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Give definition of magnetic field and give its unit.
Solution
Magnitude of force on electric charge in magnetic field is given by,
$\mathrm{F}=\mathrm{B} q u \sin \theta$ $\therefore \quad \mathrm{B}=\frac{\mathrm{F}}{q v \sin \theta}$
The magnitude of magnetic field $B$ is $1 \mathrm{SI}$ unit, when the force acting on a unit charge $(1 \mathrm{C})$,
moving perpendicular to $B$ with a speed $1 \mathrm{~m} / \mathrm{s}$ in one newton.
$SI$ unit:
Unit of $B$ $=\frac{\text { Unit of F }}{q v \sin \theta}$
$=\frac{1 \mathrm{~N}}{1 \mathrm{C} \times 1 \mathrm{~ms}^{-1} \times 1}=\frac{1 \mathrm{~N}}{1 \mathrm{Cs}^{-1} \times 1 \mathrm{~m}}$
$\quad=1 \mathrm{NsC}^{-1} \mathrm{~m}^{-1}$ is also called Tesla
$\quad=1 \frac{\mathrm{N}}{\mathrm{Am}}$
$\quad=1 \mathrm{NA}^{-1} \mathrm{~m}^{-1}=1 \mathrm{Tesla}$
