The same wave travels for 2 seconds, what is its frequency?*

In waves distance is measured by wave length  and time is measured by frequency or period.

velocity ratio=wave length multiply by frequency.

HENCE, if the same wave travels for 2 econds its frequency will be 2Hz.

Explanation:

energy stored in the capacitor= 9.72 x 10⁻⁴ j

explanation:

the energy stored in the capacitor is given by

e= 1/2 c v²

e= energy stored

c= capacitance= 1.5 µc= 1.5 x 10⁻⁶f

v= potential difference=36 v

so energy stored= e= 1/2 (1.5 x 10⁻⁶) (36)²

energy stored= 9.72 x 10⁻⁴j