Question

Three bulbs 40w, 60 w and 100 w are connected to 220 v mains. Which bulb will glow brightly, if they are connected in series.

Answer 1

Since series connection is asked we need to find out which one has highest resistance.

resistance = (voltage^2)/power rating

calculating we get 40W lamp to have highest resistance.

Since they are connected in series same current flows through them and the max power drop occurs at 4oW lamp and hence it glows brightest.
that means

Resistance of 100W bulb = 220^2/100 = 484 ohms

Resistance of 60W bulb = 220^2/60 = 806.66 ohms

Resistance of 40W bulb = 220^2/40 =1210 ohms

Since they are connected in series same current flows through them and the max power drop occurs at 4oW lamp and hence it glows brightest