## Magnetic Effect Of Current And Magnetism

## Magnetic Properties Of Materials Multiple Choice Question And Answers

**Question 1. If a current I flows through a loop of area A and the strength of the pole thus generated is qm, the magnetic moment of the loop is**

- IA
- IA²
- q
_{m}A - q
_{m}A²

**Answer:** 1. IA

**Question 2. The torque acting on a bar magnet of magnetic moment M in a uniform magnetic field B will be**

- MBsinθ
- MB/sinθ
- MBcosθ
- MB/cosθ

**Answer:** 1. MBsinθ

**Question 3. When a magnet is placed in a uniform magnetic field, it experiences**

- A force but no torque
- A torque but no force
- A force and also a torque
- Neither a force nor a torque

**Answer:** 2. A torque but no force

**Read and Learn More Class 12 Physics Multiple Choice Questions**

**Question 4. In the case of a bar magnet, lines of magnetic induction**

- Start from die North Pole and end at the South Polo
- Run continuously through the bar and outside
- Emerge in circular patios from the middle of the bar
- Are produced only at the north pole like rays of light from a bulb

**Answer:** 2. Run continuously through the bar and outside

**Question 5. The basic element of magnetism is a**

- North pole
- South pole
- Dipole
- Quadrupole

**Answer:** 3. Dipole

**Question 6. While entering a paramagnetic material from the air, the spacing between the magnetic lines of force**

- Remains the same
- Decreases
- Increases
- First increases then decreases

**Answer:** 2. Decreases

**Question 7. The magnetic susceptibility of a diamagnetic material is**

- Nearly 1000
- Slightly greater than 1
- In between 0 and 1
- Less than 0

**Answer:** 4. less than 0

**Question 8. Curie temperature is the temperature above which**

- Ferromagnetic material becomes paramagnetic
- Ferromagnetic material becomes diamagnetic
- Paramagnetic material becomes diamagnetic
- Paramagnetic material becomes ferromagnetic

**Answer:** 1. Ferromagnetic material becomes paramagnetic

**Question 9. There are four light-weight-rod samples A, B, C, and D separately suspended by threads. A bar magnet is slowly brought near each sample and the following observations are noted.**

**A is feebly repelled****B is feebly attracted****C is strongly attracted****D remains unaffected**

**Which one of the following is true?**

- B is of a paramagnetic material
- C is of a diamagnetic material
- D is of a ferromagnetic material
- A is of a non-magnetic material

**Answer:** 1. B is of a paramagnetic material

**Question 10. At a place on the earth’s surface where the horizontal and vertical components of the earth’s magnetic field are equal,**

- The angle of dip is 0°
- The angle of dip is 90°
- The angle of the dip is 45°
- The angle of dip is 30°

**Answer:** 3. The angle of dip is 45°

**Question 11. At any place in the northern hemisphere of Earth, the value of the angle of the dip**

- Is positive everywhere
- Is negative everywhere
- Is zero everywhere
- May be zero, positive, or negative depending on the position of the place

**Answer:** 1. Is positive everywhere

**Question 12. If the Intensity of the geomagnetic field at a place on the magnetic equator of the earth is 20 A.m ^{-1}, die horizontal component of the geomagnetic intensity there is**

- 28 A m
^{-1} - >28 A m
^{-1} - <28 A m
^{-1} - Zero

**Answer:** 1. 28 A m^{-1}

**Question 13. If the intensity of the geomagnetic field at the magnetic poles of the earth is 32 A.m ^{-1}, the horizontal component of. die geomagnetic field intensity there is**

- 32 A.m
^{-1} - >32 A m
^{-1} - < 32 A.m
^{-1} - Zero

**Answer:** 4. Zero

**Question 14. Two short bar magnets of length 1 cm each have magnetic moments i.20 A.m ^{2} and 1.00 A m^{2} respectively. They are placed on a horizontal table parallel to each other with their N poles pointing towards the south. They have a common magnetic equator and are separated by a distance of 20.0 cm. The value of the resultant horizontal magnetic induction at the mid-point O of the line joining their centers is close to (a horizontal component of earth’s magnetic induction is 3.6 x 10^{-5}Wb.m^{-2})**

- 3.50 x 10
^{-4}Wb.m^{-2} - 5.80 x 10
^{-4}Wb m^{-2} - 3.6 x 10
^{-5}Wb.m^{-2} - 2.56 x 10
^{-4}Wb.m^{-2}

