Magnetism and Matter Test 19

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Magnetism and Matter Test 19

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Question 1
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A magnetic substance is kept in a magnetic field, it will be thrown out of the field if it is a __________ substance.

A
paramagnetic

B
ferromagnetic

C
diamagnetic

D
antiferromagnetic

Solution

Magnetic substance when kept in a magnetic field is feebly repelled or thrown out if the substance is diamagnetic. This happens because the substance is feebly magnetised in the direction opposite to that of the field. The susceptability of diamagnetic substance is negative. Paramagnetic substances are feebly attracted by a magnetic.
Ferromagnetic substances are strongly attracted by a magnet.
Question 2
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Inside the magnet, the direction of field lines is from

A
north to south pole

B
south to north pole

C
depends on position on the earth

D
none

Solution

The direction of the magnetic field is taken to be the direction in which a north pole of the compass needle moves inside it. Therefore it is taken by convention that the field lines emerge from north pole and merge at the south pole. Inside the magnet, the direction of field lines is from its south pole to its north pole. Thus the magnetic field lines are closed curves. The relative strength of the magnetic field is shown by the degree of closeness of the field lines. The field is stronger, that is, the force acting on the pole of another magnet placed is greater where the field lines are crowded. No two field-lines are found to cross each other. If they did, it would mean that at the point of intersection, the compass needle would point towards two directions, which is not possible.
Question 3
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The strength of the magnetic field of an electromagnet :

A
is always greater than that of a permanent magnet

B
is always less than that of a permanent magnet

C
can be adjusted

D
none of the above

Solution

The strength of magnetic field of electromagnetic can be adjusted by changing the current passing through it.
Question 4
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A magnet of magnetic moment 20 CGS units is freely suspended in a uniform magnetic field of intensity 0.3 CGS units. The amount of work done in deflecting it by an angle of $$30^o$$ in CGS units is

A
6

B
$$3\,\overline{3}$$

C
$$3(2-\overline{3})$$

D
3

Solution

Work done, $$W =-MB_{\mu} (1 - cos\theta )$$
$$= 20 \times 0.3(1 - cos 30^o)$$
$$=6\,1-\frac{\overline{3}}{2}=3(2-\overline{3})$$
Question 5
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All of the following magnets pictured have magnetic field lines like the ones pictured in the diagram above. For which one are the poles correctly labeled?

A

B

C

D

E

Solution

The magnetic field lines originate at the North pole $$(N)$$ and terminate at the South pole $$(S)$$ making closed loops.
Hence option B is correct.
Question 6
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A magnetic moment of an electron orbiting in a circular orbit of radius $$r$$ with a speed $$v$$ is equal to:

A
$$\displaystyle \frac { evr }{ 2 } $$

B
$$\displaystyle evr$$

C
$$\displaystyle \frac { er }{ 2v } $$

D
none of these

Solution

Current flowing in the circular orbit : $$I = e\nu$$ where , $$\nu$$ is the frequency of revolution of electron
$$\therefore$$ $$I = e \times \dfrac{v}{2\pi r}$$
Area of the circular orbit: $$A = \pi r^2$$
Magnetic moment of an electron : $$\mu =IA =\dfrac{ev}{2\pi r} \times \pi r^2 = \dfrac{evr}{2}$$
Question 7
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A bar magnet has a coercivity of $$\displaystyle 4\times { 10 }^{ 3 }{ Am }^{ -1 }$$. It is placed inside a solenoid of length $$12\ cm$$ having 60 turns in order to demagnetize it, the amount of current that should be passed through the solenoid is:

A
$$16\ A$$

B
$$8A\ $$

C
$$4\ A$$

D
$$2\ A$$

Solution

Given: $$ H=4\times 10^3 \ {Am}^{-1}$$

$$ L=12 \ cm$$

$$N=60$$

Bar magnet requires a magnetic intensity ($$ H=4\times 10^3 \ {Am}^{-1}$$) to become demagnetized.

Let $$n$$ be the number of turns per unit length.

$$\Rightarrow n=\dfrac{N}{L}=\dfrac{60}{0.12}=500 \ turns/metre$$

Now, $$H=nI$$

$$\Rightarrow I=\dfrac{H}{n}=\dfrac{4\times 10^3}{500}=8 \ A$$
Question 8
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Identify which is not a property of a ferromagnetic material.

A
It contains magnetic domains.

B
It has magnetic atoms.

C
It can become a permanent magnet.

D
It can be magnetized by a magnetic field.

E
It contains aluminum

Solution

Atoms of a ferromagnetic material have a spin associated with them and hence are magnetic in nature. In the absence of external magnetic field, groups (called domains) are formed in the material in which the spins are aligned. The spin of each domain is different and the net magnetic field is zero. On the presence of an external magnetic field, the spins of the domains realign along with the magnetic field and the material shows magnetic properties. This results in strong magnetism in the material. When a strong magnetic field is applied for a longer period of time, some domains retain their spin resulting in the retention of magnetic field in the material hence converting it to a permanent magnet. Some common ferromagnetic materials are iron, nickel, cobalt and their alloys.
Question 9
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The material used for permanent magnet has:

A
low retentivity, high coercivity

B
high retentivity, low coercivity

C
high retentivity, high coercivity

D
low retentivity, low coercivity

Solution

The materials used to make permanent magnets has high retentivity and high coercivity.
The coercivity tells you how much (reverse) magnetic field would have to be applied to demagnetize the material.
The materials with high remanence and high coercivity from which permanent magnets are made are sometimes said to be "magnetically hard" to contrast them with the magnetically soft materials from which transformer cores and coils for electronics are made.
Question 10
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The magnet is strongest near the

A
Poles of the magnet

B
Ends of the magnet

C
Centre of the magnet

D
One quarter point from the poles of the magnet

Solution

A magnet is strongest near the poles at that point all the field lines are present and its density is maximum thus strongest.