a simple question

Discussion in 'Electrical Forum discussion & Blog' started by jwelectric, Dec 1, 2008.

  1. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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 don’t 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.

    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. jwelectric

    jwelectric Electrical Contractor/Instructor

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    20 views and no replies??????????
  3. 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 a moderator: Dec 2, 2008
  4. jamiedolan

    jamiedolan New Member

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    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
  5. Jamie .,

    You got the correct answer there.

    Merci,Marc
  6. jamiedolan

    jamiedolan New Member

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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
  7. jwelectric

    jwelectric Electrical Contractor/Instructor

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    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. Alectrician

    Alectrician DIY Senior Member

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    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. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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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.
  10. jimbo

    jimbo Plumber

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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. jar546

    jar546 In the Trades

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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
  12. jamiedolan

    jamiedolan New Member

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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
  13. jamiedolan

    jamiedolan New Member

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    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
  14. seaneys

    seaneys New Member

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    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
  15. hj

    hj Moderator & Master Plumber Staff Member

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    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.
  16. jadnashua

    jadnashua Retired Defense Industry Engineer xxx

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    Keep in mind that a meter is sort of like a light bulb...when you insert it in the circuit between say ground and neutral, you've now connected them together albeit with a very high resistance. It's more realistic to have a load in the circuit (say a lamp on), then measure voltages between neutral and ground or neutral and hot, or hot and ground. A high impedance meter can introduce its own oddities.

    If you have any electronic dimmers or digital timers or photocells in the circuit, they could be connecting things together but drawing milliamps.
  17. jwelectric

    jwelectric Electrical Contractor/Instructor

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    All and all you all did very good on this experiment a couple of mistakes from a couple of posters but all in all very good.

    First in a 240 volt single phase panel there is only one phase present. Yes I know that there two voltages present but only one phase.

    When the neutral is opened or broke or unconnected the circuit becomes a series 240 volt circuit instead of the parallel 120 volt circuit that we had with the neutral attached, closed or on. As several has already pointed out Ohm’s law can be used to show the voltage drop across each bulb. If both of these bulbs had of been the same size when the neutral was opened you wouldn’t have notice any difference between them. If we remove the neutral coming into our homes the same effect will occur to our 120 volt circuits in the house.

    Current leaves one leg of the phase and returns through the other leg just as in any 240 volt circuit. In the 120 volt circuit the current will leave one leg and return half way through the phase on the center tap or the neutral. This center tap is always grounded or connect to earth at both the utility pole and again at the building served.

    I can go out side and touch the bare copper conductor that is attached to my ground rod with out feeling a thing. This grounding electrode conductor is connected to the neutral either in the meter base or to the neutral bar in the service panel.
    But if I disconnect the neutral and touch it and the neutral bar in the panel at the same time I would be dead.

    We manipulated the current flow through the bulbs by opening a conductor. We could see the results of what happens when we change the path of current flow. We determined that in a series circuit that the voltage drop will be greater across the load with the most resistance.

    Now for one more problem, let’s add one more resistor in series with this circuit of 5000 ohms. This is the average resistance of the human body when it is completely dry and has no wounds exposed to the points of contact (taken from OSHA).

    How much of a voltage drop is across the human body?

    What happens to the light bulbs?

    More so than any thing else what happens to the human body?

    Just as with the 50 watt light bulb the human body will burn brighter than the 100 watt light bulb. All of that heat will be in this human causing severe damage to the internal organs.
    Now if you think that a Do-It-Yourselfer can learn enough to keep his/her self safe by posting and reading a couple of post on the internet then I surrender all my years of experience and knowledge to nothing more than a waste of time.
    Praise God and greyhounds give me a pipe wrench cause I already know that a stack vent is dry and a vent stack is wet I am going into plumbing.

    Thank you
  18. leejosepho

    leejosepho DIY scratch-pad engineer

    Since "DIYer" is singular, the word there would be "doesn't" ...

    ... and the word there would be "well", as in:

    "It is good when everyone in class does well while trying to do some good!

    I agree with you, JW. Words and wordings are quite important.
    Last edited: Dec 2, 2008
  19. jwelectric

    jwelectric Electrical Contractor/Instructor

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    Thank you Lee for your expertise in the English language.
    Notice that the sentence above showing gratitude for your knowledge of the English language, although it conveys a message does not have an action segment or verb if you like.
    This is grammar or the structure of a language something that is taught in communication classes such as elementary school English.

    Terminology on the other hand is a study of terms. Terminology is used to define something within a discipline or specialty field.

    With your help maybe the two of us can learn how to construct a message that not only has good grammar but the proper terminology.
  20. Thatguy

    Thatguy Homeowner

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    something else to consider

    Incandescent lamps act like resistors whose resistance value is current-dependent. The cold resistance is 10x or 15x lower than the hot resistance so they sort of act to maintain constant current in a circuit like this.

    Regarding safety, and the big picture: somehow, I can't imagine how, only 1300 people per year are electrocuted in the U.S.
    Maybe it is their natural fear of something invisible and hard to understand and that can cause harm. On the other hand, with moving machinery and the like, the danger is obvious (to adults).

    1300 is "down in the noise."
    Hospitals kill 90,000/yr. Highways kill 40,000/yr. Smoking kills ~400,000/yr. Food poisoning, 5000/yr.
    Last edited: Dec 2, 2008
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