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Why doesn't my lightbulb explode?

  1. Mar 22, 2014 #1
    I hope you are having an awesome day.

    Can you tell me why this is?

    I put twenty light bulbs (rated for 100 watts, 120 volts) in series in my room. They light up. It's real bright in here. Now I unplug nineteen of them. Why isn't the remaining either 20 times brighter or just explode. In other words what scientific mechanism that, in effect, tells my non-ideal power source: 'Hey buddy, hold back on that 2000 watts that you could blast that bulb with and only give the bulb 100 watts.' There is no circuit reference ground - check. But the power is at the outlet. Why doesn't all go through the bulb like it did when all twenty bulbs were connected. After all, all 120 volts drop through the bulb. Why doesn't all 20 amps (coulombs per second) drop through the bulb as well?

    Gratefully yours,
    Johnny
     
  2. jcsd
  3. Mar 22, 2014 #2

    phinds

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    I assume your have them in parallel, not series, since if you had them in series, each bulb would be getting about 6 volts which is not enough to make it light up. Since they are in parallel, the one you don't unplug doesn't care whether or not the others are plugged in and since it's rated for 120V, it works just fine.
     
  4. Mar 22, 2014 #3
    I believe you probably had the bulbs connected in parallel.

    Each bulb was designed to draw a specific amount of energy at 120VAC. Think of water flowing through tubes or tubes with constrictions to only allow a certain amount of flow (flow by the way is the equal of current; coulombs per second) at a specific pressure(think of pressure as voltage). Now if the pressure is held constant and you parallel several or one of the calibrated tubes described earlier, then the same amount of flow travels through each, and their combined effect is a large flow (large current say 20 amps), but take all out but one, and keep in mind that the pressure is held constant, then the flow through any one tube is the same. This is the basis of introduction to electrical engineering node analysis. Pick a point connected by a wire (called a node) and any current entering that node must also flow away. Otherwise the node would be supplying current, and by definition as only a wire this cannot happen; conversely it cannot hold current either. Perhaps another visual is a yard sprinkler system, the city water is usually held near 80 psi, so as long as the flow can be maintained by proper design, then one or 4 sprinkler heads in the system, but each head still shoots the stream the same distance (keep in mind max flow is allowed at each and the pressure is held constant by good design).

    Hope this helps!
    Mitch
     
  5. Mar 26, 2014 #4
    Thanks for your replies.

    I did mean parallel not series. (apparently I am not ready to use house current yet)

    The analogy to water is useful to me. What I was misunderstanding, among other things, was that the devices were rated to handle 120 volts. Just as one water hose is capable of handling 80 psi. I didn't consider that the breakdown voltage was greater than 120 volts.
     
  6. Mar 28, 2014 #5
    pre-response to Jaus

    I was emailed a post that may not have been posted yet about how current is saved by not using it.

    What a light bulb, or a resistor in general, does is allow some current to flow through a circuit. But not all of the available current. With a water hose analogy: you can have one sprinkler drawing X milliliters of water per second. If you add another sprinkler, then the total water drawn is 2 times X. The sprinkler holes are tiny and very little water can get through the opening.

    The material in a light bulb conducts some electricity, but not too much.

    Remove one sprinkler and save half the water. Remove both sprinklers and save all the water.

    If the source of electricity is a battery, the saved energy stays in the battery. If the source is an electric generator, then the generator can be slowed by reducing work input turning the generator, or the generator can be turned off if enough people turn off their lights.


    My old 60 watt light bulb had about 20 ohm resistance according to my ohm meter. So current equals voltage squared over resistance, then: 120 squared is 14400. Dived by 20 equals: 720 watts. Did you know that the average light bulb increases its resistance by a factor of about 15 in the first 100 milliseconds as the filament warms up. No it's true.
     
  7. Mar 28, 2014 #6

    phinds

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    Yeah, that post was nonsense and the poster wisely deleted it right after posting it, so we got the email but the post was gone by the time we looked to see it in the thread.

    EDIT: oops ... I should not have said that post was nonsense. I wasn't nonsense, it just displayed a total lack of understanding of electricity.
     
  8. Mar 28, 2014 #7
    The understanding with the battery is correct about cutting down the number of connected devices. But the generator is much more complicated. The speed a generator turns is tightly coupled to the output frequency of your outlet 60Hz. It is complicated by the fact that the GRID is fed by multiple stations and all have to operate at almost EXACTLY the same frequency to keep the power flowing the right direction (that is out of the station). So other parameters are changed to meet the changing current demands out on the grid.
    But, the generalization of the analogy of seeing the amount of water out change is correct.
     
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