# Class 12th Physics – 5 Magnetism and Matter MCQs

#### Magnetic South Pole of the earth is near the ____.

Correct! Wrong!

The magnetic south pole of the earth is near the geographical North Pole. We know that N-pole, N pole repel and N-pole, S-pole attract each other. We also know that a freely suspended bar magnet gets oriented itself in the magnetic N-S direction. The North seeking end of the magnet is named N-pole and the south seeking end, the S-pole. Since earth is a magnet, it is exerting a force on the freely suspended bar magnet and making it to align in the N-S direction. Since unlike poles of two magnets attract each other, the earth's magnetic S-pole must be at the geographic North.

#### A long magnet is cut into two parts in such a way that the ratio of their lengths is 2: 1. Then ratio of pole strengths of both the sections is ___________.

Correct! Wrong!

By cutting magnetic length, the magnetic moment, M gets affected, but not the pole strength. If a magnet is cut, then each part acts as a magnet of the same pole strength. So, options (b), (c) and (d) are wrong. Hence, if a long magnet is cut into two parts in such a way that the ratio of their lengths is 2:1, the ratio of the pole strengths of both the sections is equal.

#### Retentivity of a sample of substance refers to ___________________.

Correct! Wrong!

Retentivity means the flux density or magnetic induction which remains in the sample, after the withdrawal of the external magnetic fields. So, (a) is correct. Option (b) gives susceptibility, (c) gives coercivity and (d) gives permeability of the sample. So, (b), (c) and (d) are incorrect. Hence, Retentivity of a sample of substance refers to the magnetic induction which remains with the sample, even after external magnetic field is cut off.

#### Isogonic lines on a magnetic map will have ____.

Correct! Wrong!

By definition, Isogonic lines on a magnetic map will have the same angle of declination.

#### A paramagnetic material is kept in a magnetic field. The field is increased till the magnetisation becomes constant. If the temperature is now decreased the magnetisation____________.

Correct! Wrong!

Once magnetisation becomes constant, if the temperature is decreased, only the thermal vibrations will be reduced. There would be no negative effect on magnetisation, as paramagnetic material atoms have permanent dipole moments. Hence, in the given scenario, the magnetization remains constant.

#### Magnetic meridian is a ____.

Correct! Wrong!

By definition, Magnetic meridian is a vertical plane.

#### The ___________ type of ferromagnetic material is used for coating magnetic tapes in a cassette player.

Correct! Wrong!

Sound waves and the energy they carry is stored on the cassette tape as electromagnetic signals which are read by the electronic (cassette player) circuit as-is, and are converted back to sound signals by the speaker. To store the electromagnetic signals we need strongly magnetisable and economically cheap materials. So, ceramic ferrites are used. Hence, the ceramic ferrites type of ferromagnetic material is used for coating magnetic tapes in a cassette player.

#### The magnetic flux passing through a given area is zero, if the given area is ____.

Correct! Wrong!

We know that the magnetic flux passing through a given area, φ=BA cosθ When the given area is along the magnetic field lines, the area vector is perpendicular to the magnetic field, Then θ=90o cos900 = 0 Then the magnetic flux φ=0

#### If a solution of ferromagnetic material is poured into a U-tube and one arm of this U-tube is placed between the poles of a strong magnet with the meniscus in line with the field, then the level of the solution will __________.

Correct! Wrong!

Ferromagnetic material has a tendency to move from a region of lower field concentration to a region of higher field concentration. So, the level of solution of ferromagnetic material which is placed in magnetic field rises. Hence, if a solution of ferromagnetic material is poured into a U-tube and one arm of this U-tube is placed between the poles of a strong magnet with the meniscus in line with the field, then the level of the solution will rise.

#### A wire of length 'L' carries a current I. It is bent in the form of a circle. The magnetic moment of current loop (in amp-m2) is ___.

Correct! Wrong!

The magnetic moment of current loop (in amp-m2) is IL2/4Π.
Length of the wire = Circumference of circle formed
L = 2Πr
⇒ Radius of the circle, r=L/2π
Area, A = Πr2 = L2

#### An imaginary line passing through all the places having zero dip is called the ____.

Correct! Wrong!

By definition, an imaginary line passing through all the places having zero dip is called the aclinic line.

#### The magnetic flux passing per unit area is called ____.

Correct! Wrong!

We know that, the maximum flux passing through given area, φ=BA
Then φ=BA
Thus the magnetic flux passing per unit area is called flux density.

#### The unit of magnetic susceptibility is ____.

Correct! Wrong!

Susceptibility, being the ratio intensity of magnetisation to the intensity of magnetising field, it is unit-less and dimensionless.

#### Susceptibility is positive and small for a ____.

Correct! Wrong!

For paramagnetic substance, the intensity of magnetisation I is small and positive, as these are feebly attracted by magnets. Hence, X=I/H is also small and positive for paramagnetic substance.

#### The magnetic field of earth is due to ____.

Correct! Wrong!

The earth's magnetic field is due to distribution of molten metal like materials inside the earth which rotate with the earth thereby forming a current loop and thus producing magnetic field.

#### Two magnetic lines of force _________________.

Correct! Wrong!

