400J of heat is produced in 4 sec in 4ohm resistor.calculate yhe PD across the resistor.

Actually, after the information you gave, I expected you to ask for the voltage across the resistor, so I'm not quite sure what you mean by 'PD'.  It could be the "Potential Difference" or the "Power Dissipation" across the resistor.Here are both answers:Power Dissipation:Power = Energy/time =  400 joules / 4 seconds = 100 joules/second = 100 watts.Potential Difference (voltage):Power dissipated by a resistor = (voltage)² / Resistance100 watts = (voltage)² / 4 ohmsMultiply each side by 4 ohms:400 watts-ohms = (voltage)²Voltage = square root of 400 watts-ohms = 20 volts