Magnetism and Matter Test 18

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Magnetism and Matter Test 18

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Question 1
1 / -0

Mark the correct difference between a permanent magnet and an electromagnet.

A
Permanent magnet does not require any external electric supply to produce the field. But their magnetic field is normally weaker than that of an electromagnet.

B
Hence, permanent magnets are relatively bulky in size compared to electromagnets.

C
The applications of permanent magnet in the field of engineering are quite limited.

D
All

Solution

Permanent magnet does not require any external electric supply to produce the field. But their magnetic field is normally weaker than that of an electromagnet. Hence, permanent magnets are relatively bulky in size. The strength of the field cannot be varied as per requirement. The field of these magnets are also not everlasting, it will be loosened over a period of time. They also lose their magnetism, if they get subjected to physical shock or vibration. Because of these many disadvantages, the applications of permanent magnet in the field of engineering are quite limited.
Question 2
1 / -0

Identify the incorrect statement from the following.

A
The temperature of the magnet is increased its molecules vibrate more violently.

B
Magnetic strength of a magnet decreases with increasing temperature.

C
$$760^o$$C is the Curie point of iron.

D
None

Solution

The temperature of the magnet is increased its molecules vibrate more violently. As a result the rearranged magnetic molecules of the magnet get out of their aligned position. Hence magnetic strength of a magnet decreases with increasing temperature. If the temperature is further increased the magnetic strength is further decreased and after a certain temperature the entire magnetic properties of a magnet is vanished. The temperature, at which a ferromagnetic material entirely loses its magnetic properties, is referred as Curie point. This temperature is a special characteristic of magnetic material and this is different for different magnetic materials. For example 760C is the Curie point of iron.
Question 3
1 / -0

The temperature, at which a ferromagnetic material entirely loses its magnetic properties, is referred as

A
non magnetic point

B
curie point

C
van revolt point

D
none

Solution

The temperature, at which a ferromagnetic material entirely loses its magnetic properties, is referred as Curie point. This temperature is a special characteristic of magnetic material and this is different for different magnetic materials. For example 760$$^o$$C is the Curie point of iron.
Question 4
1 / -0

Why the applications of permanent magnet in the field of engineering are quite limited?

A
The field of these magnets are not everlasting.

B
They also lose their magnetism, if they get subjected to physical shock or vibration.

C
Their magnetic field is normally weaker.

D
All

Solution

Permanent magnet does not require any external electric supply to produce the field. But their magnetic field is normally weaker than that of an electromagnet. Hence, permanent magnets are relatively bulky in size. The strength of the field cannot be varied as per requirement. The field of these magnets are also not everlasting, it will be loosened over a period of time. They also lose their magnetism, if they get subjected to physical shock or vibration. Because of these many disadvantages, the applications of permanent magnet in the field of engineering are quite limited.
Question 5
1 / -0

An electron ofcharge e moves in a circular orbit ofradius r around the nucleus at a frequency v. The magnetic moment associated with the orbital motion of the electron is

A
$${\pi ver ^2}$$

B
$${\dfrac{\pi vr^2}{e}}$$

C
$${\dfrac{\pi ve}{r}}$$

D
$${\dfrac{\pi er^2}{v}}$$

Solution
Question 6
1 / -0

Which one of the following statement is not correct about the magnetic field?

A
Inside the magnet the linesgo from north pole to south pole of the magnet

B
Tangents, to the magnetic lines give the direction of the magnetic field

C
The magnetic lines form a closed loop

D
Magnetic lines of force do not cut each other
Question 7
1 / -0

Assertion: Lenz's law violates the principle of conservation of energy.
Reason: Induced emf always opposes the change in magnetic flux responsible for its production.

A
If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

B
If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.

C
If Assertion is correct but Reason is incorrect.

D
If Assertion is incorrect but Reason is correct.

