An electric heater has a resistance of 20 ohm connected to a cell of 600 V . What amount of heat energy does the heater produce per second ?

Dear Student ,
Here in this question the voltage , V = 600 V and resistance , R = 20 ohm .
Now , the heat energy does the heater produce per second is ,
$P =\frac{V^2}{R}=\frac{{\left(600\right)}^2}{20}=18000 watt$
Regards