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If there is a electric circuit with a bulb and after i connect a battery to pass the electric current , it will suddenly glow. Lot of people tell that a current is set up instantly due to presence of electrons all over in the metal wire but won't they have to move some distance to actually gain kinetic energy which then can be converted into heat energy to make the bulb glow(consider non led)? Then why the bulb suddenly glows up ?

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A couple of things happen.

First - the bulb doesn’t actually light up “instantly” - the filament has finite heat capacity so it takes a bit of time for the heating to result in a filament with maximum brightness (which depends on temperature).

Second - the resistance of a filament typically increases with temperature- so while the filament is cold, the current will be larger. If your energy source has zero internal impedance (not true for a practical battery) then there will be a very high initial current. The only thing that limits the rise in current is the inductance of the circuit - an inductor resists change in current but inductance of a simple loop is very small.

All these things happen very quickly - so it may look like the bulb turns on instantly. But if you have a photocell and an oscilloscope, you will see it takes time. The slowest part is probably the heating of the filament.

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  • $\begingroup$ Thanks but seems you have explained why bulb actually does not glow suddenly but i have asked about the filament getting heat energy in the first place $\endgroup$
    – Cyberax
    Commented Aug 28, 2021 at 12:29
  • $\begingroup$ @Cyberax in a conductor, the density of charge carriers is very high. The “drift velocity” of electrons needed to set puma current of, say, 1 A, is tiny - on the order of mm/s (depending on the size of the wire and the material). Since electrons are actually already moving at many 100’s of m/s due to thermal energy, this net change in velocity is negligible. This is nicely explained (with a worked example) in this article $\endgroup$
    – Floris
    Commented Aug 28, 2021 at 12:34
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Here's an analogy. Imagine a bunch of marbles all lined up in a single line. You shoot another marble at the first marble in the line. Each marble in the line hits the next marble in the line until the last marble shoots away. The marble you shot would have had a certain kinetic energy, but it hit the next marble instead. The last marble ends up with the kinetic energy, not the one you shot.

Now, imagine that at the far end of the marble line is a nail stuck into a piece of wood. Now, when you shoot a marble at the near end of the line, the marbles collide with each other down the line until the last marble hits the nail, driving it into the wood. None of the marbles end up with any kinetic energy. All of the energy of the shot ends up as work driving the nail into the wood.

Electricity is similar. The electron that exits a 9V battery would have 9eV of kinetic energy it it was traveling from one terminal to another in a vacuum. But, the electron enters a conductive wire instead. There, it collides with other electrons in the wire, pushing them instead of traveling with any speed. The electrons in the wire push other electrons farther down the wire until the light bulb. There, the energy from the battery goes into heating the wire instead of pushing electrons with the same force.

Like the marbles driving the nail, the energy of the electrons coming out of the battery gets converted into heat and light instead of kinetic energy. Also like the marbles, the pushing travels down the line of marbles/electrons faster than any individual marble/electron.

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  • $\begingroup$ This is a nice analogy. You might consider including the actual velocity distribution of the electrons; when that entire cloud of electrons that was moving in random directions at velocities over 1000 m/s is suddenly drifting with a mean velocity of um/sec, what happens to the kinetic energy? It's probably a fun calculation and would certainly add to your answer. $\endgroup$
    – Floris
    Commented Sep 1, 2021 at 22:41

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