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Earth - a gigantic magnet - class-VIII

Description: earth - a gigantic magnet
Number of Questions: 23
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Tags: physics properties of a magnetic field magnetism magnetism and electromagnetism
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The magnetic south pole of the earth is situated near the 

  1. geographic south pole

  2. geographic north pole

  3. geographic east

  4. geographic west


Correct Option: B
Explanation:

'Magnetic North' is a LOCATION, so-named to distinguish it from True North -it has NOTHING to do with its magnetic POLARITY. The magnetic polarity at this location is south -which is why it attracts the north-seeking pole of a compass needle or magnet.

State whether true or false:

Magnetic south pole of the Earth lies close to geographic south pole of the Earth.

  1. True

  2. False


Correct Option: B
Explanation:

Magnetic south pole of the Earth lies close to geographic north pole of the Earth.

The shape of the earth's magnetic field resembles that of an imaginary

  1. current-carrying circular loop

  2. solenoid

  3. U-shaped magnet

  4. bar magnet


Correct Option: D
Explanation:

Answer is D.

A freely suspended magnet always points in the North-South direction even in the absence of any other magnet. This suggests that the Earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction: North and South. The shape of the Earth's magnetic field resembles that of a bar magnet of length one-fifth of the Earth's diameter buried at its center.
The South Pole of the Earth's magnet is in the geographical North because it attracts the North Pole of the suspended magnet and vice versa. Thus, there is a magnetic S-pole near the geographical North, and a magnetic N-pole near the geographical South. The positions of the Earth's magnetic poles are not well defined on the globe; they are spread over an area. The axis of Earth's magnet and the geographical axis do no coincide. The axis of the Earth's magnetic field is inclined at an angle of about 15 degrees with the geographical axis. Due to this a freely suspended magnet makes an angle of about 15 degrees with the geographical axis and points only approximately in the North-South directions at a place.

The angle of declination at a place is the angle

  1. Between the vertical plane and the geographical meridian

  2. Between the vertical plane and the magnetic meridian

  3. Between the geographical meridian and magnetic meridian

  4. Between the geographical meridian and horizontal plane


Correct Option: C
Explanation:

A freely suspended magnet always points in the North-South direction even in the absence of any other magnet. This suggests that the Earth itself behaves as a magnet which causes a freely suspended magnet (or magnetic needle) to point always in a particular direction: North and South.

The vertical plane passing through the axis of a freely suspended magnet is called magnetic meridian. The direction of Earth's magnetic field lies in the magnetic meridian and may not be horizontal. The vertical plane passing through the true geographical North and South (or geographical axis of Earth) is called geographical meridian. The angle between the magnetic meridian and the geographic meridian at a place is called declination at that place.

It is a common notion that the earth's magnetism is due to the

  1. presence of a huge permanent magnet in the interior of the earth

  2. presence of electric currents circulating in the interior of the earth

  3. influence of the sun's magnetic field

  4. influence of a nuclear explosion


Correct Option: B
Explanation:

In the Earth, the liquid metal that makes up the outer core passes through a magnetic field which causes an electric current to flow within the liquid metal. The electric current, in turn, creates its own  magnetic field—one that is stronger than the field that created it in the first place.

Even though earth is a giant magnet, all the iron does not stick to it at north and south pole because

  1. Iron is embedded in between rocks

  2. Very less iron is present near the magnetic poles of earth

  3. Force of earth's magnetism is weak

  4. Much iron is scattered away from the poles


Correct Option: C
Explanation:
The Earth has a magnetic field which is produced deep in the interior by electrical currents in the molten part of the core.At the surface of the Earth and beyond the magnetic field is weak.It is just strong enough to rotate a small magnet, called a compass needle, balanced with little friction on a pointed support.
The Earth itself is not a magnet.The magnetic field produced in the core is much more complex in form that what would be expected for a simple magnet such as a bar magnet.The illustration of the Earth as if there is a bar magnet inside are misleading.

AN iron rod burried inside earth along direction becomes a magnet

  1. N-S

  2. S-N

  3. Both

  4. None


Correct Option: A
Explanation:

An iron rod buried inside earth along N-S direction becomes a weak magnet. It is only possible when the earth itself behaves like a magnet.

The bulging of earth at the equator and flattening at the poles is due to 

  1. Centripetal force

  2. Centrifugal force

  3. Gravitational force

  4. Electrostatic force


Correct Option: B
Explanation:

The bulging of equator and flattening at poles is due to rotation of earth. The rotation of earth creates a centrifugal force which is pseudo effect counter balancing the inward acting centripetal force. This pseudo effect pushes the earth outward, bugling from earth, and flattening at poles. 

Hence correct answer is option B.

