Discussion
1. A strong magnetic field is applied on a
stationary electron. Then the electron
a) moves in the direction of the field
b) remains stationary
c) moves perpendicular to the direction of
the field
d) begins to spin
e) moves opposite to the direction of the field
2. Two long parallel straight wires carry equal
currents in opposite directions. At a point
midway between the wires, the magnetic field
they produce is:
a)zero
b)non-zero and along a line connecting the
wires
c)non-zero and parallel to the wires
d)non-zero and perpendicular to the plane of
the two wires
e)none of the above
3. A cyclotron operates with a given magnetic field
and at a given frequency. If R is the radius of
the trajectory at exit, the final particle energy
is proportional to:
a) 1/R
b) R c) R2 d) R3 e) R4
4. A uniform magnetic field is in the positive z
direction. A positively charged particle is
moving in the positive x direction through the
field. The net force on the particle can be
made zero by applying an electric field in what
direction?
a) Positive y
b) Negative y
c) Positive x
d) Negative x
e) Positive z
5. The resistance of the shunt required to allow
2% of the main current through the
galvanometer of resistance 49 is
a) 1
b) 2
c) 0.2
d) 0.1 e)
0.01
6. A long wire carrying a steady current is bent
into a circle of single turn. The magnetic field
at the centre of the coil is B. If the same wire
is bent into a circular loop of n turns, the
magnetic field at the centre of the coil for the
same current is
a) 2nB
b) 2n2B c) n2B d) nB e) B
7. You are facing a loop of wire which carries a
clockwise current of 3.0A and which surrounds
an area of 5.0 × 10−2 m2. The magnetic dipole
moment of the loop is:
a) 3.0Am2, away from you
b) 3.0Am2, toward you
c) 0.15Am2, away from you
d) 0.15Am2, toward you
e) 0.15Am2, left to right
8. The diagram shows a straight wire carrying
current i in a uniform magnetic field.
i
F
The magnetic force on the wire is indicated by
an arrow but the magnetic field is not shown.
Of the following possibilities, the direction of
the magnetic field is:
a) opposite the direction of the current
b) opposite the direction of force
c) in the direction of force
d) into the page
e) out of the page
9. An electron is launched with velocity ⃗ in a
uniform magnetic field ⃗ . The angle θ between
⃗ and ⃗ is between 0 and 90o. As a result, the
electron follows a helix, its velocity vector ⃗
returning to its initial value in a time interval
of:
a)
b)
c)
d)
e) none of these
10. A 2 C charge moving around a circle with a
frequency of 6.25 x 1012 Hz produces a
magnetic field 6.28 tesla at the centre of the
circle. The radius of the circle is
a) 2.25m b) 0.25m c) 13.0m d) 1.25 m
e) 3.25 m
11. Two wires with currents 2A and 1A are
enclosed in a
circular
loop.
Another
wire
with current 3 A
is
situated
outside
the
loop as shown.
The ∮ ⃗ ⃗⃗⃗ around the loop is
a) 0
b)
30
c) 60
d)
20
e) zero
12. An electron moving around the nucleus with
an angular momentum l has a magnetic
moment
a)
b)
c)
d)
e)
13. The force between two parallel current
carrying wires is independent of
a) their distance of separation
b) the length of the wires
c) the magnitude of currents
d) the radii of the wires
e) the medium in which they are placed
14. A proton, a deuteron and an  particle having
the same kinetic energy are moving in circular
trajectories in a constant magnetic field. If rp,
rd and r denote respectively the radii of the
trajectories of these particles then
a) r = rd>rp
b)
r = r d = r p
c) r = rp<rd
d)
r>rd>rp
e) r<rd<rp
15. The magnetic field at the mid-point between
two parallel wires carrying current in the same
direction is 10 μT. If the direction of smaller
current among them is reversed, the field
becomes 30 μT. The ratio of the larger to the
smaller current in them is
a)
b)
c)
d)
e)
Discussion
B1.
F=
qvBsinstationary
electronvelocity v =0. Then F=0
D2.
Fleming’s Left Hand Rule
C3.
Cyclotron - kinetic energy of particle =
B4.
A5.
Magnetic field = B ̂
Velocity = v ̂
Charge = +q
Magnetic force, ⃗ = q( ⃗X ⃗⃗) = q(v ̂XB ̂ )
= - qvB ̂
Electric force ⃗ = q ⃗⃗
Net force ⃗ = ⃗ + ⃗
Then q ⃗⃗ +- qvB ̂ = 0
So ⃗⃗ =vB ̂
Electric field along positive y axis
Galvanometer current (Ig) is
of main
current (I)
So = 50
Shunt, S =
C6.
=
Field at the center B =
When the same length is bent to n turns,
radius becomes times.
So new filed B =
C7.
E8.
A9.
D10.
= 1
( )
= n 2B
=
Magnetic moment of current loop,
M = NiA
Clockwise current – moment is away from
the
observer
Fleming’s Left Hand Rule
Helical path – After one Time period T =
velocity vector becomes equal to
initial value
Magnetic field B due to a charge q moving
in a
circle of radius r with a frequency ν
B=
A11.
B12.
D13.
C14.
Ampere’s circuital law ∮ ⃗⃗ ⃗⃗⃗⃗ =
where I
is
the net current passing through the
loop
Gyromagnetic ratio is the ratio of magnetic
moment to angular momentum of orbiting
electron.
The force per unit length between two
parallel
current carrying wires
Radius
of
trajectory
of
charged
particle with kinetic energy E, r =
B15.
√
If proton has mass m and charge e, then
deuteron has mass 2m and charge e and αparticle has mass 4m and charge 2e.
Then rd>rp=rα
Current in same direction,
B1 =
= 10 μT
Current in opposite direction,
B2 =
= 30 μT
Then,
= and
=2