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Homework Help: Lamps connected in series with different voltages

  1. Mar 7, 2010 #1
    1. The problem statement, all variables and given/known data

    5 lamps are connected in series (one after the other in a circle) across the terminals of an energy source. Each lamp has a voltage drop across its terminals that is double the value of the one to the left of it. Which lamp will glow the brightest?

    2. Relevant equations

    V = I x R

    3. The attempt at a solution

    I know how to apply Ohm's law to components of a circuit, and to the whole circuit. I understand that the current through all lamps is the same, and that they have different resistances and voltage drops across them, and I can calculate all that fine. It's just the brightness - I do not understand what makes a lamp brighter or dimmer.. Is it the voltage drop across the lamp that determines it brightness? Or is it the current? Thanks for any assistance!
  2. jcsd
  3. Mar 7, 2010 #2


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    Science Advisor

    Welcome to PhysicsForums!

    This question requires you to think about series circuits. Here's a hint: how much current flows through each of these lightbulbs?

    Note that power dissipated across a two-terminal element (like a resistor or a lightbulb) is calculated as: [itex]P=IV[/itex], the current through the element multiplied by the voltage drop across the element.

    EDIT: Ooops.... You already did all that! Okay, assume that each of these lightbulbs has the same efficiency, i.e. they convert the same percentage of power (dissipated across them) into light. I'd state this assumption for good measure.
    Last edited: Mar 7, 2010
  4. Mar 7, 2010 #3
    The question as you put it is one of those questions where any answer you give could be right, provided you give a reason why.

    Like Matlabdude alluded to, you have to mention the efficiency. If two bulbs have the same efficiency, (brightness/power) then of course more power will mean more brightness.

    But in addition to that, it depends on how you define brightness - you can add all sorts of stipulating factors like: how intense is the light in a certain direction (Is a laser bright if you are looking at it from the opposite end?); how intense is the light in a certain color range (is it bright if it has a lot of UV?).

    According to wikipedia, ( http://en.wikipedia.org/wiki/Brightness ) brightness is "an attribute of visual perception", so you could even get away with claiming they are equally bright to a blind person, or that what you think is the brightest may seem the darkest to a person with a strange neurological problem.

    But after all that, for the general assumptions that a reasonable person might make, the bulbs will be of the same efficiency and design, so if you increase the energy going in, you will naturally increase the energy coming out.
  5. Mar 9, 2010 #4
    Thanks for your help!
    The question made sense once it was explained, was simpler than I thought, thanks.
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