Define angle of declination at a place :

  1. angle between the vertical plane and the geographical meridian

  2. angle between the vertical plane and the magnetic meridian

  3. angle between the geographical meridian and the magnetic meridian

  4. angle between the geographical meridian and horizontal plane


Correct Option: C
Explanation:

The magnetic meridian is an equivalent imaginary line connecting the magnetic south and north poles and can be taken as the magnetic force lines along the surface of the earth. Therefore a compass needle will be parallel to the magnetic meridian.

The angle between the magnetic and the true meridian is the angle of declination, which is relevant for navigating with a compass.

Assertion : Earth's magnetic field does not affect the working of a moving coil galvanometer.
Reason : The earth's magnetic field is quite weak as compared to magnetic field produced in the moving coil galvanometer.

  1. If both assertion and reason are true and reason is the correct explanation of assertion.

  2. If both assertion and reason are true and reason is not the correct explanation of assertion.

  3. If assertion is true but reason is false.

  4. If both assertion and reason are false.


Correct Option: A
Explanation:

The field magnet used in a moving coil galvanometer is very strong. The earth's magnetic field is quite weak as compared to the magnetic field produced by the field magnet. Practically the coil rotates under the effect of the strong magnetic field due to the earth does not affect the working of the moving coil galvanometer.

The north pole of earth's magnet is near __________.

  1. geographic north

  2. the centre of earth

  3. geographic south

  4. Both (A) and (C)


Correct Option: C
Explanation:
The opposite poles of two magnets attract.
So, north pole of earth's magnet is near the geographic south. The magnetic field created by the molten core of the earth have a magnetic south pole near the geographic north pole to attract N-pole.
Hence, option : C.

A magnet is placed earth's magnetic field with south pole of the magnet pointing north. At the neutral point :

  1. The earth's magnetic field is zero

  2. The fields of the magnet and the earth are equal and in opposite direction.

  3. The magnet's magnetic field is zero.

  4. The net magnetic field is zero.


Correct Option: B,D
Explanation:
If the magnet is placed as the north pole, the magnet is aligned with south of earth magnetic field then at a neutral point, that is the point where magnetic forces are equal and opposite thus fields of both are equal and opposite and Net field is zero.

The magnetic field of earth is due to :

  1. Magnetic dipole buried at the centre of the earth

  2. Motion and distribution of some material in and outside the earth.

  3. Induction effect of the sun.

  4. Interaction of gamma rays with the crust of the earth.


Correct Option: B
Explanation:

The magnetic field of earth is due to motion and distribution of some materials in and outside the earth.
Our planets magnetic field is believed to be generated deep down in earth's core. The Coriolis force, resulting form the earth's spin, also cause swirling whirlpools. The flow of liquid iron generated electric current, which in turn produces magnetic field.
Hence, option : B.

A body of mass 100 kg falls on the earth from infinity. What will be its energy on reaching the earth? Radius of the earth is 6400 km and g = 9.8 $m/s^2$. Air friction is negligible.

  1. 6.27 x $10^9$ J

  2. 6.27 x $10^{10}$ J

  3. 6.27 x $10^{12}$ J

  4. 6.27 x $10^7$ J


Correct Option: A
Explanation:

$\begin{array}{l} u\widehat { i } +k\widehat { i } ={ u _{ f } }+{ k _{ f } } \ \Rightarrow 0+0=\frac { { -GMm } }{ R } +\frac { 1 }{ 2 } m{ v^{ 2 } } \ \Rightarrow \frac { 1 }{ 2 } m{ v^{ 2 } }=\frac { { GMm } }{ { R\times R } } \times R \ =mgR \ =100\times 9.8\times 6400\times 1000 \ =6272000\times 1000 \ =6.27\times { 10^{ 9 } }J \end{array}$

Hence,
option $(A)$ is correct answer.

A ball 'A' of mass m falls to the surface of the earth from infinity. Another ball 'B' of mass 2m falls to the earth from the height equal to six times radius of the earth then ratio of velocities of 'A' and 'B' on reaching the earth is

  1. $\sqrt (6/5)$

  2. $\sqrt (5/6)$

  3. 1

  4. $\sqrt (7/6)$


Correct Option: D
Explanation:

$\begin{array}{l} { v _{ A } }=\sqrt { \dfrac { { 2GM } }{ R }  } \left( { escape\, \, velocity } \right)  \ { v _{ B } }=\sqrt { \dfrac { { 2gh } }{ { 1+\frac { h }{ R }  } }  } =\sqrt { \dfrac { { 2gh } }{ 7 }  } \left( { h=6R } \right)  \ { v _{ B } }=\sqrt { \dfrac { { 2gh6R } }{ 7 }  } =\sqrt { \dfrac { { 12GM } }{ { 7R } }  }  \ \dfrac { { { v _{ B } } } }{ { { v _{ A } } } } =\sqrt { \dfrac { { 2GM } }{ h }  } =\sqrt { \dfrac { { 7R } }{ { 12GM } }  } =\sqrt { \dfrac { 7 }{ 6 }  }  \ Hence, \ \dfrac { { { v _{ _{ B } } } } }{ { { v _{ A } } } } =\sqrt { \dfrac { 7 }{ 6 }  }  \ \therefore \, option\, \, D\, \, is\, correct\, \, answer. \end{array}$

