1. Xtra Edge February 2012 - Career Point

1. A particle of charge Q and of negligible initial speed
is accelerated through a potential difference of U.
The particle reaches a region of uniform magnetic
field of induction B, where it undergoes circular
motion. If potential difference is doubled & B is also
doubled then magnetic moment of the circular
current due to circular motion of charge Q will
become
(A) double (B) half
(C) four times (D) remain same
2. Three long rod AA, AB, CC are moving with a
speed v in a uniform magnetic field B perpendicular
to the plane of paper as shown in figure. The
triangle formed between the three wires is always an
equilateral triangle. If resistance per unit length of
wire is λ , then the induced current in the triangle is
A B
× × × × × × ×
B0
× × × × × × ×
v v
× × × × × × ×
× × × × × × ×
C
B
× × × × × C
A
(A)
v
3λ
(C)
v
3λ
Physics Challenging Problems
This section is designed to give IIT JEE aspirants a thorough grinding & exposure to variety
of possible twists and turns of problems in physics, that would be very helpful in facing IIT
JEE. Each and every problem is well thought of in order to strengthen the concepts and we
hope that this section would prove a rich resource for practicing challenging problems and
enhancing the preparation level of IIT JEE aspirants.
By : Dev Sharma
Solutions will be published in next issue
Director Academics, Jodhpur Branch
v B0
B0 2B0 (B)
λ
3
v
B0 B0 (D)
λ
3. In the diagram shown, the wires P1Q1 and P2Q2 are
made to slide on the rails with same speed of 5m/s.
In this region a magnetic field of 1T exists. The
electric current in 9 Ω resistor is
× × P1 × × P2 × × × ×
× × × × × × × × ×
4cm × 2Ω × × × 2Ω × × × × ×
9Ω
× × × × × × × × ×
× × × × × × × × ×
Q1
Q2
v
(A) zero if both wires slide towards left
(B) zero if both wires slide in opposite direction
(C) 0.2mA if both wires move towards left
(D) 0.2mA if both wires move in opposite direction
4. A circular current loop is shown in the adjacent
figure. The magnetic field in the region is along xaxis
and its magnitude in the space is increasing
with increasing y-coordinate. The net magnetic
force on the loop is
XtraEdge for IIT-JEE 14 FEBRUARY 2012
y
(A) along + z-axis (B) along –z-axis
(C) along +y-axis (D) None of these
5. A point charge q moves from A to B along a
parabolic path (AB is latus rectum). Electric field is
along x-axis. The work done by the field is
y
B
y 2 A B
= 4ax
(A) – qEa (B) –2qEa
(C) +qeA (D) 2qEa
6. In a cylindrical zone of radius R, magnetic field →
B is
present perpendicular to the plane of the paper into it.
Magnitude of →
B is varying with time as B0(p + qt)
where p & q are positive constant. Consider a static
triangle loop OAM. Emf induced in the triangular loop
x
x
E
Set # 10