**Answer:** 4. 2.56 x 10^{-4}Wb.m^{-2}

**Question 15. The magnetic field of Earth can be modeled by that of a point dipole placed at the center of the Earth. The dipole axis makes an angle of 11.3° with the axis of the earth. At Mumbai, the declination is nearly zero. Then**

- The declination varies between 11.3°W to 11.3° E
- The least declination is 0°
- The plane defined by the dipole axis and earth axis passes through Greenwich
- Declination averaged over the earth must be always negative

**Answer:** 1. The declination varies between 11.3°W to 11.3° E

**Question 16. A paramagnetic sample shows a. net magnetization of 8 A m ^{-1} when placed in an external magnetic field of 0.6T at a temperature of 4K. When the same sample is placed in an external magnetic field of 0.2 T at a temperature of 16K, the magnetization would be**

- \(\frac{32}{3} \mathrm{~A} \cdot \mathrm{m}^{-1}\)
- \(\frac{2}{3} \mathrm{~A} \cdot \mathrm{m}^{-1}\)
- 6 a.m.
^{-1} - 2.4a.m.
^{-1}

**Answer:** 2. \(\frac{2}{3} \mathrm{~A} \cdot \mathrm{m}^{-1}\)

∴ \(I \propto \frac{B}{T} \quad ..\frac{I_2}{I_1}=\frac{B_2}{B_1} \times \frac{T_1}{T_2}\)

∴ \(I_2=I_1 \times \frac{B_2}{B_1} \times \frac{T_1}{T_2}=8 \times \frac{0.2}{0.6} \times \frac{4}{16}=\frac{2}{3} \mathrm{~A} \cdot \mathrm{m}^{-1}\)

**Question 17. A current-carrying circular loop of radius R is placed on the xy plane with the center at the origin. Half of the loop with x>0 is now bent so that it now lies on the yz plane.**

- The magnitude of the magnetic moment now diminishes
- The magnetic moment does not change.
- The magnitude of \(\vec{B}\) at (0, 0, z), (z >> R) increases
- The magnitude of \(\vec{B}\) at (0, 0, z), (z >> R) is unchanged

**Answer:** 1. The magnitude of the magnetic moment now diminishes

When the circular loop is placed in the xy plane, then

⇒ \(M=I \times \pi R^2 \text {, along }+ \text { ve } z \text {-axis }\)

When half of the loop is bent on yz. plane, the magnetic moment of this portion of the loop,

⇒ \(M_1=\frac{I \cdot \pi R^2}{2}\), along positive x-axis

The magnetic moment of the other portion of the loop (in the xy plane)

⇒ \(M_2=\frac{I \pi R^2}{2}\), along positive z-axis

∴ Resultant moment,

⇒ \(M^{\prime}=\sqrt{M_1^2+M_2^2}=\frac{\sqrt{2}}{2} \cdot I \pi R^2<M\)

**Question 18. A circular current loop of magnetic moment M in an arbitrary orientation in an external magnetic field B. The work done to rotate the loop by 30° about an axis perpendicular to its plane is**

- MB
- \(\frac{\sqrt{3}}{2} M B\)
- \(\frac{MB}{2}\)
- Zero

**Answer:** 4. Zero

The orientation of the loop does not change due to this 30° rotation.

∴ W = 0.

**Question 19. S is the surface of a lump of magnetic material.**

- Lines of \(\vec{B}\) are necessarily continuous across S.
- Some lines of \(\vec{B}\) must be discontinuous across S.
- Lines of \(\vec{H}\) are necessarily continuous across S.
- Lines of \(\vec{H}\) cannot all be continuous across S.

**Answer:**

1. Lines of \(\vec{B}\) are necessarily continuous across S.

4. Lines of \(\vec{H}\) cannot all be continuous across S.

⇒ \(\vec{B}\) is necessarily continuous across S. Outside the lump of

magnetic material, \(H=\frac{B}{\mu_0}\) and inside the lump,

⇒ \(H=\frac{B}{\mu}=\frac{B}{\mu_0 \mu_r}\)

Thus lines of \(\vec{H}\) cannot be continuous across S.

