ohm's law

Discussion in 'General Electronics Chat' started by akifnadeem, Dec 19, 2011.

  1. akifnadeem

    Thread Starter Member

    Apr 9, 2010
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    if 40W , 60W , 100W and 200W bulbs are connected in series , which bulb will burn more bright?
     
  2. KJ6EAD

    Senior Member

    Apr 30, 2011
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    A high wattage bulb has a lower hot resistance but a lower wattage bulb requires less current to incandesce so the 40W bulb will be the brightest.
     
  3. Adjuster

    Well-Known Member

    Dec 26, 2010
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    Strictly, that answer will only be true if it can be assumed that all the lamps are designed for the same supply voltage.

    This seems very likely to have been what whoever set the question had in mind, but for instance in a written answer to an assignment I would probably want say that that assumption had been made.
     
  4. paulktreg

    Distinguished Member

    Jun 2, 2008
    612
    120
    All depends on the voltage.

    The 200W if it glowed at all because it would get 50% of the available power. The 100W would get a 25%, the 60W 15% and the 40W only 10%.
     
  5. MrChips

    Moderator

    Oct 2, 2009
    12,449
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    This sounds like a homework question. Why don't you do the calculation. Work out the theoretical resistance of each lamp and calculate the current in the series connection. Then calculate the wattage of each bulb as a percentage of its rated wattage.
     
  6. Adjuster

    Well-Known Member

    Dec 26, 2010
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    This is not right if the lamps are designed to give their rated wattages at the same voltage.

    What is important to notice here is that the lamps are all in series, so they all get the same current. For a typical filament lamp, brightness depends very strongly on the current. If all the lamps were made for the same voltage, the 40W bulb would get a larger percentage of its nominal current.

    The 40W lamp would also end up with a bigger share of the total voltage across it, because it has a higher resistance than the other lamps. Hence in a series circuit the lower wattage lamp receives more watts than the higher wattage lamp. Only with the lamps connected in parallel do the wattages come out as rated.
     
  7. BJT_user

    Member

    Oct 9, 2011
    35
    8
    The 40 watt, being the highest resistance among the four, would burn the brightest because in series, all four bulbs share the same current. Since Ohms Law states that P = I^2 * R, then the highest resistance would dissipate the most power.
     
  8. akifnadeem

    Thread Starter Member

    Apr 9, 2010
    15
    0
    thnx all for your informative replies. the answer 40W seems accurate. but point is, when we will add them in series current will reduce than the rated current of bulbs. and what, if we connect all of them in parallel keeping voltage constant?
     
  9. SgtWookie

    Expert

    Jul 17, 2007
    22,182
    1,728
    It is.
    What?
    Placing bulbs in series does not reduce the rated current of the bulbs.
    Then the highest wattage bulb will have the greatest current flow, and be the brightest.
     
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