Tag: properties of a magnetic field

Questions Related to properties of a magnetic field

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.

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  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.

Show answer
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 __________.

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  1. geographic north

  2. the centre of earth

  3. geographic south

  4. Both (A) and (C)

Show answer
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 :

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  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.

Show answer
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 :

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  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.

Show answer
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.

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  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

Show answer
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

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  1. $\sqrt (6/5)$

  2. $\sqrt (5/6)$

  3. 1

  4. $\sqrt (7/6)$

Show answer
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

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  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

Show answer
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 :-

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  1. 3R

  2. 4R

  3. 5R

  4. 15R

Show answer
Correct Option: C

The actual north magnetic pole of earth

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  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 $(2)$

Show answer
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,

physics properties of a magnetic field earth - a gigantic magnet magnetic field of earth introduction to magnetic field and magnetic flux

  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

Show answer
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.