A bar magnet has coercivity \(4\times 10^{3}~\text{Am}^{-1}\). It is desired to demagnetize it by inserting it inside a solenoid \(12~\text{cm}\) long and having \(60\) turns. The current that should be sent through the solenoid is:
1. \(2\) A
2. \(4\) A
3. \(6\) A
4. \(8\) A

The unit of pole strength is

(1) $A{m}^2$

(2) Am

(3) $\frac{A^2}{m}$

(4) $\frac{A^2}{m^2}$

Two similar bar magnets P and Q, each of magnetic moment M, are taken. If P is cut along its axial line and Q is cut along its equatorial line, all the four pieces obtained have

1. Equal pole strength          2. Magnetic moment M/4

3. Magnetic moment M/2      4. Magnetic moment M

Each atom of an iron bar $(5 cm \times 1 cm \times 1 cm)$ has a magnetic moment $1.8\times {10}^{-23} A{m}^2$ . Knowing that the density of iron is $7.78\times {10}^3 kg{m}^{-3}$ atomic weight is 56 and Avogadro's  number is $6.02\times {10}^{23}$the magnetic moment of the bar in the state of magnetic saturation will be

1. 4.75 Am²                  2. 5.74 Am²

3. 7.54 Am²                  4. 75.4 Am²

Two equal bar magnets are kept as shown in the figure. The direction of resultant magnetic field, indicated by arrow head at the point P is (approximately)

1. 

2. 

3. 

4. 

A long magnetic needle of length 2L, magnetic moment M and pole strength m units is broken into two pieces at the middle. The magnetic moment and pole strength of each piece will be

1. $\frac{M}{2},\frac{m}{2}$

2. $M,\frac{m}{2}$

3. $\frac{M}{2},m$

4. M, m

Two identical bar magnets are fixed with their centres at a distance d apart. A stationary charge Q is placed at P in between the gap of the two magnets at a distance D from the centre O as shown in the figure

The force on the charge Q is 

1. zero

2. directed along OP

3. directed along PO

4. directed perpendicular to the plane of the paper

The variation of the intensity of magnetisation (I) with respect to the magnetising field (H) in a diamagnetic substance is described by the graph

1. OD                            2. OC

3. OB                            4. OA

The magnet of a vibration magnetometer is heated so as to reduce its magnetic moment by 19%. By doing this the periodic time of the magnetometer will

1. Increase by 19%       2. Decrease by 19%

3. Increase by 11%       4. Decrease by 21%

A straight wire carrying a current i is turned into a circular loop. If the magnitude of the magnetic moment associated with it in M.K.S. unit is M, the length of wire will be

1. $4\mathrm{\pi iM}$                          2. $\sqrt{\frac{4\mathrm{\pi M}}{i}}$

3. $\sqrt{\frac{4\mathrm{\pi i}}{M}}$                        4. $\frac{M\mathrm{\pi }}{4i}$