Magnetism and Matter Test 25

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Magnetism and Matter Test 25

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Weekly Quiz Competition

Question 1
1 / -0

Which of the following is not true about the advantages of electromagnet over permanent magnet?

A
Small electromagnets are used in electronic lock

B
Scrapyard cranes have powerful electromagnets that lift metal car bodies with ease.

C
Magnetic Resonance Imaging machines use very powerful electromagnets to produce highly detailed images of the human body.

D
None of the above

Solution

Magnets come in two main types: permanent magnets and electromagnets. As its name suggests, a permanent magnet is always magnetized -- think of a kitchen magnet that stays stuck to a refrigerator door for years. An electromagnet is different; its magnetism works only when powered by electricity. Although an electromagnet is more complicated than a permanent magnet, it has useful and important advantages. An electromagnet can be made very strong by increasing the number of turns in the coil, and by increasing the current passing through the coil. On the other hand, a permanent magnet cannot be made so strong.
Question 2
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In the given diagrams, compasses are used to plot the magnetic field around a bar magnetic field around a bar magnet with poles marked N (North) and S (South). Which of the following diagrams correctly shows the expected field pattern?

A

B

C

D

Solution

The magnetic field line is directed from the North pole to the South Pole outside the magnet. The arrowhead of the needle of the compass points toward the direction of the field.
So option (D) is correct.
Question 3
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A square loop of wire, side length 10 cm is placed at angle of $$45^{o}$$ with a magnetic field that changes uniformly from 0.1 T to zero in 0.7 seconds. The induced current in the loop ( its resistance is $$1\Omega$$) is

A
1.0 mA

B
2.5 mA

C
3.5 mA

D
4.0 mA

Solution

$$L=10cm=10^{-2}m$$ ,$$\theta=45$$
Initial magnetic field $$B_i=0.1T$$
Final magnetic field $$B_f=0T$$
$$\Delta t=0.7s$$ ,$$R=1 \Omega$$
Now $$\phi_B=BA \cos\theta=\dfrac{ { 10 }^{ -3 } }{ \sqrt { 2 } } $$wB
$${ \phi }_{ { B }_{ f } }=0\quad wb$$
$$\varepsilon =\dfrac{\left| \Delta { \phi }_{ B } \right| }{ \Delta t } =\dfrac{ { 10 }^{ -3 } }{ \sqrt { 2 } } \times \dfrac{ 1 }{ 0.7 } =1mV$$
$$I=\dfrac{ \varepsilon }{ R } =\dfrac{ 1mV }{ 1\Omega } =1mA$$
Question 4
1 / -0

Potential energy of a bar magnet, of magnetic moment M is placed in a magnetic filed of induction B such that it makes an angle $$\theta$$ with the direction of B is?

A
$$MB\,sin\, \theta$$

B
$$-MB\,cos\, \theta$$

C
$$MB(1-cos\theta)$$

D
$$MB(1+cos\theta)$$

Solution

$$dw=-\tau .d\theta =-MB\sin \theta .d\theta $$ where $$\theta$$ is angle between $$\vec{M}$$ and $$\vec{B}$$.
Since work done $$=$$ potential energy lost
$$=>du=-dw=MB\sin \theta .d\theta $$
$$=>u=\int MB\sin \theta .d\theta =-MB\cos \theta $$
Question 5
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Which of the following statement is correct?

A
Both dynamo and electric motor converts mechanical energy into electrical energy

B
Dynamo converts electrical energy into light energy and electric motor converts mechanical energy into electrical energy

C
Dynamo converts mechanical energy into electrical energy and electric motor converts electrical energy into mechanical energy

D
Both dynamo and electric motor converts electric energy into mechanical energy

Solution

Dynamo is used to convert mechanical energy into electric energy while the electric motor is used to convert electrical energy into mechanical energy.
Hence option 'C' is correct.
Question 6
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A copper rod of length $$l$$ rotates about its end with angular velocity $$\omega$$ in a uniform magnetic field B. The emf developed between the ends of the rod if the field is normal to the plane of rotation is:

A
$$B\omega l^2$$

B
$$\dfrac{1}{2}B\omega l^2$$

C
2 $$B\omega l^2$$

D
$$\dfrac{1}{4}B\omega l^2$$

Solution

$$dE=B\omega xdx\\E=\int _{ 0 }^{ L }{ B\omega xdx } \\ \quad =B\omega\cfrac{x^2}{2}\\ \quad=\cfrac{1}{2}B \omega l^2$$
Question 7
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What is the phenomenon shown by ferromagnetic substances whereby the magnetic flux through the medium on magnetising field as well as previous state of the substance?

A
Irradiation

B
Hysteresis

C
Inversion

D
Magnetostriction

Solution

B. Hysteresis.

A magnetic hysteresis, otherwise known as a hysteresis loop, is a representation of the magnetizing force(H) versus the magnetic flux density(B) of Ferromagnetic material. The curvature of the hysteresis is characteristic of the type of material being observed and can vary in size and shape.
Question 8
1 / -0

The direction of an induced electromotive force is always such as to oppose the cause that produces it. This is called :

A
Neumann's law

B
Lenz's law

C
Biot-Savert law

D
Faraday's third law of electromagnetic induction

Solution

B. Lenz's law
It states that the direction of the induced electromagnetic force is always such as to result in opposition to the change producing it.
Question 9
1 / -0

Dynamo is a device that converts.

A
Alternating current into direct current

B
Electrical energy into mechanical energy

C
Mechanical energy into electrical energy

D
Heat energy into electrical energy

Solution

Dynamo is a device which converts mechanical energy into electrical energy.
Question 10
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Charged particle (charge$$=q$$; mass$$=m$$) is rotating in a circle of radius $$R$$ with uniform speed $$V$$, Ratio of its magnetic moment ($$\mu$$) to the angular momentum ($$L$$) is

A
$$\cfrac { q }{ 2m } $$

B
$$\cfrac { q }{ m } $$

C
$$\cfrac { q }{ 4m } $$

D
$$\cfrac { 2q }{ m } $$

Solution

Magnetic moment $$\mu = IA=(q\nu) A$$
$$\mu = \dfrac{qV}{2\pi R}\times \pi R^2 = \dfrac{qVR}{2}$$
Angular momentum $$L = mVR$$
Ratio $$\dfrac{\mu}{L} = \dfrac{\frac{qVR}{2}}{mVR} = \dfrac{q}{2m}$$