Magnetism and Matter Test -10

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Magnetism and Matter Test -10

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Weekly Quiz Competition

Question 1
1 / -0

The materials suitable for making electromagnets should have

A
low retentivity and low coercivity

B
high retentivity and low coercivity

C
low retentivity and high coercivity

D
high retentivity and high coercivity

Solution

A materials suitable for making electromagnet is that which will become a strong magnet, when current is switched on and will lose magnetism on switching off the current. Therefore, such a material should have low retentivity and low coercivity.
Question 2
1 / -0

Curie temperature is the temperature above which:

A
a paramagnetic material becomes diamagnetic

B
a ferromagnetic material becomes diamagnetic

C
a paramagnetic material becomes ferromagnetic

D
a ferromagnetic material becomes paramagnetic

Solution

A ferromagnetic material becomes paramagnetic at a temperature above its curie temperature.
Question 3
1 / -0

Relative permittivity and permeability of a material are ε_r and μ_r respectively. Which of the following values of these quantities are allowed for a diamagnetic material?

A
ε_r= 0 ⋅ 5, μ_r = 1 ⋅ 5

B
ε_r= 0 ⋅ 5, μ_r = 0 ⋅ 5

C
ε_r= 1 ⋅ 5, μ_r = 1 ⋅ 5

D
ε_r= 1 ⋅ 5, μ_r = 0 ⋅ 5

Solution

For a diamagnetic material, relative permittivity is greater then 1 and relative permeability is less than 1.
Question 4
1 / -0

Needles N₁, N₂ and N₃ are made of a ferromagnetic, a paramagnetic and a diamagnetic substance respectively. A magnet, when brought close to them, will

A
attract N₁ and N₂ strongly but repel N₃

B
attract N₁ strongly, N₂ weakly and repel N₃ weakly

C
attract N₁ strongly, but repel N₂ and N₃ weakly

D
attract all three of them

Solution

A magnet attracts ferromagnetic substance strongly, a paramagnetic substance weakly and a diamagnetic substance is repelled by it.
Question 5
1 / -0

If a diamagnetic substance is brought near north or south pole of a bar magnet, it is

A
repelled by the north pole and attracted by the south pole

B
attracted by both the poles

C
attracted by the north pole and repelled by the south pole

D
repelled by both the poles

Solution

The diamagnetic substances are feebly repelled by a magnet
Question 6
1 / -0

A bar magnet of magnetic moment M is cut into two parts of equal lengths. The magnetic moment and pole strength of either part is:

A
M, m/2

B
M/2, m

C
M/2, m/2

D
M, m

Solution

Let m and 2l be the pole strength and the length of the given bar magnet. Then, magnetic moment of the magnet,
M = m × 2 l
When the magnet is cut into two equal parts, each part will be a magnet having pole strength m (unchanged) and length l.
Therefore, the magnetic moment of each part will be ml i.e. M/2.
Question 7
1 / -0

A magnet of moment 4Am$$^{2}$$ is kept suspended in a magnetic field of induction $$5\times 10^{-5}T$$. The workdone in rotating it through 180$$^{0}$$ is

A
$$4\times 10^{-4}J$$

B
$$5\times 10^{-4}J$$

C
$$2\times 10^{-4}J$$

D
$$10^{-4}J$$

Solution

Torque acting on a magnetic dipole of moment $$M$$ kept in uniform magnetic field=$$MBsin\theta$$
Therefore work done in rotating a magnet of moment $$M$$ by an angle $$\theta=MB(1-\cos\theta)=MB(1-(-1))=2MB$$
$$=2\times 4\times 5\times 10^{-5}=4\times 10^{-4}J$$
Question 8
1 / -0

To explain ferromagnetism, we use

A
orbital electron hypothesis

B
spinning electron hypothesis

C
domain theory

D
Langevin theory

Solution

According to domain theory, during Magnetization the individual magnetic moment of the atom are aligned and they point in the same direction, and the paramagnetic materials have the same characteristic.
Question 9
1 / -0

A bar magnet of magnetic moment $$2Am^{2}$$ is free to rotate about a vertical axis passing through its center. The magnet is released from rest from east - west position. Then the KE of the magnet as it takes N-S position is
$$(B_{H}=25\mu T)$$

A
$$25 \mu J $$

B
$$50\mu J$$

C
$$100\mu J$$

D
$$12.5\mu J$$

Solution

$$m=2Am^{2}$$
$$B_{H}=25\mu T$$
$$\theta _{1}=\pi /2$$
$$\theta _{2}=0$$
$$\mu _{1}=-mB\cos\pi /2$$
$$=0$$
$$\mu _{2}=-mB\cos0^o$$
$$=-2\times 25\mu T$$
$$k_{1}=0$$
$$k_{2}-k_{1}=U_{1}-U_{2}$$
$$k_{2}=50\mu J$$
Question 10
1 / -0

In the hystersis cycle, the value of H needed to make the intensity of magnetisation zero is called

A
retentivity

B
coercivity

C
Lorentz force

D
Pseudo force

Solution

retentivity, We reduce H to make it zero,whereas B$$\neq$$0 (standard result of magnetism)
Question 11
1 / -0

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

A
only a is correct

B
only b is correct

C
both a and b are correct

D
both a and b are wrong

Solution

Because the suceptibility of the soft iron is very high. and its coercivity is very low,it is very less corrosive.
$$\chi$$ >> 1 (Because it is a ferromagnetic substance.)
Question 12
1 / -0