**Question 20. If the earth’s magnetic field is supposed to be due to a magnetic dipole placed at the center of the earth, then the angle of dip at a point on the geographic equator**

- Is always zero
- Can be zero at specific points
- Can be positive or negative
- Depends on existing conditions

**Answer:**

2. Can be zero at specific points

3. Can be positive or negative

4. Depends on existing conditions

**Question 21. A long solenoid has 1000 turns per meter and carries a current of 1 A. It has a soft iron core of μ _{r} = 1000. The core is heated beyond Curie temperature T_{C}.**

- The \(\vec{H}\) field in the solenoid is almost unchanged, but the \(\vec{B}\) field reduces drastically.
- The \(\vec{H}\) and \(\vec{B}\) fields in the solenoid are-, nearly unchanged.
- The magnetization in the core reverses direction.
- The magnetization in the core diminishes by a factor of 10
^{8}.

**Answer:**

1. The \(\vec{H}\) field in the solenoid is almost unchanged, but the \(\vec{B}\) field reduces drastically.

4. The magnetization in the core diminishes by a factor of 10^{8}.

H = nI and B = μ_{0} μ_{r} nI

As μ_{r} changes B will change but H will hot.

When the core is heated beyond Curie’s point, it behaves like a paramagnetic material.

⇒ \(\frac{\chi_{\text {ferro }}}{\chi_{\text {para }}}=\frac{10^3}{10^{-5}}=10^8\)

**Question 22. The gyromagnetic ratio of an electron in an H-atom according to the Bohr model, is**

- Independent of which orbit it is in
- Negative
- Positive.
- Increases with quantum number n

**Answer:**

1. Independent of which orbit it is in

2. Negative

Gyromagnetic ratio = \(\frac{e}{2 m}\) = constant

Since e is negative, the gyromagnetic ratio is also negative.

**Question 23. The magnetic field at a distance × along the axis of a short bar magnet is**

- Inversely proportional to x
^{2} - Inversely proportional to x
^{3} - Proportional to the dipole moment of the magnet
- Twice the field at the same distance along the perpendicular bisector of the axis of the magnet

**Answer:**

2. Inversely proportional to x^{3}

3. Proportional to the dipole moment of the magnet

4. Twice the field at the same distance along the perpendicular bisector of the axis of the magnet

**Question 24. In the classification of magnetic materials based on their behavior.**

- The magnetic permeability of paramagnets is more than 1
- The magnetic permeability of diamagnets is negative
- The magnetic permeability of diamagnets does not depend on temperature
- The magnetic permeability of paramagnets does not depend on temperature

**Answer:**

1. Magnetic permeability of paramagnets is more than 1

3. Magnetic permeability of diamagnets does not depend on temperature

**Question 25. The magnetic moment of a particle with charge q rotating in a circular orbit of radius r and velocity v is p. Then**

- \(p \propto q\)
- \(p \propto v\)
- \(p \propto \frac{1}{r}\)
- p and the angular momentum of the particle is always in the opposite direction

**Answer:**

1. \(p \propto q\)

2. \(p \propto v\)

**Question 26. At a place on the surface of the earth, the angle of dip is θ, the intensity of the earth’s magnetism is I and the horizontal and vertical components of the earth’s magnetism are H and H’ respectively. Then**

- H’ = Htanθ
- H’ = Icosθ
- I = Hsecθ
- The value of I is infinity both at the magnetic north and south pole

**Answer:**

1. H’ = Htanθ

3. I = Hsecθ

**Question 27. Each molecule of iron or nickel behaves as a magnetic dipole. The origin of this magnetism is**

- The electric charge of the molecular electrons
- The orbital rotation of the electron about the nucleus
- The spin of the electrons
- Orbital motion and spin of the electrons

**Answer:**

1. Electric charge of the molecular electrons

2. Orbital rotation of the electron about the nucleus

**Question 28. The magnetic moment of a straight iron wire of length l is p. It is bent in the shape of a semicircle. Then**

- The magnetic moment is p
- Magnetic moment is \(\frac{2 p}{\pi}\)
- Magnetic length is l
- Magnetic length is \(\frac{l}{\pi}\)

**Answer:**

2. Magnetic moment is \(\frac{2 p}{\pi}\)

4. Magnetic length is \(\frac{l}{\pi}\)

**Question 29. Relation between the magnetic field vector \(\vec{B}\) and magnetic intensity \(\vec{H}\) at a point in a magnetic field is \(\vec{B}=\mu \vec{H}\) where μ is the magnetic permeability of the medium in which the point is situated. The magnetic permeability of vacuum μ _{0} = 47T x 10^{-7} H m^{-1}. Thus the relative magnetic permeability of the medium \(\mu_r=\frac{\mu}{\mu_0}\). **

