Ferromagnetism

Q. The magnetic moment of a short dipole is, $1{\text{Am}}^2$. What is the magnitude of the magnetic induction in air at $10\text{cm}$ from the center of the dipole on a line making an angle of ${30}^{\circ }$ with the axis of the dipole?

1. $\frac{\sqrt{13}}{2}\times {10}^{-4}\text{T}$
2. $\frac{\sqrt{13}}{4}\times {10}^{-4}\text{T}$
3. $\sqrt{\frac{13}{2}}\times {10}^{-4}\text{T}$
4. $\sqrt{13}\times {10}^{-4}\text{T}$

Q. The coercivity of a small magnet where the ferromagnet gets demagnetized is $3\times {10}^{3}A{m}^{-1}$ . The current required to be passed in a solenoid of length 10 cm and number of turns 100, so that the magnet get demagnetized when inside the solenoid, is :

1. 6A
2. 30 mA
3. 60 mA
4. 3A

Q.

Is${\mathrm{Fe}}_3{\mathrm{O}}_4$ ferromagnetic?

Q. A bar magnet of magnetic moment $M$ is bent in the form of an arc which makes an angle ${60}^{\circ }$ at the centre of arc. The $\%$ change in the magnetic moment is

1. $9\%$ increase
2. $9\%$ decrease
3. $4.5\%$ decrease
4. $4.5\%$ increase

Q. The area enclosed by the hysteresis loop is a measure of

1. Permeability
2. Retentivity
3. Coercivity
4. Energy loss per cycle

Q. A magnet of magnetic moment $M$ and length $2l$ is bent at its mid point such that the angle of bending is ${60}^{\circ }$. The new magnet moment is-

1. $M$
2. $2M$
3. $\frac{M}{2}$
4. $\frac{M}{\sqrt{2}}$

Q.

Ferromagnetism is different from paramagnetism because

1. The atoms of a ferromagnetic material have net dipole moment whereas that of paramagnetic materials don’t.

2. Ferromagnetic material gets attracted to stronger field whereas paramagnetic materials don’t.

3. Ferromagnetic material gets attracted to weaker field whereas paramagnetic materials don’t.

4. Ferromagnetic materials form domains whereas paramagnetic materials don’t.

Q. What happens when a magnetic dipole is placed in a non uniform magnetic field?

1. There will not be any force or a torque acting on the dipole.
2. Only force but no torque acts on the dipole
3. Force is zero, only torque will act on the dipole
4. Both force and torque act on the dipole

Q.

Is Titanium Paramagnetic Or Diamagnetic?

Q. Assertion $\left(A\right)$ : When a magnet is placed in a non-uniform magnetic field , it experiences both translatory and rotatory motion.

Reason $\left(R\right)$ : The poles of the bar magnet experiences resultant force and torque.

1. Both $\left(A\right)$ and $\left(R\right)$ are true, $\left(R\right)$ is the correct explanation of $\left(A\right)$
2. Both $\left(A\right)$ and $\left(R\right)$ are true, $\left(R\right)$ is not the correct explanation of $\left(A\right)$
3. $\left(A\right)$ is true but $\left(R\right)$ is false.
4. $\left(A\right)$ is false but $\left(R\right)$ is true.

Q. Temperature above which a ferromagnetic substance becomes paramagnetic is called

1. Critical temperature
2. Debye's temperature
3. Curie temperature
4. Boyle's temperature

Q. In the hysteresis cycle, the value of straight $H$ needed to make the intensity of magnetization zero is called -

1. Retentivity
2. Coercivity
3. Lorentz force
4. Saturation point

Q. Essential difference between electrostatic shielding by a conducting shell and magnetostatic shielding is due to

1. lines of $\overrightarrow{B}$ can also end but conductors cannot end them.
   
   
2. electrostatic field lines can end on charges and conductors have free charges.
   
   
3. shells of high permeability materials can be used to divert lines of $\overrightarrow{B}$ from the interior region.
4. lines of $\overrightarrow{B}$ cannot end on any material and perfect shielding is not possible
   
   
   
   

Q.

