A parallel plate capacitor with circular plates of radius $1\: m$ has a capacitance of $1\: nF$. At $t = 0$, it is connected for charging in series with a resistor $R= 1\: M\omega$ across a $2V$ battery. Calculate the magnetic field at a point $ P$, halfway between the centre and the periphery of the plates, after $t = 10–3s$. (The charge on the capacitor at time $t$ is $q (t) = CV [1 – exp (–t/\tau)]$, where the time constant $\tau$ is equal to $CR$.)

Question

 

$\begin {array} {1 1} (a)\;0.74 \times 10^{-13}T & \quad (b)\;7.4 \times 10^{-15}T \\ (c)\;0.74 \times 10^{-15}T & \quad (d)\;7.4 \times 10^{-13}T \end {array}$