Solution

Lenz's law (that the direction of induced emf is always such as to oppose the change that cause it) is direct consequence of the law of conservation of energy,
Question 8
1 / -0

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 $${ Am }^{ 2 }$$):

A
$$2.16\times { 10 }^{ -23 }$$

B
$$3.21\times { 10 }^{ -22 }$$

C
$$3.21\times { 10 }^{ -24 }$$

D
$$1.26\times { 10 }^{ -23 }$$

Solution

Given : $$\nu = 10^{16}$$ rev. per sec
$$r =0.05$$ $$ nm = 5 \times 10^{-11}$$ $$m$$
Area of the circular orbit $$A = \pi r^2 = 3.14 \times (5 \times 10^{-11})^2 = 7.85 \times 10^{-21}$$ $$m^2$$
Magnetic moment $$\mu =IA$$ where $$I = e\nu$$
$$\implies$$ $$\mu = e\nu A =1.6\times 10^{-19} \times 10^{16} \times 7.85 \times 10^{-21}$$
$$\mu = 1.26 \times 10^{-23}$$ $$Am^2$$
Question 9
1 / -0

The ratio of magnetic dipole moment of an electron of charge $$e$$ and mass $$m$$ in Bohr's orbit in hydrogen atom to its angular momentum is

A
$$\dfrac{e}{m}$$

B
$$\dfrac{m}{e}$$

C
$$\dfrac{2m}{e}$$

D
$$\dfrac{e}{2m}$$

Solution

Angular momentum,

$$L = {m}_{e}vr$$ .........(i)

Therefore, orbital motion of electron is equivalent to a current,

$$I = e\left({1}/{T}\right)$$

Period of revolution of electron is given by

$$T = \dfrac{2\pi r}{v}$$

$$\therefore I = e\left(\dfrac{1}{{2\pi r}{v}}\right) = \dfrac{ev}{2\pi r}$$

Area of electron orbit, $$A = \pi {r}^{2}$$

Magnetic dipole moment of the atom

$$M = \dfrac{ev}{2\pi r} \times \pi {r}^{2} = evr$$

Using equation (i), we have

$$M = \left(\dfrac{e}{2{m}_{e}}\right)L$$
Question 10
1 / -0

Why is an electromagnet more advantageous?

A
It produces a strong magnetic field

B
The magnitude of the magnetic field can be easily changed

C
Reversing the current changes the polarity of the magnet

D
All of the above

Solution

An electromagnet is a temporary strong magnet made from a piece of soft iron when current flows in the coil wound around it. It is an artificial magnet and loses its magnetism when the current is removed. The magnitude of magnetic field can be altered by increasing or decreasing the current. Also the polarity can be reversed by changing the direction of current. Thus all these properties make it more advantageous.

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Magnetism and Matter Test 18

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Magnetism and Matter Test 18

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Question 1

Permanent magnet does not require any external electric supply to produce the field. But their magnetic field is normally weaker than that of an electromagnet.

Hence, permanent magnets are relatively bulky in size compared to electromagnets.

The applications of permanent magnet in the field of engineering are quite limited.

All

Solution

Question 2

The temperature of the magnet is increased its molecules vibrate more violently.

Magnetic strength of a magnet decreases with increasing temperature.

$$760^o$$C is the Curie point of iron.

None

Solution

Question 3

non magnetic point

curie point

van revolt point

none

Solution

Question 4

The field of these magnets are not everlasting.

They also lose their magnetism, if they get subjected to physical shock or vibration.

Their magnetic field is normally weaker.

All

Solution

Question 5

$${\pi ver ^2}$$

$${\dfrac{\pi vr^2}{e}}$$

$${\dfrac{\pi ve}{r}}$$

$${\dfrac{\pi er^2}{v}}$$

Solution

Question 6

Inside the magnet the linesgo from north pole to south pole of the magnet

Tangents, to the magnetic lines give the direction of the magnetic field

The magnetic lines form a closed loop

Magnetic lines of force do not cut each other

Question 7

If both Assertion and Reason are correct and Reason is the correct explanation of Assertion.

If both Assertion and Reason are correct, but Reason is not the correct explanation of Assertion.

If Assertion is correct but Reason is incorrect.

If Assertion is incorrect but Reason is correct.

Solution

Question 8

$$2.16\times { 10 }^{ -23 }$$

$$3.21\times { 10 }^{ -22 }$$

$$3.21\times { 10 }^{ -24 }$$

$$1.26\times { 10 }^{ -23 }$$

Solution

Question 9

$$\dfrac{e}{m}$$

$$\dfrac{m}{e}$$

$$\dfrac{2m}{e}$$

$$\dfrac{e}{2m}$$

Solution

Question 10

It produces a strong magnetic field

The magnitude of the magnetic field can be easily changed

Reversing the current changes the polarity of the magnet

All of the above

Solution

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