Answer
$231.3V$
Work Step by Step
We can find the required potential difference as follows:
$Q=mc_w(T_f-Ti)$
$\implies Q=(4.6kg)(4186J/Kg.C^{\circ})(32^{\circ}C-22^{\circ}C)$
$Q=192.556\times 10^3J$
and $P=\frac{Q}{\Delta t}$
$\implies P=\frac{192.556\times 10^3J}{900s}$
$P=214W$
Now $P=\frac{V^2}{R}$
This simplifies to:
$V=\sqrt{PR}$
We plug in the known values to obtain:
$V=\sqrt{214W\times 250\Omega}$
$V=231.3V$
