Calculation

[Strathead]

Thank you. Unfortunately some want to show their superior intellect and want to take an elementary level question showing basic ohms law to some rocket science level. What they fail to realize is everyone has to crawl before they walk.

To the OP, your question was answered by post # 5 and most anything beyond that was beyond what the course question was looking for.

To make the others happy maybe the question should have left "lamp" out of the equation but get over it.

Agreed. I have a hard enough time getting my 2nd year students to remember the difference between 120/240 and 208/120, to care whether the represented resistance jn an equation is perfectly fixed. It was not a pooRly written question. It was a simple question designed to help AVERAGE electricians understand what happens when a neutral is lost. I take offense at gar’s statement because I use a similar example to convey that the dim bulb will get brighter and the bright bulb will get dimmer. For most electricians that is enough to get them through their career.

 

[HEYDOG]

221228-2137 EST

Hfalz1:

This is a very poorly written question. Synchro provided a good answer.

For a tungsten filament bulb you can use it with any input voltage up to it's rated voltage, and a ways above that rating. Average lifetime of a bulb goes down quickly with voltage above its rating. The shortening of life is greatly accelerated above the bulb's rated voltage.

A 120 V 100 W bulb has a hot resistance of about 144 ohms from 100 / 120 = 0.84 A. At room temperature and no current the resistance of the 100 W bulb is about 10.7 ohms. A ratio of about 14 to 1.

With the two bulbs in series, and 240 V applied to the series string the lower wattage bulb will have a much higher voltage across it than the higher wattage unit.

As an experiment I used a 60 W bulb in series with a 100 W bulb, and with total voltage adjusted to put 120 V across the 60 W bulb the total voltage is 146 V. Or a 26 V drop on the 100 W bulb.

The person that created this question is ignorant and incompetent, and should not be teaching.

Agreed. I have a hard enough time getting my 2nd year students to remember the difference between 120/240 and 208/120, to care whether the represented resistance jn an equation is perfectly fixed. It was not a pooRly written question. It was a simple question designed to help AVERAGE electricians understand what happens when a neutral is lost. I take offense at gar’s statement because I use a similar example to convey that the dim bulb will get brighter and the bright bulb will get dimmer. For most electricians that is enough to get them through their career.

I am in total agreement. I taught Electrical Trades to high school juniors and seniors in a Vocational school for 18 years. I used examples like this two lamp one many times to teach what happens when a neutral becomes open. They were just getting their feet wet. At that point I felt it wasn’t necessary to go beyond that. To call the teacher ignorant and shouldn’t be teaching is an insult to a lot of Teachers out there. You have to learn how to crawl before you can walk.

 

[RumRunner]

…. What they fail to realize is everyone has to crawl before they walk.

………………..

I disagree with that analogy.

No one would attempt to teach someone beyond their cognitive capability. . . let alone get someone to solve an equation in Calculus if that someone doesn’t have an idea how to solve a problem in Trigonometry and Algebra.
A newborn that is learning how to crawl can’t read or even recognize numerals from alphabets.

That’s a CLICHE that needs to be wiped out from the face of the earth.

A non-sensical metaphor.

 

[ggunn]

…. What they fail to realize is everyone has to crawl before they walk.

………………..

I disagree with that analogy.

No one would attempt to teach someone beyond their cognitive capability. . . let alone get someone to solve an equation in Calculus if that someone doesn’t have an idea how to solve a problem in Trigonometry and Algebra.
A newborn that is learning how to crawl can’t read or even recognize numerals from alphabets.

That’s a CLICHE that needs to be wiped out from the face of the earth.

A non-sensical metaphor.

Also, my grandson learned to walk before he crawled anywhere. He never really crawled much at all.

 

[HEYDOG]

Also, my grandson learned to walk before he crawled anywhere. He never really crawled much at all.

It’s sounds like you should have been and instructor. this whole conversation started over a basic circuit and turned into how ignorant the person was for writing such a question. Totally disrespectful. Obviously you have not taught before. When you have a class full of students everyone learns at a different pace. Some are visual learners and some have great math and reading skills where others not so much. If your a great Teacher you learn to teach all students where there at and continue to build on that with more complex questions. As an instructor I have used many many books written by experts in the field. You can always throw what ifs into almost any question written. I have always enjoyed this sight that is offered here free of charge and feel that it is a great place to exchange information and learn new things. There is no reason to be disrespectful. I might have should addressed this to the post above you!

 

[HEYDOG]

…. What they fail to realize is everyone has to crawl before they walk.

………………..

I disagree with that analogy.

No one would attempt to teach someone beyond their cognitive capability. . . let alone get someone to solve an equation in Calculus if that someone doesn’t have an idea how to solve a problem in Trigonometry and Algebra.
A newborn that is learning how to crawl can’t read or even recognize numerals from alphabets.

That’s a CLICHE that needs to be wiped out from the face of the earth.

A non-sensical metaphor.

You take the analogy to literally! See the post above!

 

[Strathead]

You take the analogy to literally! See the post above!

Oddly, his calculus example, validates the metaphor.

 

[junkhound]

Am with Boeseker and Gar on this issue.

An 'error' or misconception in early instruction can stay with a person all their life. Have had post grad BSEE folks commit analysis and design errors due to some misconception carried over from early exposure to inaccurate 'the teacher said' events.

One of the reasons I read this forum is to get an understanding of the different ways folks approach problems. This thread a good example. Carry on <G>

 

[ggunn]

There is no real world device I know of that would hold the power constant in the example circuit when the neutral is disconnected from the source. The point of the exercise is the calculation of voltage and current when the power is held constant, not that any thing in the real world would actually do that. I cannot imagine that anyone with half a brain faced with this problem would carry the concept that light bulbs would behave that way into more advanced studies. Jeez, this is a stupid argument.

