### Question

In a series circuit with two or more bulbs, which bulb lights first when the circuit is closed?

They will all light up at exactly the same time. Thing of the flow of electricity like a metal chain. When you pull at one end of the chain the entire chain moves. The electrons are linked in a similar way, if one begins to move through the circuit, it will push the electron in front of it and pull the electron behind it. Since all the electrons around the circuit move instantaneously at the same time by the "chain" principle all the lights should light up at the same moment.

In a little more detail, the lights in the circuit act as resistors. They convert the electrical energy from your circuit into heat energy (as do all resistors). The light bulb gets so hot that it glows. In the chain analogy, the lightbulbs would be like sandpaper that the chains have to rub against as they move. Just like in the circuit when the chain is pulled over the sandpaper the sandpaper will heat up, and if you had several separate pieces of sandpaper along the chain, they would all heat up at the same rate just like the lightbulbs would all light up at the same time.

This is all assuming that you mean that the two or more lightbulbs are identical in wattage.

When a series circuit is turned on, the amount of current is equal in all circuit elements and begins at the same time. Only miniscule deviations to these basic conditions occur since the current wave moves between the two bulbs at the speed of light, creating a delay of a few nanoseconds (billionths of a second). The most important factors determining the heating of a filament to incandescence are thermal mass and electrical resistance of the filament. Thermal mass, which relates power input for a given temperature rise, depends on the actual mass and type of material used in the filament. Generally, the larger the thermal mass, the slower the filamentï¿½s temperature will rise. On the other hand, the larger the resistance, the faster the filament will heat up because the power transferred to the filament is equal to the square of the current times the resistance (Ohm's Law).

Taking all these factors into account, the light containing the filament with lowest thermal mass and highest resistance will "turn on" fastest. The difference in turn-on time would generally not be noticeable for standard household incandescent bulbs, since they turn on very quickly. A slide projector bulb has a much thicker filaments resulting in higher thermal mass and lower resistance. There is typically a noticeable delay when a projector bulb is turned on.

There are several less important factors that may affect turn-on time, such as type and pressure of the gas used to fill the light bulb, and physical structure and processing of the filament material. Slight variations in these factors would not produce noticeable differences when the lights were turned on, but sensitive, high-speed optical detectors could still measure the differences.
Answered by: Scott Wilber, President, ComScire - Quantum World Corporation

Assuming that all the bulbs are identical, incandescent-style ones, they will all light up at the same time. An analogy between an electric circuit and a long line of train boxcars helped me years ago understand why this is so.

In a long line of train cars, situated such that any "slack" is taken up at all the linkages, they all are at rest, ready to move once the engine car (locomotive) imparts a force to the train. Once the engine car jerks forward, all the cars react simultaneously to that jerk. (keep in mind that for the purpose of this analogy, the box cars are infinitely stiff, and no deformation of the metal occurs). However, while the force (jerk) is felt very quickly, the cars move very (very) slowly.

In the analogy, think of the energy of the engine car as an electric circuit's EMF (voltage) source, and think of the box cars as the electrons (current). Only when the EMF is introduced to the circuit does the current begin to move. The resistance in the circuit (resistance in the wires and in the bulbs' filaments) will limit the speed at which the current flows. (This resistance is like the friction in the wheels of the train). However, when the EMF is present to the circuit, all electrons begin moving simultaneously, and thus all the bulbs light up at the same time.

Think of how water flows in a hose. In order for 1 gallon of water to flow out of the end of the hose, 1 gallon of water must flow into the hose. Unless the hose has a hole in it or is deforming, water cannot accumulate in the hose. The same is so with the electrons flowing in the circuit. For every electron that flows into the circuit, one must flow out. The water pressure is like the voltage (EMF) that "pushes" the electrons thru the circuit. The diameter of the hose will determine how much resistance the water will encounter.
Answered by: Dan Hopcus, B.S., Sr Electro-Mechanical Engineer, Tucson AZ

### Support US

Our server costs have gone up and our advertising revenue has gone down. You do the math! If you find our site useful, consider donating to keep us going. Thanks!

### Science Quote

'We should take care not to make intellect our god; it has powerful muscles but no personality.'

Albert Einstein
(1879-1955)