Final answer:
In a series circuit, all bulbs go out when one bulb goes out because the electrical pathway is interrupted, stopping the flow of electric current. The operating voltage for each bulb in a string of 40 identical bulbs operating at 120 V is normally 3 V per bulb.
Step-by-step explanation:
When bulbs are connected in a series circuit and one bulb burns out, the electrical pathway is interrupted. This results in all bulbs going out because the flow of electric current stops. In a series circuit, the current has only one path to flow through, and if that path is broken, the current can't reach the other components. This is unlike a parallel circuit, where each component has its own separate path to the power source. When a string of holiday lights that are connected in series is considered, if one of the bulbs burns out and creates an open circuit, the result is that the entire string of lights will go out.
Additionally, for series strings with old bulbs that function like an open switch when they fail, the operating voltage for each of the 40 identical bulbs would normally be the total voltage divided by the number of bulbs, which is 120 V / 40 = 3 V per bulb. In the case of newer versions where the bulbs short when they fail, the rest of the 39 bulbs will remain lit and the operating voltage will now be 120 V / 39, which is slightly more than 3 V per bulb. If a bulb with a lower resistance would now have a slightly higher operating voltage but remain lit.