Question 7.11: Find the self-inductance of a toroidal coil with rectangular...

Find the self-inductance of a toroidal coil with rectangular cross section (inner radius a, outer radius b, height h), that carries a total of N turns.

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The magnetic field inside the toroid is (Eq. 5.60)

B(r)=\begin{cases} \frac{\mu _{0}NI}{2\pi}\hat{\phi },      for   points   inside   the    coil, \\ 0,                     for   points   outside   the   coil,\end{cases}        (5.60)

B\frac{\mu _{0}NI}{2\pi s}.

The flux through a single turn (Fig. 7.34) is

\int{B.da}=\frac{\mu _{0}NI}{2\pi}h \int_{a}^{b}{\frac{1}{s}ds }=\frac{\mu _{0}NIh}{2\pi}\ln \left(\frac{b}{a}\right) .

The total flux is N times this, so the self-inductance (Eq. 7.26) is

\phi = L I.        (7.26)

L=\frac{\mu _{0}N^{2}h}{2\pi}\ln \left(\frac{b}{a}\right) .                (7.28)

7.34

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