Magnetic Force(12th Grade > Physics ) Questions and answers for exam Preparation

12th Grade > Physics

MAGNETIC FORCE MCQs

Total Questions : 15 | Page 1 of 2 pages

Question 1. A charge particle having charge q is accelerated through a potential difference V and then it enters a perpendicular magnetic field in which it experiences a force F. If V is increased to 5V, the particle will experience a force

F
5F
F5
√5F

Discuss Question

Answer: Option D. -> √5F
:
D

\frac{1}{2} m v^{2} = q V \Rightarrow v = \sqrt{\frac{2 q V}{m}}

. Also

F = q v B

\Rightarrow F = q B \sqrt{\frac{2 q V}{m}}

hence

F \propto \sqrt{V}

which gives

F^{'} = \sqrt{5} F

Question 2. An electron and a proton enter a magnetic field perpendicularly. Both have same kinetic energy. Which of the following is true?

Trajectory of electron is less curved
Trajectory of proton is less curved
Both trajectories are equally curved
Both move on straight line path

Discuss Question

Answer: Option B. -> Trajectory of proton is less curved
:
B
By using

r = \frac{\sqrt{2 m K}}{q B}

; For both particles

q \to

same,

B \to

same,

K \to

same
Hence

r \propto \sqrt{m} \Rightarrow \frac{r_{e}}{r_{p}} = \sqrt{\frac{m_{e}}{m_{p}}}

∵ m_{p} > m_{e}

r_{p} > r_{e}

Since radius of the path of proton is more, hence its trajectory is less curved.

Question 3. Electrons move at right angle to a magnetic field of

1.5 \times 10^{- 2}

Tesla with a speed of

6 \times 10^{7}

m/s. If the specific charge of the electron is

1.7 \times 10^{11}

C/kg, then the radius of the circular path will be?

2.9 cm
3.9 cm
2.35 cm
3 cm

Discuss Question

Answer: Option C. -> 2.35 cm
:
C

r = \frac{m v}{q B} \Rightarrow \frac{v}{(q / m) . B} = \frac{6 \times 10^{7}}{1.7 \times 10^{11} \times 1.5 \times 10^{- 2}} = 2.35 \times 10^{- 2} m = 2.35 c m

Question 4. A coil carries a current and experiences a torque due to a magnetic filed. The value of the torque is 80% of the maximum possible torque. The angle between the magnetic field and the normal to the plane of the coil is

30∘
53∘
45∘
37∘

Discuss Question

Answer: Option B. -> 53∘
:
B
T=B I A n (0.8)

S i n θ = \frac{4}{5} θ = 53^{\circ}

Question 5. The magnetic field is perpendicularly into the plane of the paper and a few charged particles are projected in it. Which of the following statement is true?

A represents proton and B and electron
Both A and B represent protons but velocity of A is more than that of B
Both A and B represents protons but velocity of B is more than that of A
Both A and B represent electrons, but velocity of B is more than that of A

Discuss Question

Answer: Option C. -> Both A and B represents protons but velocity of B is more than that of A
:
C
Both particles are deflecting in same direction so they must be of same sign.(i.e., both A and B represents protons)
By using

r = \frac{m v}{q B} \Rightarrow r \propto v

From given figure radius of the path described by particle B is more than that of A.
Hence

v_{B} > v_{A}

Question 6. An electron (mass =

9 \times 10^{- 31} k g,

charge

= 1.6 \times 10^{- 19}

C ) whose kinetic energy is

7.2 \times 10^{- 20}

J is moving in a circular orbit in a magnetic field of

9 \times 10^{- 5} w e b e r / m^{2}

. The radius of the orbit is

1.25 cm
2.5 cm
12.5 cm
25.0 cm

Discuss Question

Answer: Option B. -> 2.5 cm
:
B

r = \frac{\sqrt{2 m K}}{q B} = \frac{\sqrt{2 \times 9 \times 10^{- 31} \times 7.2 \times 10^{- 20}}}{1.6 \times 10^{- 19} \times 9 \times 10^{- 5}} = 2.5 c m

Question 7. A metallic block carrying current i is subjected to a uniform magnetic induction B as shown in the figure. The moving charges experience a force F given by ……. which results in the lowering of the potential of the face …….? Assume the speed of the carriers to be v.

eVB^k, ABCD
eVB^k, ABCD
−eVB^k, ABCD
−eVB^k, EFGH

Discuss Question

Answer: Option C. -> −eVB^k, ABCD
:
C
As the block is of metal, the charge carriers are electrons; so for current along positive x-axis, the electrons are moving along negative x-axis, i.e.

⃗ v = - v i

and as the magnetic field is along the y-axis, i.e.

⃗ B = B^j

⃗ F = q (⃗ v \times ⃗ B)

for this case yield

⃗ F = (- e) [- v^i \times B^j]

i.e.,

⃗ F = e v B^k

[A s^i \times^j =^k]

As force on electrons is towards the face ABCD, the electrons will accumulate on it an hence it will acquire lower potential.

Question 8. A rectangular coil 6 cm long and 2 cm wide is placed in a magnetic field of 0.02 T. If the loop contains 200 turns and carries a current of 50 mA, the torque on the coil when its area vector is perpendicular to the field will be?

2.4×10−4Nm
2.4 Nm
0.24 Nm
0.024 Nm

Discuss Question

Answer: Option A. -> 2.4×10−4Nm
:
A
T = BiAn
=

0.02 \times 50 \times 10^{- 3} \times 12 \times 10^{- 4} \times 200 = 24 \times 10^{- 5} = 2.4 \times 10^{- 4} N m

Question 9. A vertical wire carrying a current in the upward direction is placed in a horizontal magnetic field directed towards north. The wire will experience a force directed towards