Physics Test

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Physics Test - 3

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Question 1
3 / -1

An electron, a proton and an alpha particle having the same kinetic energy are moving in circular orbits of radii r_e, r_p, r_α respectively in a uniform magnetic field B. The relation between r_e, r_p, r_α is:

A
r_e < r_α < r_p

B
r_e > r_P = r_α

C
r_e < r_p = r_α

D
r_e < r_p < r_α

Solution

Solution
Question 2
3 / -1

The magnetic field inside a solid conducting long wire at distance r from its axis is given as B = B₀r³ where B0 is constant. Which of the following relations correctly represents current enclosed in the loop of radius r shown in figure:

A

B

C

D

Solution

Solution
Question 3
3 / -1

Two short bar magnets with magnetic moments 400 ab-amp cm² and 800 ab-amp cm² are placed with their axis in the same straight line with similar poles facing each other and with their centers at 20 cm from each other. Then the force of repulsion is

A
12 dyne

B
6 dyne

C
800 dyne

D
150 dyne

Solution

Solution
Question 4
3 / -1

A magnetic needle lying parallel to a magnetic field required W units of work to turn it through 60°. The torque required to maintain the needle in this position will be

A
√3 W

B
W

C

D
2W

Solution

Solution

Work done by magnetic field is given by

Torque experienced by the needle in final position is

Hence, the torque needed to maintain the needle in this position is √3W.
Question 5
3 / -1

A coil PQRS carrying a current i A is placed in a powerful horse shoe magnet NS of uniform magnetic field as shown in the figure. If A is the area of the coil and θ is the inclination of the plane of the coil with the magnetic field in equilibrium, then, the deflecting couple will be,

A
Bi A cot θ

B
Bi A cos θ

C
Bi A cosec θ

D
Bi A sin θ

Solution

Solution

When θ is the angle made by the magnetic field with the BA cosθ plane of the coil, the field component parallel to the coil is Deflecting couple = torque on the loop = Bi A cosθ.
Question 6
3 / -1

A charge moving with velocity v in X - direction is subjected to a field of magnetic induction in negative X - direction. As a result, the charge will

A
remain unaffected

B
start moving in a circular path Y - Z plane

C
retard along X - axis

D
move along a helical path around X - axis

Solution

Solution

The force acting on a charged particle in magnetic field is given by
Question 7
3 / -1

An electron moving around the nucleus with an angular momentum l has a magnetic moment

A

B

C

D

Solution

Solution
Question 8
3 / -1

An iron rod of volume 10⁻³ m³ and relative permeability 1000 is placed as core in a solenoid with 10 turns cm⁻¹. If a current of 0.5 A is passed through the solenoid, then the magnetic moment of the rod will be :

A
50 × 10²Am²

B
5 × 10²Am²

C
500 × 10²Am²

D
0.5 × 10²Am²

Solution

Solution

Given, Volume of iron rod is, V = 10⁻³ m³

Relative permeability, μ_r = 1000

No of turns per unit length of solenoid is, n = N/l = 10 turns cm⁻¹= 1000

Current through the solenoid is, i = 0.5 A

Magnetic moment of an iron core solenoid is given by,
Question 9
3 / -1

Three long wires, with identical currents, either directly into or directly out of the page, are placed at three corners of a square in four different arrangements as shown. The correct order of the magnitude of the net magnetic field at the centre of the square is

A
B_I = B_IV > B_II = B_III

B
B_II = B_III = B_I = B_IV

C
B_I = B_IV < B_II = B_III

D
B_I > B_III > B_II > B_IV

Solution

Solution

In figure II and III field due to three wires add up. Hence, the magnitude is same. In figure I and IV field due to two wires cancel and the field is only due to third wire.

∴ B_I = B_IV < B_II = B_III
Question 10
3 / -1

A uniform magnetic needle is suspended from its centre by a thread. Its upper end is now loaded with a mass 50 mg , and the needle becomes horizontal. If the strength of each pole is 98.1 A cm and g = 981 cm s⁻², then the vertical component of earth's magnetic induction is

A
0.50 G

B
0.25 G

C
0.005 G

D
0.05 G

Solution

Solution

As shown in figure,

mass M = 50mg = 50 × 10⁻³g

Strength of each pole,

m = 98.1 amp cm , g = 981 cm s⁻², V = ?