# Which bulb lights up first?

Joined Dec 4, 2009
328
Three bulbs are connected in series, and, a DC voltage is applied. Which bulb lights up first? Electrons move from -ve to positive, and, at speed of light, about a foot per ns.
Simple question posed to me.

#### Lundwall_Paul

Joined Oct 18, 2011
236
Are they CFL bulbs in cold weather? If they are none will light.

#### kubeek

Joined Sep 20, 2005
5,796
A fraction of a ns will the ones closer to -ve "light up faster" or actually notice the current faster. If you take into account how long it takes for the filament to heat up, they start glowing at the same time.

#### KMoffett

Joined Dec 19, 2007
2,918
Actually the "effect" of electron movement is near the speed of light, but the electron movement is much slower. Since all of the electrons essentially move together along the series circuit, like boxcars on a train, all of the bulb filaments are exposed to the movement of electrons at the same time.
http://amasci.com/miscon/speed.html

Ken

#### BillB3857

Joined Feb 28, 2009
2,571
Actually the "effect" of electron movement is near the speed of light, but the electron movement is much slower. Since all of the electrons essentially move together along the series circuit, like boxcars on a train, all of the bulb filaments are exposed to the movement of electrons at the same time.
http://amasci.com/miscon/speed.html

WOW! The reference you sited above (paragraph near the end about the salt crystal) sure changed my perception. I was thinking more along the lines of hydraulics with a non compressible fluid and having instantaneous equal flow at any point along the circuit. Your train analogy sure seems to be a good fit with the slack in the couplings allowing each car to move a slight distance before the next car moves.

#### PaulEE

Joined Dec 23, 2011
474
Actually the "effect" of electron movement is near the speed of light, but the electron movement is much slower. Since all of the electrons essentially move together along the series circuit, like boxcars on a train, all of the bulb filaments are exposed to the movement of electrons at the same time.
http://amasci.com/miscon/speed.html

Ken
I second that answer. I believe I saw this in an MIT opencourseware lecture video on circuits.

#### w2aew

Joined Jan 3, 2012
219
...so the answer is - the bulb that has the highest cold filament resistance will likely light up first - because it will begin to dissipate the most power first, thus increasing the filament resistance more, thus beginning to starve the others of current. Of course, this assumes that all the filaments have the same thermal time constants, etc...

#### kubeek

Joined Sep 20, 2005
5,796
...so the answer is - the bulb that has the highest cold filament resistance will likely light up first - because it will begin to dissipate the most power first, thus increasing the filament resistance more, thus beginning to starve the others of current. Of course, this assumes that all the filaments have the same thermal time constants, etc...
They are in series, so the hotter one starves others of voltage.