71 (ε/R) = I’ + I” and I’ = 3I”, so I” = 0.17 (ε/R) and I’ = 0.53(ε/R)
I’
I”
I
F1
F3
F4
0.87 m
I
Parallel
Series
