Earth's Magnetism

Q.

What is flux density formula?

Q. A wheel with $10$ metallic spokes each $0.5\text{m}$ long is rotated with a speed of $120\text{rev/min}$ in a plane normal to the earth's magnetic field at the place. If the magnitude of the magnetic field is $0.4\text{G}$, what is the induced emf between the axle and the rim of the wheel ?

1. $1.256\times {10}^{-3}\text{V}$
2. $6.28\times {10}^{-4}\text{V}$
3. $1.256\times {10}^{-4}\text{V}$
4. $6.28\times {10}^{-5}\text{V}$

Q. Two concentric circular coils of 10 turns each are situated in the same plane. Their radii are 20 cm and 40cm and they carry currents of 0.2A and 0.3A respectively in opposite directions. If ${\mu }_0$ is permeability of vacuum, then the magnetic field in$Wb/m^2$at the common centre is

1. $\frac{35{\mu }_0}{4}$
2. $\frac{5{\mu }_0}{4}$
3. $\frac{{\mu }_0}{80}$
4. $\frac{7{\mu }_0}{80}$

Q. What is curie point?

Q.

Give one example, where electrical energy changes into mechanical energy.

Q. If density(d), frequency (f)and velocity (v) are taken as fundamental quantities write the dimensions of force and angular momentum

Q. Two bar magnets oscillate in a horizontal plane in earth’s magnetic field with time periods of $3\text{s}$ and $4\text{s}$ respectively. If their moments of inertia are in the ratio of $3:2$, then the ratio of their magnetic moments will be

1. $27:16$
2. $2:1$
3. $1:3$
4. $8:3$

Q. The reduction factor of a tangent galvanometer may be defined as the current passing through it to produce a deflection of

1. ${90}^{\circ }$
2. ${45}^{\circ }$
3. ${30}^{\circ }$
4. ${22.5}^{\circ }$

Q. II73. An electric dipole coincides on Z-axis and its mid-pointis on origin of the coordinates system. The electric fieldat an axial point at a distance z from origin is E andelectric field at an equatorial point at a distance y fromE(z)origin is Ev). Here, z = y >> a, soE(z)/E(y) =.?

Q.

How to demagnetize a permanent magnet using current?

Q. A galvanometer coil has $500$ turns, and each turn has an average area of $3\times {10}^{-4}{\text{m}}^2$. If a torque of $1.5\text{Nm}$ is required to keep this coil parallel to a magnetic field when a current of $0.5\text{A}$ is flowing through it, the strength of the field \$\text{(in T)}$ is .

Q.

A magnetic dipole in a constant magnetic field has

1. zero potential energy when the torque is minimum

2. zero potential energy when the torque is maximum

3. minimum potential energy when the torque is maximum

4. maximum potential energy when the torque is maximum

Q.

A circular loop is kept in that vertical plane which contains the north-south direction. It carries a current that is north at the topmost point. Let A be a point on the axis of the circle to the east of it and B be a point on this axis to the west of it. The magnetic field due to the loop

1. is towards east at A and towards west at B

2. is towards west at A and towards east at B

3. is towards east at both A and B

4. is towards west at both A and B

Q.

If the earth's magnetic field has a magnitude $3.4\times {10}^{-5}$ T at the magnetic equator of the earth, what would be its value at the earth's geomagnetic poles ?

Q. Eddy currents are used in

1. Induction furnace
2. Electromagnetic brakes
3. Speedometers
4. All of these

Q. A horizontal rod of mass 10 gm and length 10 cm is placed on a smooth plane inclined at an angle of ${60}^{\circ }$ with the horizontal, with the length of the rod parallel to the edge of the inclined plane. A uniform magnetic field of induction B is applied vertically downwards. If the current through the rod is 1.73 ampere, then the value of B for which the rod remains stationary on the inclined plane is

1. $Noneoftheabove$
2. $1.73Tesla$
3. $\frac{1}{1.73}Tesla$
4. $1Tesla$

Q.

One fourth of a sphere of radius R is removed as shown. An electric field E exists parallel to x – y plane. Find the flux through remaining curved part.

1. πR²E

2. 

3. 

4. none of these

Q. At a certain place, the horizontal component of earth’s magnetic field is $\sqrt{3}$ times the vertical component. The angle of dip (in degrees) at that place is

Q.

How do we demagnetize a bar magnet?

Q.

