A short bar magnet placed in a horizontal plane has its axis aligned along the magnetic north-south direction. Null points are found on the axis of the magnet at $14\; cm$ from the centre of the magnet. The earth's magnetic field at the place is $0.36\; G$ and the angle of $dip$ is zero. What is the total magnetic field (in $G$) on the normal bisector of the magnet at the same distance as the null-point (i.e., $14 \;cm$) from the centre of the magnet? (At null points, field due to a magnet is equal and opposite to the horizontal component of earth's magnetic field.)
Earth's magnetic field at the given place, $H=0.36\, G$ The magnetic field at a distance $d$, on the axis of the magnet is given as:
$B_{1}=\frac{\mu_{0}}{4 \pi} \frac{2 M}{d^{3}}=H\dots(i)$
Where, $\mu_{0}=$ Permeability of free space
$M=$ Magnetic moment The magnetic field at the same distance $d$, on the equatorial line of the magnet is given as:
$B_{2}=\frac{\mu_{0} M}{4 \pi d^{3}}=\frac{H}{2}$ [Using equation $(i)$]
Total magnetic field, $B=B_{1}+B_{2}$ $=H+\frac{H}{2}$
$=0.36+0.18=0.54 \,G$
Hence, the magnetic field is $0.54 \,G$ in the direction of earth's magnetic field.
Vibration magnetometer before use, should be set
Vibration magnetometer is used for comparing
A tangent galvanometer has a coil with $50$ $turns$ and radius equal to $4$ $cm$. A current of $0.1 $ $A$ is passing through it. The plane of the coil is set parallel to the earth's magnetic meridian. If the value of the earth's horizontal component of the magnetic field is $7 \times {10^{ - 5}}$ $tesla$ and ${\mu _0} = 4\pi \times {10^{ - 7}}\, weber/amp \times m$, then the deflection in the galvanometer needle will be.....$^o$
The error in measuring the current with a tangent galvanometer is minimum when the deflection is about.....$^o$
To compare magnetic moments of two magnets by vibration magnetometer, 'sum and difference method' is better because