Maximum P.E. of magnet of moment M situated in a magnetic field of induction B, is

A
$$\dfrac {1}{2}MB$$

B
$$\dfrac {M}{B}$$

C
$$2MB$$

D
$$MB$$

Solution

$$PB=-MB\cos\theta $$
$$\text{PE is max if}$$ $$\theta = \pi $$
$$\therefore PB_{max}=MB$$
Question 13
1 / -0

A short bar magnet placed with its axis at $$30^{o}$$ with a uniform external magnetic field of $$0.16\ T$$ experiences a torque of magnitude $$0.032 Nm$$ If the bar magnet is free to rotate, its potential energies when it is in stable and unstable equilibrium are respectively

A
$$-0.064\ J, +0.064\ J$$

B
$$-0.032\ J, +0.032\ J$$

C
$$+0.064\ J, -0.128\ J$$

D
$$0.032\ J, -0.032\ J$$

Solution

$$\theta =30^{o}$$
$$B=0016\ T$$
$$z=0.032$$
$$z=mB\sin\theta $$
$$0.032=m\times \dfrac{1}{2}\times 0.16$$
$$m=0.4\ Am$$
$$U=-\vec{m}.\vec{B}$$
$$U_{max}=mB = 0.4\times 0.16=0.064\ T$$
$$U_{min}=-mB = -0.4\times 0.16= -0.064\ T$$
Question 14
1 / -0

The value of susceptibility for super conductor is

A
0

B
$$\infty $$

C
+1

D
-1

Solution

-1, Because superconductor are perfect diamagnetic substances.
Question 15
1 / -0

The work required to rotate a magnetic needle by 60$$^{0}$$ from equilibrium position in a uniform magnetic field is W. The torque required to hold it in that position is

A
$$\dfrac{\sqrt{3}}{2}$$W

B
W

C
$$\dfrac{W}{2}$$

D
$$\sqrt{3}W$$

Solution

$$\theta _{1}=0$$
$$\theta _{2}=60$$
$$U_{1}=-mBcos0$$
$$=-mB$$
$$U_{2}=-mBcos60^{0}$$
$$=-\dfrac{mB}{z}$$
$$w=U_{2}-U_{1}$$
$$=\dfrac{mB}{z}$$
$$z=mBsin\theta $$
$$=mBsin60$$
$$=\dfrac{mB}{z}\sqrt{3}$$
$$z=w\sqrt{3}$$
Question 16
1 / -0

A coil of area $$A$$, turns $$N$$ and carrying current $$i$$ is placed with its face parallel to the lines of magnetic induction $$B$$. The work done in rotating the coil through an angle of $$180^{0}$$ is

A
$$iNAB$$

B
$$2iNAB$$

C
$$\dfrac{iNAB}{2}$$

D
$$Zero$$

Solution

The potential energy of a system of magnetic moment $$\vec{M}$$ placed in magnetic field $$\vec{B}$$ is $$U=-\vec{M}.\vec{B}$$
Hence the total work done in rotating the coil by $$180^{\circ}$$ is
$$U_2-U_1=(MB)-(-MB)$$
$$=2MB=2iNAB$$

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Magnetism and Matter Test -10

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Question 1

low retentivity and low coercivity

high retentivity and low coercivity

low retentivity and high coercivity

high retentivity and high coercivity

Solution

Question 2

a paramagnetic material becomes diamagnetic

a ferromagnetic material becomes diamagnetic

a paramagnetic material becomes ferromagnetic

a ferromagnetic material becomes paramagnetic

Solution

Question 3

εr = 0 ⋅ 5, μr = 1 ⋅ 5

εr = 0 ⋅ 5, μr = 0 ⋅ 5

εr = 1 ⋅ 5, μr = 1 ⋅ 5

εr = 1 ⋅ 5, μr = 0 ⋅ 5

Solution

Question 4

attract N1 and N2 strongly but repel N3

attract N1 strongly, N2 weakly and repel N3 weakly

attract N1 strongly, but repel N2 and N3 weakly

attract all three of them

Solution

Question 5

repelled by the north pole and attracted by the south pole

attracted by both the poles

attracted by the north pole and repelled by the south pole

repelled by both the poles

Solution

Question 6

M, m/2

M/2, m

M/2, m/2

M, m

Solution

Question 7

$$4\times 10^{-4}J$$

$$5\times 10^{-4}J$$

$$2\times 10^{-4}J$$

$$10^{-4}J$$

Solution

Question 8

orbital electron hypothesis

spinning electron hypothesis

domain theory

Langevin theory

Solution

Question 9

$$25 \mu J $$

$$50\mu J$$

$$100\mu J$$

$$12.5\mu J$$

Solution

Question 10

retentivity

coercivity

Lorentz force

Pseudo force

Solution

Question 11

only a is correct

only b is correct

both a and b are correct

both a and b are wrong

Solution

Question 12

$$\dfrac {1}{2}MB$$

$$\dfrac {M}{B}$$

$$2MB$$

$$MB$$

Solution

ε_r= 0 ⋅ 5, μ_r = 1 ⋅ 5

ε_r= 0 ⋅ 5, μ_r = 0 ⋅ 5

ε_r= 1 ⋅ 5, μ_r = 1 ⋅ 5

ε_r= 1 ⋅ 5, μ_r = 0 ⋅ 5

attract N₁ and N₂ strongly but repel N₃

attract N₁ strongly, N₂ weakly and repel N₃ weakly

attract N₁ strongly, but repel N₂ and N₃ weakly