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  • Can you figure this out?

    For now assume that we have 3 medium base lamp (light bulb) sockets and they are connected in series. In each of them is a 40 Watt, 120 Volt light bulb. We apply 240 Volts to the completed circuit.

    What happens?
    A) All three light up to equal brightness

    B) None light up

    C) KaBoom

    D) None of the above as something else happens ________________________________

    ================================================== ============================================

    And now for the fun part: Lets change one of the three light bulbs and replace it with a 60 Watt 120 Volt light bulb.
    Now what happens when 240 Volts is applied to the circuit?

    A) All three light up and at equal or about equal brightness

    B) Two are dimmer and one is brighter

    C) One is dimmer and two are brighter

    D) KaBoom again

    E) None of them light up, or they all light up very bright and burn out quickly

    F) None of the above _____________________________________________


    Special note: Please don't bother doing math. Please just pick the answers and if any are "None of the above" then just give a basic answer. It's not a class or engineer's test but rather just to find out who has the basics of what actually would happen in both cases.
    Last edited by Woussko; 05-24-2009, 09:46 PM.

  • #2
    Re: Can you figure this out?

    A
    C


    Any particular reason you are asking us to excercise ohm's law and it's derivatives, like Kirchoffs, on this holiday weekend.????

    Comment


    • #3
      Re: Can you figure this out?

      sound like somebody is cramming for a test

      G3

      Comment


      • #4
        Re: Can you figure this out?

        Well, the first half, the answer is "A", but not for the reason you gave. Since resistance does not change, solve for the internal resistance of each lamp, solved for "I total" at 240 volts,then used I^2R for wattage. To make certain the lamps don't burn out at total current at 240 volts, I then solved for current for each lamp under normal conditions at 120v to see what the current is supposed to be for one lamp (0.33 amps.) Well, for your curcuit, it doesn't since it is at 0.22 amps, and it drops 80 volts accross each lamp.
        ...................

        At times like these, I like to think back to what an old electrician once told me,
        "GOD DAMMIT! If there was a hard way to do something, YOU'D FIND IT!"

        ................

        As for the second half.
        The answer is "C", two are brighter, and one is dimmer.

        Though which ones get brighter suprises me. Folks, the two that get a little brighter are the two 40 watt lamps, and the dimmer one is the 60 watt lamp. Amasingly enough (And MrsSeatdown, that is NOT an error in spelling!), the two 40 watt lamps use up 22.5 watts while the 60 watt lamp uses 15. And once again, I checked the current the lamps are meant for to make sure they don't burn up.

        FYI for those who care, the two 40 watt lamps drop 90 volts, the 60 watt lamp drops 60 volts.
        I total = 0.25 amps.
        R total = 960

        I'm still trying to wrap my head around why a 60 watt lamp has 240 ohm resistance while the 40 watt lamp has 360 ohm resistance.
        I know it has to do with the current and the I^2R formula for power, but just trying to accept it is what's bugging me, it just LOOKS wrong for some reason.


        ...I should have gone into engineering...
        Last edited by tailgunner; 05-24-2009, 10:10 PM.

        Comment


        • #5
          Re: Can you figure this out?

          Sorry to anyone that tried to figure out the values. I needed to put the last part on before posting. I wasn't trying to have anyone do math, but rather just see who would be able to get the general idea of what would happen when power was applied.

          Let's try part 3 now and see who can get it right. Again, please just select the most correct answer choice. There's no need to come up with values or exactly what will happen.

          In socket A I will install a 15 Watt 120 Volt light bulb. In socket B I'll install a 100 Watt light bulb and in socket C I'll install a 60 Watt 120 Volt light bulb. Please remember they are connected in series. This time I'll apply 240 Volts AC sine wave power to the circuit.


          What do you think will happen?

          A) All will light up to the same brightness and stay lit until power is shut off

          B) Only one lamp (light bulb) will light up bright enough to produce any usable light and the other two will just glow dim red.

          C) Zappa KaBoom - One or more light bulb(s) will light up super bright and go POOF

          D) While there will be Voltage across the filaments, none will produce visible light. All three will just glow dim red.

          E) All will light up but not to the same brightness. None will be super bright or go poof.


          Please just select one of the above answers. Don't go grunting & groaning away trying to figure out exactly what will happen.

          After there are more replies, I'll post the member names that have the correct answers. Again, please don't worry about exacts here.
          Last edited by Woussko; 05-25-2009, 11:25 AM.

          Comment


          • #6
            Re: Can you figure this out?

            To the third question, C may be the most correct. The 15 watt bulb will be operating at closer to 30 watts, which will shorten its life, can't say for sure though that it would immediately go Kabooom. But the 15 will be bright and the others quite dim until K-hour.


            Remember that you cannot be using the Watt ratings of the bulbs directly in your 240 volt calculations, because that number is derived at 120 volts. But you can use the relative difference in resistance of the bulbs as a reference.

            Comment


            • #7
              Re: Can you figure this out?

              I pick C, 15W bulb goes poof

              Comment


              • #8
                Re: Can you figure this out?

                So far those of you that did select answers have it correct. Good job

                As for question C, I don't expect any big KaBoom but the 15 Watt light bulb will light up very bright and in a short time fail.
                Last edited by Woussko; 05-26-2009, 11:07 AM.

                Comment


                • #9
                  Re: Can you figure this out?

                  Originally posted by tailgunner View Post

                  I'm still trying to wrap my head around why a 60 watt lamp has 240 ohm resistance while the 40 watt lamp has 360 ohm resistance.
                  I know it has to do with the current and the I^2R formula for power, but just trying to accept it is what's bugging me, it just LOOKS wrong for some reason.


                  ...I should have gone into engineering...
                  Higher resistance = lower current = less power. A low resistance bulb will draw a lot of current and have a higher wattage. It starts screwing with your head when you hook them up in series because they will act backwards (high wattage bulbs dim, low wattage bulbs bright).

                  The real answer to these questions is don't hook up lamps in series!

                  Comment


                  • #10
                    Re: Can you figure this out?

                    Using Ohm's law (E=IR) if the voltage (E) stays constant and the resistance (R) increases, then then the current (I) decreases. Give that some thought when you are trying to get to sleep tonight. It should help the concept sink in (as well as helping you to fall asleep!) Hope that helps. Don

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