Question

A photographic flash unit consists of a xenon-filled tube. It gives a flash of average power $2000W$ for $0.04s$. The flash is due to discharge of a fully charged capacitor of $40\mu F$. The voltage to which it is charged before a flash is given by the unit is

• A. $1500V$
• B. $2000V$
• C. $2500V$
• D. $3000V$

Answer 1

The correct option is B $2000V$

Energy due to this photographic flash is given by:
$E_P=P\times t$
$\therefore$ $E_P=2000\times 0.04J$
Also, we know that energy stored in a fully stored capacitor is given by $E_C=\frac{1}{2}C{V}^2$.
Now, we know as the energy given by flashlight is due to discharge from capacitor
$⟹$ $E_P=E_C$
$⟹$ $2000\times 0.04$ = $\frac{1}{2}\times 40\times {10}^{-6}\times V^2$
$⟹$ $V^2=4\times {10}^6$
$⟹$ $V=2\times {10}^3=2000V$
$\therefore$ The voltage to which it is charged before a flash is produced is $2000V$