Core of electromagnets are made of ferromagnetic material which has

1. low permeability and high retentivity

2. high permeability and low retentivity

3. low permeability and low retentivity

4. high permeability and high retentivity

Q. Figure shows a cross section of a large metal sheet carrying an electric current along its surface as shown in the figure. The current in a strip of width $dl$ is $kdl$, where $k$ is a constant. The magnetic field at a point $\text{P}$ distance $y$ from the metal sheet will be

1. ${\mu }_{0}k$
2. $\frac{{\mu }_{0}k}{2}$
3. $\frac{{\mu }_{0}k}{4}$
4. $\frac{3}{2}{\mu }_{0}k$

Q. Two short magnets of equal dipole moments $M$ are fastened perpendicularly at their centres, as given in the figure. The magnitude of the magnetic field at a distance $d$ from the centre of the bisector of the right angle is -

[Point $\text{P}$ lies on the equatorial axis of both magnets]

1. $\frac{{\mu }_{0}M}{4\pi d^3}$
2. $\frac{\sqrt{2}{\mu }_{0}M}{4\pi d^3}$
3. $\frac{2\sqrt{2}{\mu }_{0}M}{4\pi d^3}$
4. $\frac{2{\mu }_{0}M}{4\pi d^3}$

Q.

Are antiferromagnetic materials magnetic?

Q. The variation of magnetic susceptibility ($\chi$) with a magnetising field for a paramagnetic substance is

1. 
2. 
3. 
4. 

Q.

What substance makes a better permanent magnet?

Q. A long horizontal wire A is rigidly fixed and an another wire B which is placed directly below and parallel to wire A . Wire B remains suspended in air due to magnetic attraction. If direction of current is reversed in any one wire than due to gravity instantaneous acceleration of free wire B is ??(where g is acceleration due to gravity)

Q. The magnetic susceptibility is negative for
(a) paramagnetic materials only
(b) diamagnetic materials only
(c) ferromagnetic materials only
(d) paramagnetic and ferromagnetic materials

Q. A small rod of bismuth is suspended freely between the poles of a strong electromagnet. It is found to arrange itself at right angles to the magnetic field. This observation establishes that bismuth is

1. Diamagnetic
2. Paramagnetic
3. Ferromagnetic
4. Anti-ferromagnetic

Q. If relative permeability of iron is 2000. Its absolute permeability in S.I units is

1. $8\pi \times {10}^{-3}$
2. $8\pi \times {10}^{-4}$
3. $800/\pi$
4. $8\pi \times {10}^9/\pi$

Q.

At Curie temperature, in ferromagnetic materials

1. the atomic dipoles get aligned
2. the atomic dipoles lose alignment
3. magnetism is zero
4. the atomic dipoles remains stationary

Q. The core of electromagnet is made of soft iron, because
a) susceptibility of soft iron is very high
b) coercivity of soft iron is very low

1. only a is correct
2. only b is correct
3. both a and b are correct
4. both a and b are wrong

Q. Electromagnets are made of soft iron because soft iron has

1. high susceptibility and low retentivity
2. low susceptibility and high retentivity
3. low susceptibility and low retentivity
4. high susceptibility and high retentivity

Q. Curie temperature is the temperature above which

1. Paramagnetic material becomes ferromagnetic material
2. Ferromagnetic material becomes diamgnetic material
3. Ferromagnetic material becomes paramagnetic material
4. Paramagnetic material becomes diamagnetic material

Q. state 10 uses of electromagnet.

Q.

Answer the following questions:

(a) Why does a paramagnetic sample display greater magnetisation (for the same magnetising field) when cooled?

(b) Why is diamagnetism, in contrast, almost independent of temperature?

(c) If a toroid uses bismuth for its core, will the field in the core be (slightly) greater or (slightly) less than when the core is empty?

(d) Is the permeability of a ferromagnetic material independent of the magnetic field? If not, is it more for lower or higher fields?

(e) Magnetic field lines are always nearly normal to the surface of a ferromagnet at every point. (This fact is analogous to the static electric field lines being normal to the surface of a conductor at every point.) Why?

(f ) Would the maximum possible magnetisation of a paramagnetic sample be of the same order of magnitude as the magnetization of a ferromagnet?

Q. What are permanent magnets? Give one example.