Light bulbs in series

A 100 W light bulb is placed in series with a 40 W light bulb. When they are connected to a 120-volt wall socket, which bulb will be brighter?

1. The 100 W bulb
2. The 40 W bulb
3. Neither, they will be equally bright

The brightness is related to the power (not the power stamped on the bulb, the power actually being dissipated in the bulb). The current is the same through the bulbs, so consider:

P = I² R

We already showed that the resistance of the 100 W bulb is 144 Ω at 120 volts. A similar calculation shows that the 40 W bulb has a resistance of 360 Ω at 120 volts. Neither bulb has 120 volts across it, but the key is that the resistance of the 40 W bulb is larger, so it dissipates more power and is brighter.

What happens to the 40 W bulb when the 100 W bulb is unscrewed from its socket?

1. It gets brighter
2. It stays the same
3. It gets dimmer (but stays on)
4. It goes off