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Why does bulbs glow bright in parallel?

  1. Sep 13, 2013 #1


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    I have searched google and got no satisfying answer.Someone said that the overall resistance is less so as the voltage is same,the current increases.I don't think this is correct because the current for a single bulb is reduced.Can you give a satisfying answer?:confused:
  2. jcsd
  3. Sep 13, 2013 #2
    two bulbs connected in parallel would have significantly lower resistance even if the current has to be divided upon them , consider each one of 15 ohms and a voltage of 30 volts
    if they were connected in series , total resistance would be 30 ohms and so intensity will be 1 amp
    while if the bulb was connected in parallel , the total resistance would be 7.5 ohms , total intensity will be 4 amps
    divided by 2 for each lamp , 2 amps for each lamp !
    the intensity is divided , but also the resistance has hugely been reduced
  4. Sep 13, 2013 #3
    Consider a source of potential difference 'V' and two bulbs of equal resistance 'R'.When they are connected in series the current through them is 'V/2R'.When they are connected in parallel the NET current is '2V/R' and the current through EACH of the bulbs is 'V/R'.So basically the splitting of current doesn't decrease the current through the bulbs by a larger value.
  5. Sep 13, 2013 #4
    The brightness of a bulb is related to the power transformed .Power =V2/R.

    Consider two bulbs having resistances R each connected across a battery having voltage V.

    1)When in Series - The equivalent resistance is 2R .Hence P1=V2/2R

    2)When in Parallel - The equivalent resistance is R/2 .Hence P2=2V2/R

    Clearly P2>P1
  6. Sep 13, 2013 #5
    Because when the bulbs are connected in parallel across a voltage drop, you have the full voltage drop across each of the bulbs. When you have the bulbs connected in series, you have half the full voltage drop across each of the bulbs. So, in parallel, the current through each of the bulbs is twice as high as when they are in series.
  7. Sep 14, 2013 #6


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    Yours is one of those questions which is at the same time right and also wrong. http://physicsforums.bernhardtmediall.netdna-cdn.com/images/icons/icon6.gif [Broken]

    The brightness of any particular bulb is determined by the voltage across that bulb.

    (A) If you have a pair of identical bulbs, and power them in parallel from a 12V battery each bulb will glow with a certain brightness.

    (B) If you connect those bulbs in series and power the combination from a 24V battery each will glow with a brightness identical to that in part (A). Because each bulb still "sees" 12V across it then the current is identical with that in part (A).
    Last edited by a moderator: May 6, 2017
  8. Sep 14, 2013 #7


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    Assume that the power source is constant voltage, like the electricity going into your house. The power source will adjust the voltage to compensate for reduced resistance, increasing current. So with a constant voltage source, the amount of current going through each bulb in parallel is the same. Two bulbs in parallel are brighter than one bulb, three bulbs in parallel are brighter than two bulbs in parallel, ... .
  9. Sep 14, 2013 #8


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    Say we have two 110VAC bulbs, each in a box with its own brightness meter and a switch and an adequate voltage source that can power both bulbs.

    Turn on 1 bulb, its meter has a reading. Turn on the other bulb, its meter reads the same. The first meter does not change when you turn on the second bulb (no interaction).

    Each bulb puts out X lumens when connected to 110V. Connecting 2,3,4,5,6 bulbs in parallel does not change the 110V so each bulb behaves as if the others are not there.

    If the 110V is "not adequate" then there could be interactions. Like your house lights dimming when the air conditioning is turned on.
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