The magnetic field dimensions in M, L, T, and C (coulomb) are given as

1. $\left[{\mathrm{MLT}}^{-1}{\mathrm{C}}^{-1}\right]$

2. $\left[{\mathrm{MT}}^2{\mathrm{C}}^{-2}\right]$

3. $\left[{\mathrm{MT}}^{-1}{\mathrm{C}}^{-1}\right]$

4. $\left[{\mathrm{MT}}^{-2}{\mathrm{C}}^{-1}\right]$

Q. Answer the following questions: (a) The earth’s magnetic field varies from point to point in space. Does it also change with time? If so, on what time scale does it change appreciably? (b) The earth’s core is known to contain iron. Yet geologists do not regard this as a source of the earth’s magnetism. Why? (c) The charged currents in the outer conducting regions of the earth’s core are thought to be responsible for earth’s magnetism. What might be the ‘battery’ (i.e., the source of energy) to sustain these currents? (d) The earth may have even reversed the direction of its field several times during its history of 4 to 5 billion years. How can geologists know about the earth’s field in such distant past? (e) The earth’s field departs from its dipole shape substantially at large distances (greater than about 30, 000 km). What agencies may be responsible for this distortion? (f ) Interstellar space has an extremely weak magnetic field of the order of 10–12 T. Can such a weak field be of any significant consequence? Explain. [Note: Exercise 5.2 is meant mainly to arouse your curiosity. Answers to some questions above are tentative or unknown. Brief answers wherever possible are given at the end. For details, you should consult a good text on geomagnetism.]

Q. At a certain place the angle of dip is ${30}^{\circ }$ and the horizontal component of earth's magnetic field is $0.5G$ . The earth's total magnetic field (in $G$ ), at that certain place, is :

1. $\sqrt{3}$
2. $\frac{1}{\sqrt{3}}$
3. $1$
4. $\frac{1}{2}$

Q. A neutral point is obtained at the centre of a vertical circular current carrying coil. The angle between the plane of the coil and the magnetic meridian is

1. $0^{\circ }$
2. ${45}^{\circ }$
3. ${60}^{\circ }$
4. ${90}^{\circ }$

Q. Magadh express takes $16$ hours to cover the distance of $960\text{km}$ between Patna and Ghaziabad. The rails are separated by $130\text{cm}$ and the vertical component of the earth's magnetic field is $4.0\times {10}^{-5}\text{T}$. Find the average emf induced across the width of the train.

Assume uniform non-stop motion.

1. $4.3\times {10}^{-4}\text{V}$
2. $2.1\times {10}^{-4}\text{V}$
3. $5.5\times {10}^{-4}\text{V}$
4. $8.7\times {10}^{-4}\text{V}$

Q. Earth's magnetic field always has a horizontal component except at or Horizontal component of earth's magnetic field remains zero at

1. An altitude of ${60}^{\circ }$
2. Equator
3. Magnetic poles
4. A latitude of ${60}^{\circ }$

Q. A short bar magnet of moment $4{\text{Am}}^2$ is placed in the magnetic merdian with its south-pole pointing geographic north. The distance between two null points is found to be $20\text{cm}$.

$\left(i\right)$The value of horizontal component of the earth's magnetic field at the place is

1. $4\times {10}^{-4}\text{T}$
2. $6\times {10}^{-4}\text{T}$
3. $8\times {10}^{-4}\text{T}$
4. $10\times {10}^{-4}\text{T}$

Q. At a place, the horizontal and vertical intensities of earth’s magnetic field is 0.30 gauss and 0.173 gauss, respectively. The angle of dip (in degrees) at this place is

Q. Two long conductors (shaded parts) of radius $r$ are arranged as shown in figure. Their axes are separated by a distance $d$. Current of equal density $J$ flow in both conductors, directed into the page in one conductor and directed out-of-the page in other. The magnitude and direction of magnetic field at point $A$ are

Diagram

1. $\frac{{\mu }_{0}Jd}{2}$, in $+y$ direction
2. $\frac{{\mu }_{0}J{d}^2}{2\pi r}$, in $-y$ direction
3. $\frac{{\mu }_{0}Jd}{2}$, in $-y$ direction
4. $\frac{{\mu }_{0}J{r}^2}{2\pi d}$, in $+y$ direction

Q. Derive the expression for horizontal and vertical components of the earths magnetic field.

Q. Oscillation performed by a body dropped in a tunnel passing through the centre of earth