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Thread: a simple question

  1. #1
    Electrical Contractor/Instructor jwelectric's Avatar
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    Default a simple question

    In lieu of resent events and comments on this site something came to mind from a question from another site.

    It has been said that proper terminology is not a factor in giving advice on a DIY web site. It has also been implied that the DIYer donít need to fully understand just what is taking place in a circuit in order to work with electricity.

    Here is a problem for some of you to think about and see if you can come up with an answer.

    I have a 100 watt and a 50 watt light bulb connected to opposite legs of a multiwire circuit. I open the neutral of these two circuits which bulb will burn brighter?

    This is a question I ask in the first semester of an electrical class. We then go to the lab and see just what happens.
    Unless you can answer this question and explain why then you should be asking yourself if you should be messing with an electrical circuit.
    Should you come in contact with a 120 volt circuit the same will happen to you as happens with these two bulbs.

  2. #2
    Electrical Contractor/Instructor jwelectric's Avatar
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    Default

    20 views and no replies??????????

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    Electrician frenchelectrican's Avatar
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    I have a 100 watt and a 50 watt light bulb connected to opposite legs of a multiwire circuit. I open the neutral of these two circuits which bulb will burn brighter?

    This is a question I ask in the first semester of an electrical class. We then go to the lab and see just what happens
    C'est simple one word Ohm Laws you can not bend , beat or do houdi on this one.

    My answer is 50 watt bulb will burn much brighter while the 100 watter will be dim or not even glowing.

    Merci,Marc
    Last edited by Terry; 12-02-2008 at 09:55 AM.

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    DIY Junior Member jamiedolan's Avatar
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    Quote Originally Posted by jwelectric View Post
    20 views and no replies??????????
    It took me a few minutes to set this up and give it a try. I of course knew that I would be feeding 240 to the bulbs once the neutral was open, and I suspected that the bulb with a filliment with less resistance would be brighter, the 40 w bulb in my test setup.

    And I was correct, the 40 watt bulb became much brighted once the neutral was opened. The 100w bulb did not go out, but became dim.

    So the test confirmed what I suspected would happen.

    Did I understand what happened properly, that the 40 became brighter instead of the 100 because it's filiment has less resistance?

    Jamie

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    Electrician frenchelectrican's Avatar
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    Jamie .,

    You got the correct answer there.

    Merci,Marc

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    DIY Junior Member jamiedolan's Avatar
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    Thanks for the confirmation Marc.

    ok JW, I am ready for the next question / experiment, don't make it too hard...

    Jamie

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    Electrical Contractor/Instructor jwelectric's Avatar
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    Quote Originally Posted by jamiedolan View Post
    It took me a few minutes to set this up and give it a try. I of course knew that I would be feeding 240 to the bulbs once the neutral was open, and I suspected that the bulb with a filliment with less resistance would be brighter, the 40 w bulb in my test setup.

    And I was correct, the 40 watt bulb became much brighted once the neutral was opened. The 100w bulb did not go out, but became dim.

    So the test confirmed what I suspected would happen.

    Did I understand what happened properly, that the 40 became brighter instead of the 100 because it's filiment has less resistance?

    Jamie
    But the two burned as they were supposed to before the neutral was opened did they not.

    Why did they react the way they did?

    What changed?

    The wattage of the bulbs did not change did it?

    Why did they change?

  8. #8

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    Unless you can answer this question and explain why then you should be asking yourself if you should be messing with an electrical circuit
    I disagree.

    I thought that the bulbs would burn out @ 240 volts.

    I've been messing with electrical circuits most of my adult life.

  9. #9
    Retired Defense Industry Engineer jadnashua's Avatar
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    The two bulbs end up looking like they are in series. The larger the resistance gets a bigger voltage drop across it, so the smaller bulb which had to have a greater resistance to only use the smaller amount of power now gets a larger voltage across it and burns brighter. It might not last all that long because the voltage should be more than 120V. This assumes that the neutrals between the two light sockets are still connected, providing a path for the current to complete the circuit. If one bulb was twice the wattage of the other, the higher wattage bulb should show 80vac across it, and the smaller bulb 160vac (i.e., 2/3 verses 1/3).

    power....bigger bulb....neutral to other socket....smaller bulb...other power lead. Series connection.
    Jim DeBruycker
    Important note - I'm not a pro
    Retired Defense Industry Engineer; Schluter 2.5-day Workshop Completed 2013, 2014

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    Plumber jimbo's Avatar
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    What is different is that you now have 2 resistances in series with the total 240 volts. You will have a certain current flowing ( I think it is 0.55555 amps) and whatever that current is, it will be the same through both bulbs. The bulb with the lower original wattage rating has a higher resistance. But since it is now in a simple series circuit, the higher resistance bulb will consume more watts. The 100 watt bulb will be glowing dimmer than "normal" and the 50 watt bulb will be brighter than "normal".

