exam 1s 16-17 - University of the Philippines College of...

University of the Philippines
College of Science
Physics 72
Set A
First Long Exam (Makeup)
First Semester, AY 2016–2017
Name: Instructor:
Section/Class Schedule: Student Number:
Course: College:

First Long Exam (Makeup) First Semester, AY 2016–2017 Physics 72
INSTRUCTIONS: Choose the best answer and shade the corresponding circle on your answer
sheet. To change your answer, cross-out and sign your original answer and then shade your
new answer. No computational devices allowed (e.g. calculators, mobile phones). Any form
of cheating in examinations or any act of dishonesty in relation to studies, such as plagiarism,
shall be subject to disciplinary action. Following instructions is part of the exam.
Useful formulas:
Area Volume
Sphere (radius= r ) 4 πr
2 4
3
πr
3
Cylinder (radius= r , height= h ) 2 πr
2
+ 2 πrh πr
2
h
sin( π/ 3) = cos( π/ 6) =
√
3 / 2
cos( π/ 3) = sin( π/ 6) = 1 / 2
sin( π/ 4) = cos( π/ 4) =
√
2 / 2
Useful constants:
e - 1 . 60 × 10
- 19
C
m
e
9 . 1 × 10
- 31
kg
0
8 . 854 × 10
- 12
C
2
/Nm
2
k 8 . 988 × 10
9
Nm
2
/C
2
1. Tatsulok. Three charges are at the corners of an equi-
lateral triangle of side length 0 . 5 m as shown. What is
the location of the net electric field produced by these
charges at the center of the triangle?
A. + x -direction
B. - x -direction
C. + y -direction
D. - y -direction
Solution: fields equal magnitude; for charges on the x-axis, x-components cancel
2. X. A negative charge of magnitude Q is uniformly distributed along the + y -axis between
y = 0 and y = l as shown. Which of the following correctly gives the x -component of the
net electric field at point P ?
A – 2

First Long Exam (Makeup) First Semester, AY 2016–2017 Physics 72
Z
l
0
ydy
( x 2 + y 2 )
3 / 2
Z
l
0
ydy
( x
2
+ y
2
)
3 / 2
Z
l
0
xdy
( x 2 + y 2 )
3 / 2
Z
l
0
xdy
( x 2 + y 2 )
3 / 2
dE =
kQ
l
dy
x 2 + y 2
(1)
x-component:
dE
x
= - dE
x
p
x
2
+ y
2
(2)
3. - QC. Charge - Q is uniformly distributed around a quarter circle of radius r with its center
of curvature at point P, as shown. What is the x -component of the net electric field at point
P?
dE =
k
r 2
Q
2 πr ·
1
4
rdθ (3)
A – 3

First Long Exam (Makeup) First Semester, AY 2016–2017 Physics 72
x-component:
E
x
=
Z
π
2
0
dE cos θ (4)
4. Tatlong hindi bibe. Three identical conductors X, Y, and Z of initial charges +4 q , - 8 q ,
and - 2 q , respectively, are brought into contact. A point charge +6 q is brought near, but
not in contact with, conductor X as shown. The conductors are then separated from each
other while the point charge is still near X. Which of the following is a possible final charge
distribution for each conductor?
A – 4

First Long Exam (Makeup) First Semester, AY 2016–2017 Physics 72