Bar magnet as an equivalent solenoid; magnetic field lines — practice questions

Question 1 · difficulty L1 · recall

Magnetic field lines of a bar magnet form

• A. closed continuous loops
• B. open lines ending in empty space
• C. radial lines from both poles
• D. circles only in the equatorial plane

Question 2 · difficulty L1 · understanding

Outside a bar magnet, magnetic field lines conventionally run from

• A. south pole to north pole
• B. north pole to south pole
• C. centre to both poles
• D. east to west only

Question 3 · difficulty L2 · understanding

The magnetic potential due to a magnetic dipole at a point on its axis situated at a distance of $$20 \mathrm{~cm}$$ from its center is $$1.5 \times 10^{-5} \mathrm{~T} \mathrm{~m}$$. The magnetic moment of the dipole is _________ $$A \mathrm{~m}^2$$. (Given : $$\frac{\mu_o}{4 \pi}=10^{-7} \mathrm{Tm} A^{-1}$$ )

Question 4 · difficulty L2 · understanding

In a uniform magnetic field, the magnetic needle has a magnetic moment 9.85 $$\times$$ 10 $$-$$2 A/m 2 and moment of inertia 5 $$\times$$ 10 $$-$$6 kg m 2 . If it performs 10 complete oscillations in 5 seconds then the magnitude of the magnetic field is _______________ mT. [Take $$\pi$$ 2 as 9.85]

Question 5 · difficulty L2 · understanding

Given below are two statements : one is labelled as Assertion (A) and the other is labelled as Reason (R). Assertion (A): Magnetic monopoles do not exist. Reason (R): Magnetic field lines are continuous and form closed loops. In the light of the above statements, choose the most appropriate answer from the options given below:

• A. Both (A) and (R) are correct and (R) is the correct explanation of (A)
• B. (A) is not correct but (R) is correct
• C. (A) is correct but (R) is not correct
• D. Both (A) and (R) are correct but (R) is not the correct explanation of (A)

Question 6 · difficulty L2 · understanding

A magnetic dipole experiences a torque of $80 \sqrt{3} \mathrm{~N} \mathrm{~m}$ when placed in uniform magnetic field in such a way that dipole moment makes angle of $60^{\circ}$ with magnetic field. The potential energy of the dipole is :

• A. $$\text { }-40 \sqrt{3} \mathrm{~J}$$
• B. -80 J
• C. 80 J
• D. -60 J

Question 7 · difficulty L2 · understanding

The magnetic moment of a bar magnet is $$0.5 \mathrm{~Am}^2$$. It is suspended in a uniform magnetic field of $$8 \times 10^{-2} \mathrm{~T}$$. The work done in rotating it from its most stable to most unstable position is:

• A. $$4 \times 10^{-2} \mathrm{~J}$$
• B. $$16 \times 10^{-2} \mathrm{~J}$$
• C. $$8 \times 10^{-2} \mathrm{~J}$$
• D. Zero

Question 8 · difficulty L2 · understanding

A bar magnet is released from rest along the axis of a very long vertical copper tube. After some time the magnet will

• A. move down with almost constant speed
• B. move down with an acceleration equal to $$\mathrm{g}$$
• C. move down with an acceleration greater than $$\mathrm{g}$$
• D. oscillate inside the tube

Question 9 · difficulty L2 · understanding

A bar magnet with a magnetic moment $$5.0 \mathrm{Am}^{2}$$ is placed in parallel position relative to a magnetic field of $$0.4 \mathrm{~T}$$. The amount of required work done in turning the magnet from parallel to antiparallel position relative to the field direction is _____________.

• A. zero
• B. 1 J
• C. 2 J
• D. 4 J

Question 10 · difficulty L2 · understanding

A bar magnet having a magnetic moment of 2.0 $$\times$$ 10 5 JT $$-$$1 , is placed along the direction of uniform magnetic field of magnitude B = 14 $$\times$$ 10 $$-$$5 T. The work done in rotating the magnet slowly through 60$$^\circ$$ from the direction of field is :

• A. 14 J
• B. 8.4 J
• C. 4 J
• D. 1.4 J