Adding an Additional Bulb to a Series Circuit: What Happens?

When you add a bulb to the circuit in series with the first bulb, both bulbs will be dimmer than before. This is because adding a bulb in series increases the total resistance of the circuit, which in turn decreases the current flowing through the circuit. Since the power output of a bulb is related to the current and the voltage across it, the decrease in current results in less power dissipation in each bulb, making them dimmer.

If we consider holiday light strings as an example, older versions with bulbs wired in series create an open circuit when one bulb burns out, causing all bulbs to go off. If the string operates on 120 V and has 40 bulbs, the normal operating voltage for each bulb would be 120 V / 40 bulbs = 3 V per bulb. However, newer versions are designed so that if one bulb fails, it creates a short circuit instead, allowing the rest of the bulbs to remain lit. If one burns out in this newer version, and there are 39 remaining bulbs, each would now operate at 120 V / 39 bulbs ≈ 3.08 V per bulb.