**To define the magnetic field at a point in a medium another vector needs to be mentioned, which is magnetization \(\vec{M}\). The magnetic moment per unit volume around a point in a magnetic field is known as the magnetization of the point. In most cases \(\vec{M} \propto \vec{H} \text { or, } \vec{M}=k \vec{H}\); this k is called the magnetic susceptibility of the medium. The relation among these three vectors is expressed as**

**⇒ \(\vec{B}=\mu_0(\vec{H}+\vec{M})\)**

**1. The magnetic permeability of a medium is 1.26 x 10 ^{-6}H m^{-1}. The magnetic susceptibility of the medium is**

- -0.03
- -0.003
- 0.03
- 0.003

**Answer:** 4. 0.003

**2. Magnetisation at a point in the medium is 0.002 A.m ^{-1}. The intensity at that point is (in A.m^{-1})**

- 2.52 x 10
^{-9} - 6 x 10
^{-6} - 0.667
- 1.5

**Answer:** 3. 0.667

**3. Magnetic field at the same point (in Wb.m ^{-2}) is**

- 4.2 x 10
^{-7} - 8.4 x 10
^{-7} - 4.2 x 10
^{-6} - 8.4 x 10
^{-6}

**Answer:** 2. 8.4 x 10^{-7}

**Question 30. When current flows in a closed or almost closed path (helical), it is known as a current loop. Any current loop is equivalent to a magnetic dipole whose dipole moment \(\vec{p}=I \vec{A}\), where I am the current and \(\vec{A}\) is the area vector of the loop. If there are N number of turns instead of one, then magnetic moment \(\vec{p}=N I \vec{A}\). When this loop is placed in an external magnetic field S, the torque acting on the loop, \(\vec{G}=\vec{p} \times \vec{B}\). On the other hand, a small bar magnet is also a magnetic dipole whose length vector is \(\vec{r}\) which is in the south-to-north direction. The magnetic dipole moment of the magnet \(\vec{p}=\overrightarrow{q r}\), where q is the pole strength of the magnet.**

**The force acting on q placed in an external magnetic field of strength \(\vec{B} \text { is } \vec{F}=q \vec{B}\). The magnetic field (B) due to a bar magnet at a point placed at a distance x from the center of the magnet (x>>r) is given by**

**⇒ \(B=\frac{\mu_0 q r}{4 \pi x^3} \sqrt{3 \cos ^2 \theta+1}\)**

**where θ = angle between r and the line joining P with the mid-point of the magnet and = permeability of vacuum or air = 4π x 10 ^{7} H m^{-1}.**

**1. The diameter of a circular conducting coil of 10 turns is 10 cm. A current of 10 mA flows through the coil. The magnetic moment of the coil is**

- 7.85 Am
^{2} - 0.785 A m
^{2} - 3.14 x 10
^{3}A m^{2} - 7.85 x 10
^{-4}A.m^{2}

**Answer:** 4. 7.85 x 10^{-4} A.m^{2}

**2. If the magnetic moment of a 10 cm long bar magnet is equal to that of the above coil, then the strength of the magnet is**

- 7.85 A.m
- 7.85 x 10
^{-2}A m - 7.85 x 10
^{-3}Am - 7.85 x 10
^{-5}A.m

**Answer:** 3. 7.85 x 10^{-3} Am

**3. The coil in question (1) is placed with its axis along an external magnetic field of 10 ^{-4} Wb.m^{-2}. What is the torque acting on the coil?**

- Zero
- 7.85 N.m
- 7.85 x 10
^{-4}N.m - 7.85 x 10
^{-8}N.m

**Answer:** 1. Zero

**4. What is the magnetic field developed at a point on the perpendicular bisector of the magnet in question (2) the distance of the point from the magnet is 2 m.**

- 9.8 x 10
^{-21}Wb.m^{-2} - 1.96 x 10
^{-11}Wb.m^{-2} - 9.8 x 10
^{-6}Wb.m^{-2} - 1.96 x 10
^{-5}Wb.m^{-2}

**Answer:** 1. 9.8 x 10^{-12} Wb.m^{-2}

**5. The magnetic field at a point 2 m away along the axis of the bar magnet is**

- 9.8 x 10
^{-12}Wb.m^{-2} - 1.96 x 10
^{-11}Wb.m^{-2} - 9.8 x 10
^{-6}Wb.m^{-2} - 1.96 x 10
^{-5}Wb.m^{-2}

