$A$ hollow cylinder has length $l$,radius $r$,and thickness $d$,where $l >> r >> d$,and is made of a material with resistivity $\rho$. $A$ time-varying current $I$ flows through the cylinder in the tangential direction. Assume the current is always uniformly distributed along the length of the cylinder. The cylinder is fixed so that it cannot move; assume that there are no externally generated magnetic fields during the time considered for the problems below. Assume current at $t = 0$ to be $I_0$. What is the current $I(t)$ for $t > 0$?

  • A
    $I = I_0 e^{-\frac{\rho}{2\mu_0 r d} t}$
  • B
    $I = I_0 e^{-\frac{2\rho}{\mu_0 r d} t}$
  • C
    $I = I_0 e^{-\frac{2\mu_0 r d}{\rho} t}$
  • D
    $I = I_0 e^{-\frac{\mu_0 r d}{2\rho} t}$

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