Two magnetic lines of force cannot intersect. If they intersect, then at the point of intersection, there would be two different directions of magnetic intensity, one along the tangent of each line of force, which is not possible. Hence, nowhere can the lines intersect.

#### The angle of dip is the angle ____.

Correct! Wrong!

By definition, the angle of dip is the angle between the earth's magnetic field direction and the horizontal direction.

#### The device used to measure magnetic flux is ____.

Correct! Wrong!

Fluxmeter is the device used to measure magnetic flux.

#### At a certain place, the horizontal component of the earth's magnetic field is 3 times the vertical component. The angle of dip at the place is _________.

Correct! Wrong!

The angle of dip at the place is 60°. The earth's magnetic field, Be can be resolved into two components. Horizontal component, H = Be cosθ, parallel to the earth's surface and the vertical component, V = Be sinθ, along the radially outward direction from the earth's centre.
Dividing both,
Tanθ = V/H (or) V = H tan θ
Here, θ is the angle made by earth's magnetic field is with the horizontal.
Given V = √3H in the problem, θ = 60°.

#### At magnetic poles, the angle of dip is ________.

Correct! Wrong!

At magnetic poles, the angle of dip is 90°. Angle of dip is the angle made by the resultant earth's magnetic field with the horizontal. At the magnetic poles, the resultant earth's magnetic field has only a radial component which is perpendicular to the horizontal. So, the dip angle there is 90°.

#### The relation between CGS and SI unit of magnetic flux is ____.

Correct! Wrong!

The SI unit of magnetic flux is Weber denoted as Wb
The CGS unit of magnetic flux is Maxwell denoted as Mx
The relation between them is 1Wb=108Mx

#### The incorrect statement regarding the lines of force of the magnetic filed B is _________________.

Correct! Wrong!

Magnetic lines of force are always closed having the same beginning and ending point. Outside the magnet, the lines of force run from N - pole to S - pole and inside, they run from S - pole to N - pole. So, (c) is incorrect. The number of lines of force passing through unit perpendicular area gives the magnetic intensity at a point. So, (a) is correct. Magnetic lines of force are always closed. So, (b) is correct. No two magnetic lines of force ever intersect for the reason if they do so; point B gets two directions at one point which is absurd. Hence, (d) is correct.

#### The Earth's magnetic field at a certain place has a horizontal component 0.3 gauss and the total strength is 0.5 guass. The angle of dip is ____.

Correct! Wrong!

The angle of dip is tan−1(4/3).
If total field = B, Horizontal component, H and dip angle, δ,
then H = B cos δ (or) cosδ=HB=0.3/0.5=35
So, tanδ=4/3 (or) δ=tan-1(4/3).

#### At Curie point, a ferromagnetic material becomes _______________.

Correct! Wrong!

At Curie point, ferromagnetic substance becomes paramagnetic, as, Ferro magnetism decreases with rise in temperature.

#### According to Gauss' law of magnetism, the magnetic flux passing through a closed surface is ____.

Correct! Wrong!

According to the Gauss law of magnetism. The magnetic flux through a closed surface is zero. Since the inward flux is equal to the outward flux, the net flux passing through the surface is zero.

#### The magnetic materials that have negative magnetic susceptibility are ____ materials.

Correct! Wrong!

The magnetic materials that have negative magnetic susceptibility are diamagnetic materials. Diamagnetic substances are feebly repelled by the magnets. Their magnetisation I is small, negative and temperature independent. Since susceptibility Xm=I/H and I is negative, Xm is also negative for diamagnetic.

#### A freely suspended magnetic needle makes an angle with the horizontal because ____.

Correct! Wrong!

A freely suspended magnetic needle makes an angle with the horizontal because of the earth's magnetism. The freely suspended needle aligns in the direction of the earth's magnetic field making some angle with the horizontal at every place on the earth's surface. This angle is called the dip angle.

#### In the case of a bar magnet, the lines of magnetic induction ____________ .

Correct! Wrong!

Magnetic lines of force are closed always. They have no separate beginning and separate ending points. So, option (a) is wrong and for the same reason (d) is wrong. The much of the magnetism, in case of bar magnets, is concentrated at its poles which are at the two ends of the magnets and magnetism is weak at the centre. So, option (c) is wrong. Lines of force run continuously through the bar inside and outside. Hence (b) is correct.

#### A magnetic needle is kept in a non uniform magnetic field. It experiences _______________.

Correct! Wrong!

In a uniform magnetic field, B has a constant value and so a needle kept in the uniform magnetic field at some angle is acted upon by a Torque and rotates. But, when the field is non-uniform, the magnitude of B changes from point to point and hence, the two poles of the needle experience different forces. This gives rise to a non-zero net force on the needle, in addition to torque.

#### Of the options given, __________ is most paramagnetic.

Correct! Wrong!

Unpaired electrons, if exist, then the substance exhibits paramagnetism.
Chromium contains six unpaired electrons (3d5, 4s1). So, it is most paramagnetic.
Bismuth has three unpaired electrons (6s2, 6p3). So it is relatively less paramagnetic.
Similarly, Antimony has three unpaired electrons 5s2, 6p3. So, it is even less paramagnetic.
Water has no unpaired electrons. Hence it is not paramagnetic at all.

Class 12th Physics - 5 Magnetism and Matter MCQs
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