  11. #11

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    Can I take a stab at it??

    OK, under normal circumstances the 100w bulb would draw .8A and the 50w bulb would draw .4A PROVIDING the feed is from separate phases.

    The return to the grounded conductor (neutral) would only be .4A when both bulbs are on.

    Then you take away the neutral (you bad boy) and make the circuit a 240vac series circuit.

    The resistance of the bulbs does not change but since they are different it changes the voltage because they are in series.

    The 100w bulb with 144 ohms is now drawing approximately 80 volts therefore making it very dim since it is way below its voltage rating.

    The 50w bulb with 288 ohms is now drawing approximately 160 volts, way above its rating and will probably burn out soon.

    The total draw on this circuit is now .6 amps at 240vac where with the shared neutral at 120vac it was only .4 total.

    The idea here is that the resistance and amperage changed the voltage in a series circuit.

    That was a good question and I only hope that I got it right.

    Thanks Jw
    http://www.inspectpa.com/forum/forum.php
    My answers are based mostly on the ICC codes. Advice given is my personal opinion and every person performing work should acquire a permit from his/her jurisdiction and get the work inspected. My opinions are not directions to follow for DIYs or professionals

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    DIY Junior Member jamiedolan's Avatar
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    But the two burned as they were supposed to before the neutral was opened did they not.
    Yes they did burn correctly at first, just as they would on a regular MWBC.
    Why did they react the way they did?
    Because each leg of the MWBC landed on a different leg of the panel, so feeding in power, without the neutral was equivlent to hooking up a 240v connection to the lamps.

    What changed?

    The wattage of the bulbs did not change did it?

    Why did they change?

    It no longer had a neutral to use as a return current path, so the current then started acting as a 240 connetion, where the neutral is not used as a regular return path. Now that is the point I get confused as to exactly what is going on beyond this point, can you point me in the right direction so I have a better understanding of what is happening here?

    Thanks
    Jamie

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    DIY Junior Member jamiedolan's Avatar
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    Quote Originally Posted by jadnashua View Post
    The two bulbs end up looking like they are in series. The larger the resistance gets a bigger voltage drop across it, so the smaller bulb which had to have a greater resistance to only use the smaller amount of power now gets a larger voltage across it and burns brighter. It might not last all that long because the voltage should be more than 120V. This assumes that the neutrals between the two light sockets are still connected, providing a path for the current to complete the circuit. If one bulb was twice the wattage of the other, the higher wattage bulb should show 80vac across it, and the smaller bulb 160vac (i.e., 2/3 verses 1/3).

    power....bigger bulb....neutral to other socket....smaller bulb...other power lead. Series connection.
    This is what I thought, it ends up being like a series setup in a way. I need to go get out a meter and actually take readings off of each bulb. I am not sure though if I am going to see reduce votlage when taking a reading between the 2 lights on the "neutral" connection or if I am going to see increased voltage there.

    I suspect that testing from line side of the 40 watt bulb to the neutral side when running as 240, I am going to see the higher 160+volts. Then on the other side on the 100 watt bulb, I suspect that I am going to read a lower 80voltish reading on that one. I suspect that whatever the 2 readings are that they will both add up to a total of 240v. (i.e. one will be 160v, one will be 80v total 240v)...

    Jamie

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    DIY Senior Member seaneys's Avatar
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    Quote Originally Posted by jwelectric View Post
    In lieu of resent events and comments on this site something came to mind from a question from another site.

    .....

    Unless you can answer this question and explain why then you should be asking yourself if you should be messing with an electrical circuit.
    Should you come in contact with a 120 volt circuit the same will happen to you as happens with these two bulbs.
    It's a neat question that is often used in second semester physics for engineers. I'd be very disappointed if a sophomore in engineering did not get the question right. They might need a circuit diagram to understand what you were trying to say.

    I'm also very certain that I would not want most undergraduate engineers near my wiring. Maybe a few that grew up building stuff.

    Steve

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    Moderator & Master Plumber hj's Avatar
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    Default bulbs

    That once happened in the entire house when I disconnected an electric water heater. ALL the light bulbs which were turned on burned out, along with the TV, radio, and other appliances which were running at the time.

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