Phys%2072%201st%20LE%20AY14-15%202nd%20Sem - University of...

University of the Philippines
College of Science
Physics 72
Set A
First Long Exam
Second Semester, AY 2014–2015
Name: Instructor:
Section/Class Schedule: Student Number:
Course: College:

First Long Exam Second Semester, AY 2014–2015 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. Square. Consider the point charge distribution in the fig-
ure. What is the magnitude of the net electric force on the
charge + q
0
that lies in the center of the square?
A.
4 kqq
0
a 2
B.
8 kqq
0
a 2
C.
2 k ( q + Q ) q
0
a
2
D.
4 k ( q + Q ) q
0
a 2
2. E and F . A source charge + | Q | and a test charge -| q
0
| are separated by a distance r .
Which of the following is/are TRUE about the electric field E and electric force F
E
of
+ | Q | on -| q
0
| ?
I. F
E
and E have the same direction.
II. F
E
and E have opposite directions.
III. | F
E
| and | E | are inversely proportional to one another.
A – 2

First Long Exam Second Semester, AY 2014–2015 Physics 72
3. Magnitude. Consider the diagram showing the equipo-
tential lines around two identical positive charges. What
is the relationship of the magnitude of electric field at the
different points?
4. dipole. An electric dipole is oriented in the uniform elec-
tric field with magnitude 200
N
/
C
as shown. What is the
magnitude and direction of the net torque experienced
by the electric dipole?
5. Plastic and Fur. A plastic rod ( + ) is rubbed on fur ( - ). Two balls X and Y are suspended
by insulating threads. The observations are as follows:
I. Both X and Y are attracted to the plastic rod.
II. X is attracted to Y.
III. Y is repelled by the fur.
Which of the following is TRUE regarding the system?
A. X: neutral B. Y: neutral C. X: ( - ) D. Y: ( + )
A – 3

First Long Exam Second Semester, AY 2014–2015 Physics 72