**magnetic intensity**.

- The magnetic field produced by the external source of current is called magnetising field.
- The magnetising field depends on external free currents and geometry of current carrying conductor.
- Magnetic intensity at a point in a magnetic field is defined as the number of magnetic lines of force passing normally per unit area about that point taken in free space in the absence of any substance.
- In vacuum the ratio of magnetic induction $\left(\mathrm{B}_{0}\right)$ and magnetic permeability $\left(\mu_{0}\right)$ is called magnetising field H i.e. $H =\frac{ B _{0}}{\mu_{0}}$
- In a toroidal solenoid the magnetic induction of field produced in material of Toroid is

$B=\mu n I$ so Magnetising field $H =\frac{ B }{\mu}= nI$

The magnetic intensity may be defined as the number of ampere turns flowing round a unit length of toroidal solenoid to produce that magnetic field in the solenoid.

**6. Unit of H**

**In S$\mathbf{S} \mathbf{I}$ system ** $H =\frac{ B }{\mu}=\frac{\text { tesla }}{\text { tesla meter }- amp ^{-1}}=$ ampere-meter $^{-1}\left( Am ^{-1}\right)$

$H =\frac{ B _{0}}{\mu_{0}}=\frac{\frac{ F }{ q _{0} v }}{\mu_{0}}=\frac{ F }{ q _{0} v \mu_{0}}=\frac{ N }{ C \left( ms ^{-1}\right) TmA ^{-1}}= Nm ^{-2} T ^{-1}$

$H =\frac{ N }{ m ^{2} T }=\frac{ N }{ Wb }= NWb ^{-1}= Jm ^{-1} Wb ^{-1}$

**In CGS System, ** unit of H is oerested

- 1 oerested $=\frac{1 \text { gauss }}{\mu_{0}}=\frac{10^{-4} T }{4 \pi \times 10^{-7} TmA ^{-1}}=\frac{1000}{4 \pi} Am ^{-1}=80 Am ^{-1}$

**7. It is a vector quantity with dimensions $M^{0} L^{-1} T^{0} A^{1}$. Its direction is from north pole outwards.**

**8. B and H for different situations**

(a) Solenoid $B =\mu_{0} nI$ H = n$\mathbf{I}$with $n=\frac{N}{L}$ = no. of turns per unit length.

(b) Toroid $B =\mu_{0} nI \quad H = nI$ with $n =\frac{ N }{2 \pi R }$

(c) Plane coil $B =\frac{\mu_{0} nI }{2 R } \quad H =\frac{ nI }{2 R }$

(d) Current carrying element $dB =\frac{\mu_{0} I \overrightarrow{ d \ell} \times \overrightarrow{ r }}{4 \pi r ^{3}}$ $dH =\frac{ I (\overrightarrow{ d }(\times \overrightarrow{ r })}{4 \pi r ^{3}}$

9. The magnetic intensity is independent of nature of medium.Also Read: Properties of Paramagnetic & Diamagnetic Materials

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