Question

You obtain a 50-and a 100-W light bulb. Instead of connecting them in the normal way, you devise a circuit that places them in series across normal household voltage. If each one is an incandescent bulb of fixed resistance, which statement about these bulbs is correct? Both bulbs glow with the same brightness, but more than their normal brightness. The 100-W bulb glows brighter than the 50-W bulb. The 50-W bulb glows more brightly than the 100-W bulb, Both bulbs glow with the same brightness, but less than their normal brightness.

Answer 1

The bulb will glow brighter if power dissipated through it is greater. When in parallel, the voltage is the same for both the bulbs hence the bulb with higher resistance will grow dimmer as $P=V^2/R$ .
Higher wattage means lower resistance hence higher power dissipation, hence in parallel the 100 W bulb will grow brighter.

On the other hand in series the current is the same hence the bulb with higher resistance will grow brighter as $P=I^2R$ .

Higher wattage means lower resistance hence higher power dissipation, hence in parallel the 50 W bulb will grow brighter.

Third option is correct.