**Answer:** 2. 1.96 x 10^{-11}Wb.m^{-2}

**6. The magnetic dipole moment of a particle of charge Q and mass m revolving in a circular path of radius r with a velocity v is**

- \(\frac{Q_{v r}^m}{m}\)
- \(\frac{Q}{2 m} v r\)
- Qvr
- \(\frac{1}{2} Q v r\)

**Answer:** 4. \(\frac{1}{2} Q v r\)

**Question 31. If current I flow in a coil of area A and the number of turns n, the magnetic moment of the coil is**

- nIA
- n²IA
- \(\frac{n I}{A}\)
- \(\frac{n I}{\sqrt{A}}\)

**Answer:** 1. nIA

**Question 32. The ratio of magnetic intensities at points at equal distances on the end-on position and broad-side-on position of a short bar magnet is**

- 2:1
- 1: 2
- 3:1
- 1: 1

**Answer:** 1. 2: 1

**Question 33. The relative magnetic permeability of a diamagnetic substance is**

- Zero
- Slightly greater than 1
- Slightly less than 1
- Slightly less than zero

**Answer:** 3. Slightly less than 1

**Question 34. Two similar bar magnets of magnetic moment M each are attached at a right angle with each other at their ends. The magnetic moment of the system will be.**

- M
- 2M
- \(\frac{M}{\sqrt{2}}\)
- √2M

**Answer:** 4. √2M

If the length of each magnet is L and the pole strength is m,

M = mL

The effective length of the system of the two magnets,

⇒ \(L^{\prime}=\sqrt{L^2+L^2}=\sqrt{2} L\)

∴ Magnetic moment = m. \(\sqrt{2} L=\sqrt{2} M\)

The option 4 is correct.

**Question 35. The intensity of magnetization of a bar magnet is 5.0 x 10 ^{4} A.m^{-1}. The magnetic length and the area of the cross-section of the magnet are 12 cm and 1 cm² respectively. The magnitude of the magnetic moment of this bar magnet is (in SI unit)**

- 0.6
- 1.3
- 1.24
- 2.4

**Answer:** 1. 0.6

The volume of the bar magnet,

V = 0.12 x 1 x 10^{-4}

= 1.2 x 10^{-5}m^{3}

The intensity of magnetization,

⇒ \(M=\frac{p_m}{V} \quad\left[p_m=\text { magnetic moment }\right]\)

∴ pm = MV

= 5.0 x 10^{4} x 1.2 x 10^{-5}

= 0.6 A.m^{2}

Option 1 is correct.

**Question 36. An electron in a circular orbit of radius 0.05 nm performs 10 ^{16} revolutions per second. The magnetic moment due to this rotation of electron is (in A.m^{2})**

- 2.16 x 10
^{-23} - 3.21 x 10
^{-22} - 3.21 x 10
^{-24} - 1.26 x 10
^{-23}

**Answer:** 4. 1.26 x 10^{-23}

Magnetic moment

= IA

= qnA

= (1.6 x 10^{-19}) x 10^{16} x π(0.05 x 10^{-5})^{2}

= 1.26 x 10^{-53} A.m^{2}

The option 4 is correct.

**Question 37. If \(\chi\) stands for the magnetic susceptibility of a substance, p for its magnetic permeability, and pQ for the permeability of free space, then**

**For a paramagnetic substance:**\(\chi\) > 0, μ >0**For a paramagnetic substance:**\(\chi\) > 0 , μ > μ_{0}**For a diamagnetic substance:**\(\chi\) > 0 , μ < 0**For a ferromagnetic substance:**\(\chi\) > 1 > μ > μ_{0}

**Answer:** 2 and 4 are correct.

For both paramagnetic and ferromagnetic substances, μ > μ_{0}.

But for paramagnetic substance \(\chi\) > 0 and ferromagnetic substance \(\chi\) > 1.

If 0 < μ < μ_{0}, the substance is not paramagnetic.

Options 2 and 4 are correct.

**Question 38. The coercivity of a small magnet where the ferromagnet gets demagnetized is. 3 x 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 gets demagnetized when inside the solenoid is**

- 6A
- 30 mA
- 60 mA
- 3 A

**Answer:** 4. 3 A

⇒ \(B=\mu_0 n I \quad \text { or, } \frac{B}{\mu_0}=n I \quad \text { or, } H=\frac{N I}{L} \quad\left[∵ B=\mu_0 H\right]\)

∴ \(I=\frac{H L}{N}=\frac{3 \times 10^3 \times 10 \times 10^{-2}}{100}\)

= 3A

The option 4 is correct.