The value of g decreases inside the surface of Earth because

  1. A force of upward attraction is applied by the shell or earth above

  2. The shell of earth above exerts no net force

  3. The distance from the centre of the Earth decreases

  4. The density of the material at the centre of the Earth very small


Correct Option: B,C
Explanation:

for a point inside  there is no nor force of outside shell

also
$F = \dfrac{{GMx}}{{{R^3}}}.m$        inside the surface where $x$ is distance from centre.
$\therefore$ as we go inside the distance from centre decrease
$\therefore$ $\vec g$ decrease 
hence, option $(B)$ and $(C)$ both are correct.

The height at which the weight of a body becomes $1/16^{th}$, its weight on the surface of earth (radius R), is :-

  1. 3R

  2. 4R

  3. 5R

  4. 15R


Correct Option: C

The actual north magnetic pole of earth

  1. Is situated near the geographic south pole.

  2. Is north pole of the magnetic substance of the earth's core.

  3. Attracts the $N$ pole of the compass needle.

  4. Both $(1)$ and $(2)$


Correct Option: D
Explanation:

The reason for the magnetic field of the earth is the magnetic substance in the earth’s core. 

So the pole of the earth is the pole of the magnetic substance in the earth’s core.

The actual north magnetic pole of earth is situated near the geographic south pole because the unlike poles attract each other.

 

Atoms or molecules of liquid or gaseous mate-rial are in rapid motion and are not aligned. Thus,

  1. Fluids can be easily magnetised

  2. Fluids can never be magnetised

  3. Liquid iron rotating about an axis in earth's core is an exception

  4. Liquid iron rotating about an axis in earth's core is difficult to be magnetised


Correct Option: C
Explanation:

As the material gets heated or it is in its liquid or gaseous state, the atoms or molecules are in rapid motion and are not aligned. Thus, fluids are seldom magnetic. An exception is when a magnetic material such as iron is in its liquid state and is continuously rotating around the axis; the atoms can be aligned in one direction, even though they are in liquid rapid motion.

When a Magnetic needle is suspended freely, it always aligns itself in the direction of magnetic north and south pole of the earth, the north pole of the needle gets attracted towards the magnetic south pole of the earth and the south gets repelled equally away from it. So two equal and opposite forces acting on the needle should cancel each other which does not happen because the:

  1. Force of attraction and repulsion are equal but not exactly in opposite direction

  2. Force of attraction and repulsion are not exactly equal even though in opposite direction

  3. Force of attraction and repulsion are not equal and not in opposite direction

  4. Force of attraction and repulsion are exactly equal and opposite in direction


Correct Option: B
Explanation:

Though two equal and opposite forces are acting on the needles, they don't cancel each other as they act on two different points and they generate torque which later at proper alignment reaches equilibrium such as

$\sum F=0$ and

$\sum M=0$


The place where the angle of dip is 90 are known as

  1. Geographical poles of the earth

  2. Magnetic equator

  3. Magnetic poles of the earth

  4. None of the above


Correct Option: C
Explanation:

Angle of dip at a point on earth is the angle between earth's magnetic field at given point and the horizontal line of magnetic meridian.

Since the magnetic lines of force emerge and terminate at the poles of Earth, the magnetic lines of force at the poles are perpendicular to the surface there. Hence the angle of dip is $90^{\circ}$.

The north pole of earth's magnet is near

  1. Geographic north

  2. At the center of earth

  3. Geographic south

  4. In any direction


Correct Option: C
Explanation:

Keep this well in mind that the magnetic poles are opposite in position as the geographic poles.

Magnetic north is at geographic south and vice versa.
Geographic north is the part which includes Greenland and south is Antartica.
When we hang a bar magnet then its poles are attracted to the opposite magnetic poles of the earth. We know that opposite poles attract each other.
That's the reason why the bar's north pole turns towards the geographic north of the earth and the south to south.

The earth's field departs from its dipole shape substantially at large distance (greater than about $3000$ km). The responsible factor for this distortion is then

  1. motion of ions in earth's ionosphere

  2. motion of ions in earth's atmosphere

  3. motion of ions in earth's lithosphere

  4. motion of ions in the space.


Correct Option: A
Explanation:

Due to motion of ions in earth's ionosphere the earth's magnetic field gets modified.

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