A 2 00 a b 5 00 a c 10 0 a d 20 0 a solution b μ i 2

A. 2 . 00 A B. 5 . 00 A C. 10 . 0 A D. 20 . 0 A
Solution: B =
μ
0
I
2 πd
for a current-carrying infinite wire. Thus, I =
2 πdB
μ
0
= 10 . 0 A
12. Equilateral Triangle. Consider three infinite current
carrying wires shown. The wires, carrying the same
amount of current I
0
, are each separated by a distance
s . If the field point P is at the midpoint of wires A and
C, what is the magnitude of the magnetic field strength
at P?
A.
2 μ
0
I
0
πs
+
μ
0
I
0 √
3 πs
B.
μ
0
I
0 √
3 πs
C.
2 μ
0
I
0
πs
D.
2 μ
0
I
0
πs
-
μ
0
I
0 √
3 πs
A – 6

Second Long Exam First Semester, AY 2014–2015 Physics 72
Solution: A and C cancel out. B only contributes.
13. Infinite Wires. Which of the following figures correctly shows the current direction on an
infinite wire, and the corresponding magnetic field direction at the indicated field point?
Solution: ANSWER: D
A.
μ
0
I
0
4 πR
dθ
ˆ
k
B.
μ
0
I
0
4 πR
dθ ˆ 
C. -
μ
0
I
0
4 πR
dθ ˆ 
D. -
μ
0
I
0
4 πR
dθ
ˆ
k
Solution: Note that d s × ˆ r is directed into the page, i.e. to the -
ˆ
j -direction. Also, note
that d s and ˆ r are perpendicular with each other. Using Biot-Savart law, we obtain C.
14. Current Loop. An infinitesimal current element, sub-
tending an angle dθ , is located at some angle θ on the
current-carrying circular loop shown. The loop is cen-
tered at the origin. Which of the following is the correct
expression for the magnetic field contribution at the ori-
gin, due to this element?
A – 7

Second Long Exam First Semester, AY 2014–2015 Physics 72
15. BeeDiEl. Shown are 3 integration paths (X, Y, Z) with enclosed currents shown. What is
the correct order of
R
B · d l ?
Note the magnetic field direction and integration path direction.
For the next two items, consider a circular current loop of
radius 5 . 0 m in the yz plane with a 2 . 5 A current as shown.
16. Sa Gitna. What is the magnitude and direction of the
magnetic field at the center of the loop?
17. Feel mo ba? What is the direction of the magnetic force experienced by an electron
moving in the + x direction as it enters the center of the loop?
A – 8

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