5 e kot an electron is initially at the origin with a

5. e-kot. An electron is initially at the origin with a velocity v = v
0
ˆ ı . If there is a uniform
magnetic field B = B
0
ˆ
k , what is the trajectory of the electron?
6. p-kot. Consider a proton with a velocity v = v
0
ˆ
k entering a uniform magnetic field
B = B
0
(ˆ ı + ˆ  ) . What is the cyclotron radius?
A.
√
2 m
p
v
0
eB
0
B.
m
p
v
0
eB
0
C.
m
p
v
0
√
2 eB
0
D.
m
p
v
0
2 eB
0
E. delete
A – 4

Second Long Exam First Semester, AY 2016–2017 Physics 72
7. I-kot. Consider a quarter-circle wire with radius R and
current I directed as shown. If there is a uniform mag-
netic field B = B
0
( -
ˆ
k ) , what is the y -component of the
net magnetic force on the wire?
8. Torque and B-eans. A loop carries a current I as shown
and is immersed in a uniform magnetic field B = B
0
ˆ  .
What is the direction of the torque on the loop?

Second Long Exam First Semester, AY 2016–2017 Physics 72
9. Loop-ception. Consider a current-carrying loop placed
inside a long solenoid as shown. Which of the following
is TRUE about the loop?
A. It is in unstable equilibrium.
B. It has a minimum potential energy.
C. It experiences a maximum torque.
D. It will rotate about the axis of the solenoid.
E. delete
Solution: The loop experiences no torque, thus it will not rotate about the axis of the
solenoid. The magnetic dipole moment and magnetic field are anti-parallel. Thus, it is
at unstable equilibrium.
10. Paano ako lulugar? Consider two bar magnets and four compasses whose directions are
indicated as shown. From left to right, what are the indicated polarities?
A – 6

Second Long Exam First Semester, AY 2016–2017 Physics 72
For the next two items, consider four infinitely long, straight wires carrying the same current I
with directions as shown. Point P is at the center of the square with side length S .