 

[gar]

230103-1003 EST

ggunn:

Where in the original post was there any mention of "power" being held constant? What is held constant is the total line to line voltage.

And by implication, since tungsten filament bulbs are the load, the relative bulb resistances are very different between each other when the center wire is open vs closed.

.

.

.

 

[ActionDave]

Am with Boeseker and Gar on this issue. .....

And you, Boeseker, and gar are wrong.

There is nothing wrong with using light bulbs instead of fixed resistors to illustrate the concept put forward in the OP.

 

[Besoeker3]

And you, Boeseker, and gar are wrong.

There is nothing wrong with using light bulbs instead of fixed resistors to illustrate the concept put forward in the OP.

I disagree. The concept may illustrate the concept but the voltages are simply incorrect. No ifs, no buts. Just wrong.

 

[LarryFine]

I think Synchro's response in post #3 is right to the point, as well as mine when I said fixed resistors should have been used, even if only to make the discussion consistent for all knowledge levels.

For a beginner level, the OP's question is adequate. I believe it would have been better if an explanation of the bulb-vs-resistor difference, which is itself educational, had been included as a footnote.

It's not a matter of which point of view is more technically accurate or politically correct. I think that everyone's position is valid. We are all welcome to publish our own version of electrical lessons.

 

[ActionDave]

I disagree. The concept may illustrate the concept but the voltages are simply incorrect. No ifs, no buts. Just wrong.

I know how much you hate hearing you are wrong but you're wrong.

Not about the math but about everything that is practical in communicating basic electrical.

 

[LarryFine]

I disagree. The concept may illustrate the concept but the voltages are simply incorrect. No ifs, no buts. Just wrong.

I agree technically, which is why I mentioned resistors early on. Bulbs and resistors of different values will definitely produce different voltage imbalances when placed in series.

What suggests that the reader is meant to see the bulbs as fixed resistances is that (1) filament resistances are not easily predictable, and (2) the smaller bulb would not last very long.

 

[drcampbell]

Summary:

The precise team: If you're introducing an elementary concept to an elementary audience, don't clutter up your examples with real-world subtleties that confuse the students, confound the lesson, and leave unfounded incorrect impressions that will haunt your students for life.

The lax team: Don't worry about any of that. Go ahead and write ambiguous examples and quiz questions. Clear communication isn't important and your first-year students aren't smart enough to pick up on subtleties anyway. Confusion & ambiguity are probably good preparation for later life.

Oh, gawd, make it stop!

- - -

In fairness, nobody said those lightbulbs in the example were tungsten-filament incandescent bulbs. They might have filaments made of unobtanium, which maintains a constant resistance (and AC impedance) regardless of changing voltage, current, power or temperature. Or they might be LEDs with smart power supplies that maintain constant power consumption regardless of input voltage. (unless they get a dimming command via blue teeth)

 

[gar]

230103-1128 EST

Following is some measured data. I have not gone above 120 V on any bulb in the experiment because I don't need to destroy any bulbs.

All bulbs are standard, probably about 1000 hour average life.

Calculated values, but will be very close to actual values.
100 W at 120 V = 144 ohms, 0.833 A.
60 W at 120 V = 240 ohms, 0.500 A.
50 W at 120 V = 288 ohms, 0.417 A.

Next with the neutral open, and not going above 120 V on the lower wattage bulb. In other words the total voltage was adjusted to make the lower wattage bulb 120 V.

For the 60 W at 120 V the 100 W read a voltage drop of 46 V, or 92 ohms instead of 144 ohms for the 100 W bulb. Total source voltage is 166 V.
For the 50 W at 120 V the 100 W read a voltage drop of 34 V, or 81.5 ohms for the 100 W bulb. This is a total source voltage of 154 V.

.

 

[roger]

Ok, so have any of you that are on this bandwagon "The person that created this question is ignorant and incompetent, and should not be teaching." seen the instructors answer sheet? Pretty strong insult without knowing anything about the quiz, the answer, or him/her.

Your assumptions of the answer maybe correct, maybe not correct, but in either case to call someone you have never talked to or know anything about ignorant and in incompetent is pretty much uncalled for. JMHO

 

[kwired]

I think Synchro's response in post #3 is right to the point, as well as mine when I said fixed resistors should have been used, even if only to make the discussion consistent for all knowledge levels.

For a beginner level, the OP's question is adequate. I believe it would have been better if an explanation of the bulb-vs-resistor difference, which is itself educational, had been included as a footnote.

It's not a matter of which point of view is more technically accurate or politically correct. I think that everyone's position is valid. We are all welcome to publish our own version of electrical lessons.

If this were basically a beginners level lesson on what happens when there is an open neutral in a multiwire circuit- it is plenty adequate of a demonstration of that.

If instructor wishes s(he) can throw in the fact that in real world incandescent lamp resistance does actually vary depending on filament temperature.

I think I already mentioned earlier in this thread that when we first learning basic circuit calculations and doing classroom as well as lab work on the topic we do normally disregard resistance of conductors though in real world there is some resistance in those as well.

When you go to more advanced real world problems that resistance does become more important at times.

 

[Besoeker3]

I know how much you hate hearing you are wrong but you're wrong.

Not about the math but about everything that is practical in communicating basic electrical.

Oh chill ! I don't hate anything and I very much enjoy this forum. The concept you are promoting is correct. But the mathematics is wrong. The tutor should have known that and should have sorted that.