What happens when a second bulb is added to the circuit, so that we now have one battery and two bulbs all connected in series, in one single loop?
When the circuit is completed, both bulbs light up. However, this time they are not as bright as the single bulb: they are now equally dim. How can we explain this observation using the electric circuit model?
The effect of adding a second bulb in series is to increase the difficulty in causing a flow in the circuit: the overall
resistance of the circuit increases. The resistance previously provided by the thin filament wire of just one bulb is now doubled due to the presence of two.
This increase in resistance reduces the drift speed of charges everywhere in the circuit. Fewer charges per second pass any point in the circuit so the size of the electric current is reduced.
It is worth emphasising here that the total number of charges moving around the circuit has neither been reduced nor increased.
The drift of the charges have simply been slowed down all around the circuit by adding resistance. Adding a bulb in series always increases the resistance.