**Question 39. Hysteresis loops for two magnetic materials A and B are given below. These materials are used to make magnets for electric generators, transformer cores, and electromagnet cores.**

- A for electric generators and transformers
- A for electromagnets and B for electric generators
- A for transformers and B for electric generators
- B for electromagnets and transformers

**Answer:** 4. B for electromagnets and transformers

**Question 40. A magnetic needle of magnetic moment 6.7 x 10 ^{-2} A.m^{2} and moment of inertia 7.5 x 10^{6} kg.m^{-2} is performing simple harmonic oscillations in a magnetic field of 0.01 T. Time taken for 10 complete oscillations is**

- 6.65 s
- 8.89 s
- 6.98 s
- 8.76 s

**Answer:** 1. 6.65 s

Time period, \(T=2 \pi \sqrt{\frac{I}{M B}}=2 \pi \sqrt{\frac{7.5 \times 10-6}{6.7 \times 10^{-2} \times 0.01}}\)

= 6.65 x 10^{-1}

∴ Time taken for 10 complete oscillations

= 6.65 x 10^{-1} x 10

= 6.65 s

The option 1 is correct.

**Question 41. The dipole moment of a circular loop carrying a current J, is m and the magnetic field at the center of the loop is B _{1}. When the dipole moment is doubled by keeping the current constant, the magnetic field at the center of the loop is B_{2}. The ratio \(\frac{B_1}{B_2}\) is**

- √2
- \(\frac{1}{\sqrt{2}}\)
- 2
- √3

**Answer:** 1. √2

We know, dipole moment, m = IA [I is loop current, A is area of the loop]

∴ m = I x πR^{2} [R is the radius of the loop]

According to the question, in the second case dipole moment,

⇒ \(m^{\prime}=2 m=I \times \pi(\sqrt{2} R)^2\)

∴ In the second case, the radius of the loop will be √2R

⇒ \(\text { Now, } \frac{B_1}{B_2}=\frac{\frac{\mu_0 I}{2 R}}{\frac{\mu_0 I}{2 \times \sqrt{2} R}}=\sqrt{2}\)

Option 1 is correct.

**Question 42. The magnetic susceptibility is negative for**

- Paramagnetic material only
- Ferromagnetic material only
- Paramagnetic and ferromagnetic materials
- Diamagnetic material only

**Answer:** 4. Diamagnetic material only

**Question 43. The following figures show an arrangement of bar magnets in different configurations. Each magnet has magnetic dipole moment \(\vec{m}\). Which configuration has the highest net magnetic dipole moment?**

**Answer:** 3.

For the configuration 1, total magnetic dipole moment,

⇒ \(\sqrt{m^2+m^2}\)

= \(\sqrt{2} m\)

= 1.41m

For the configuration 2, total magnetic dipole moment,

= m-m

= 0

For the configuration 3, total magnetic dipole moment,

⇒ \(=\sqrt{m^2+m^2+2 m^2 \cos 30^{\circ}}\)

= 1.93m

For the configuration 4, total magnetic dipole moment,

⇒ \(\sqrt{m^2+m^2+2 m^2 \cos 60^{\circ}}\)

= 1.73m

The option 3 is correct.

**Question 44. Two reasons for using soft iron as the material for electromagnets.**

- Low permeability and high retentivity
- High permeability and low retentivity
- Low permeability and low retentivity field
- High permeability and high retentivity

**Answer:** 2. High permeability and low retentivity

The magnetic permeability of soft iron is very high. So, when current flows through it, the electromagnet becomes very strong.

On the other hand, as its retentivity is low, the magnetism disappears as soon as the current flow is stopped.

The option 2 is correct.

**Question 45. A thin diamagnetic rod is placed vertically between the poles of an electromagnet. When the current in the electromagnet is switched on, then the diamagnetic rod is pushed up, out of the horizontal magnetic field. Hence the rod gains gravitational potential energy. The work required to do this comes from**

- The lattice structure of the material of the rod
- The magnetic field
- The current source
- The induced electric field due to the changing magnetic field

**Answer:** 3. The current source

The electrical energy of the electric source is converted into stored gravitational potential energy in the rod.

The option 3 is correct.