Two bulbs 40 W and 60 W and rated voltage 240 V are connected in series across a potential difference of 420 V . Which bulb will work at above its rated voltage?

Step-by-step solution

aligned Resistance of 40 Wbulb & = 240 240 40 & =1440 aligned aligned & Its safe current = 240 1440 & =0.167 ~A & aligned Resistance of 60 ~W bulb & = 240 240 60 & =960 aligned aligned Its safe current = 240 960 =0.25 ~A When connected in series to 420 V supply, then the current aligned i= 420 1440+960 & = 420 2400 & =0.175 ~A aligned Thus, current is greater for 40 W bulb, so it will fuse.