# Which bulb will light up first in a series circuit? [duplicate]

Let's just imagine a long series circuit with three bulbs in it and a switch. Which bulb will light up first when the switch is closed? One nearer to the positive terminal or the one nearer to the negative terminal? Is the flow of electrons or flow of current related to it?

• Why wouldn’t you think that all the bulbs won’t almost simultaneously light up? Dec 3 '19 at 6:38
• My friend is claiming that he saw bulbs light up one after another?? I'm not sure if that can be true Dec 3 '19 at 6:40
• Do you have any explanation for your friend’s alleged observation? Does your friend? Dec 3 '19 at 6:45
• I don't think so. However there has to some reason that why the bulbs light all at once? There has to be a law governing it and I want to know that. Dec 3 '19 at 6:47

First of all let’s assume all three bulbs are of the same type technology (LCD, incandescent, fluorescent) as I’m not sure if there may be delays inherent to different technologies. Then they should illuminate simultaneously. The reason is the electric field is established simultaneously throughout the circuit (near the speed of light) and therefore electrons also start moving simultaneously throughout the circuit.

Even though the speed of individual electrons is so slow (the drift velocity) that an individual electron may never even travel the entire distance of the circuit (depending on the length of the circuit and the operating time) all the electrons start moving nearly simultaneously in a chain reaction response to the applied field.

Hope this helps

There will be no visible difference. When the circuit is switched on there is a very short transient period before steady state conditions are reached. The transient effects include the propagation of the charge flow along the conductor and the start-up performance of the lights. The propagation of the charge flow depends on the impedance of the wires, but for practical purposes on a normal lighting circuit you can assume it propagates at close to the speed of light, which means that any time differences between the current flowing at each of the bulbs will be of the order of nanoseconds. That means that if your lights are incandescent bulbs the current will be running throughout the circuit long before the lights have reached visible brightness. More significantly, the time difference will be utterly negligible compared with the response time of your eye and brain.

I see this happening every time I switch on the lights in my bedroom. They are 4 downlights; 2 of them are old halogens and the other 2 have been replaced by LEDs. The LEDs switch on (and especially off) much faster than halogens.

The reason is that the halogens have a fat filament that takes the best part of a second to get to full brightness; switching off takes several seconds. The LEDs on the other hand are semiconductors that switch on or off in microseconds. Overall, the time difference is easily noticed.

Of course, my bulbs are in parallel but putting them in series would give the same effect. If the bulbs in series are the same type, then they will light up almost simultaneously. The only difference is the time taken by the electric field to travel down the wire. At close to ligthspeed, that is far too fast to be noticeable.

As the other answers said, the electric field, which is zero inside the wires when the circuit is open, will start to build up inside the conductor once the circuit becomes closed. This build up will propagate to almost the speed of light, but it starts at the switch. So even if undetectable to the naked eye, the light closer to the switch will turn on first, and so on.

• But the OP's friend claims to have observed the bulbs lighting up one after the other. That was the motivation for post. So I'm assuming your answer says that can't happen. Dec 3 '19 at 14:01
• @BobD I agree, this answer was just a note on the fact that even if it cannot happen, it actually would in an ideal case. In the real case lamps might turn on at different times just because of differences in how the lamp is made,or defects
– user65081
Dec 3 '19 at 14:09