Calculation

[gar]

221229-2029 EDT

roger:

The sloppiness of the question that started this thread, if something different was meant than presented, does not justify a sloppy answer.

If the intent of the question was to relate to resistors of relatively low temperature coefficient of resistance, then the loads should have been drawn as resistors, not as bulbs.

You want the reader to make a grossly incorrect interpretation of the question by your comment. With the grossly different values that occur in a real circuit of incandescent bulbs that simple response can not be justified. You would not treat a code question that way. You would be very precise.

.

 

[ggunn]

221229-2029 EDT

roger:

The sloppiness of the question that started this thread, if something different was meant than presented, does not justify a sloppy answer.

If the intent of the question was to relate to resistors of relatively low temperature coefficient of resistance, then the loads should have been drawn as resistors, not as bulbs.

You want the reader to make a grossly incorrect interpretation of the question by your comment. With the grossly different values that occur in a real circuit of incandescent bulbs that simple response can not be justified. You would not treat a code question that way. You would be very precise.

.

I believe his point (and mine) was that in an entry level course the question is meant to be simple. Yes, it could have been more precise with resistors instead of lamps, but so what? Newton's Laws were imprecise when compared to Einstein's work, but in entry level physics classes, Newton is what is taught.

Of course the answer would have been more precise if it were a code question, but it wasn't. It was close enough for a first approximation and it got the point across, so move along. Nothing more to see here.

 

[Besoeker3]

I believe his point (and mine) was that in an entry level course the question is meant to be simple. Yes, it could have been more precise with resistors instead of lamps,

And the teacher/tutor should have known that. Inexcusable in my opinion..

 

[gar]

221230-1235 EDT

ggunn:

A tungsten incandescent lamp when excited from a fast enough AC waveform that is way above its thermal averaging time constant is very much equivalent to DC excitation of the same bulb. 60 Hz, really 120 Hz thermal, for most incandescent bulbs is fast enough to be equivalent to DC in the bulb's thermal response.

Thus, the average resistance and instantaneous resistance are very close to the same.

When a tungsten filament light bulb is powered its resistance will vary a large amount over its useful reliable working voltage range. A 100 W tungsten filament bulb has an average resistance of about 144 ohms at 120 V input. At near zero volts it is close to 10 ohms. This is a 14 to 1 variation in resistance over a useful working range of the bulb voltage. There is no way you should consider this a constant resistance vs voltage relative to a normal 60 Hz power source. A carbon filament bulb is quite different.

Why do you want teach very sloppy thinking?

.

 

[ggunn]

221230-1235 EDT

ggunn:

A tungsten incandescent lamp when excited from a fast enough AC waveform that is way above its thermal averaging time constant is very much equivalent to DC excitation of the same bulb. 60 Hz, really 120 Hz thermal, for most incandescent bulbs is fast enough to be equivalent to DC in the bulb's thermal response.

Thus, the average resistance and instantaneous resistance are very close to the same.

When a tungsten filament light bulb is powered its resistance will vary a large amount over its useful reliable working voltage range. A 100 W tungsten filament bulb has an average resistance of about 144 ohms at 120 V input. At near zero volts it is close to 10 ohms. This is a 14 to 1 variation in resistance over a useful working range of the bulb voltage. There is no way you should consider this a constant resistance vs voltage relative to a normal 60 Hz power source. A carbon filament bulb is quite different.

Why do you want teach very sloppy thinking?

.

You realize that I know all that, right? You don't try to teach calculus to a third grader. The point of the exercise was the very simple math, not the thermal effects on the resistance of tungsten, or carbon fiber, or whatever. The fact that real live light bulbs do not behave that way is irrelevant.

I've said my piece; you guys carry on.

 

[Besoeker3]

You realize that I know all that, right? You don't try to teach calculus to a third grader. The point of the exercise was the very simple math, not the thermal effects on the resistance of tungsten, or carbon fiber, or whatever. The fact that real live light bulbs do not behave that way is irrelevant.

But the tutor could have/should have use simple fixed resistors.

 

[jim dungar]

But the tutor could have/should have use simple fixed resistors.

Why?
A novice, just learning the basic electrical formulas, would have no idea that a light bulb does not behave as a fixed resistor.

I mean if you really want to be pendantic, the resistance of the conductors should be also been considered, but most 'school' problems ignore it or say it is lumped into the 'load' value provided.

 

[infinity]

Why?
A novice, just learning the basic electrical formulas, would have no idea that a light bulb does not behave as a fixed resistor.

I agree, there is no need to take a simple concept and make it complex. Maybe we should measure that contact resistance of the lamp to the socket to ensure that both lamps are screwed in with the same amount of torque.

 

[Besoeker3]

Why?
A novice, just learning the basic electrical formulas, would have no idea that a light bulb does not behave as a fixed resistor.

The novice maybe but the tutor should not have failed that simple test. So simple to fixed it with fixed resistors. Job done.

 

[drcampbell]

... Newton's Laws [are] imprecise when compared to Einstein's work, but in entry level physics classes, Newton is what is taught. ...

In my freshman physics classes, the professors were most emphatic to let us know that the class was a limited subset of physics that is generally regarded as obsolete, only approximately correct, and then only for heavy and slow objects.

Yes, the teacher should have known better and not prepared a quiz question with such glaring problems.

 

[RumRunner]

You realize that I know all that, right? You don't try to teach calculus to a third grader. The point of the exercise was the very simple math, not the thermal effects on the resistance of tungsten, or carbon fiber, or whatever. The fact that real live light bulbs do not behave that way is irrelevant.

I've said my piece; you guys carry on.

. . . . Boring conversation anyway. . . .



You are a mean and ornery dude like a gun-slinging cowboy-- mounted on a white steed with a slim cigar between his teeth.

Happy New Year !!

 

[ggunn]

. . . . Boring conversation anyway. . . .



You are a mean and ornery dude like a gun-slinging cowboy-- mounted on a white steed with a slim cigar between his teeth.

Happy New Year !!

Thanks; I'll take that as a compliment.

 

[Dennis Alwon]

You guys are nuts. lol For beginners a resistor is fine but the light bulb is a visual reality to what is going on. Also the teacher may have been trying to get the students to figure out the resistance to solve the problem rather than give it to them directly.

If these are beginner level students I think the question is fine and perhaps afterward the teacher can explain that the resistance of a lightbulb changes as it gets hot.

The concept is what is trying to be taught. Now if these are EE students then perhaps this is not a great question. I am assuming it is like elec 101

 

[Besoeker3]

You guys are nuts. lol For beginners a resistor is fine but the light bulb is a visual reality to what is going on. Also the teacher may have been trying to get the students to figure out the resistance to solve the problem rather than give it to them directly.

If these are beginner level students I think the question is fine and perhaps afterward the teacher can explain that the resistance of a lightbulb changes as it gets hot.

The concept is what is trying to be taught. Now if these are EE students then perhaps this is not a great question. I am assuming it is like elec 101

But it would be equally simple to used fixed resistors.

 

[Besoeker3]

A addendum and a very long time ago - probably around sixteen then.
My physics teacher taught is about simple resistance calculations. Quite a long time before I became an electrical engineer.

 

[Dennis Alwon]

But it would be equally simple to used fixed resistors.

If you wanted a beginner to physically see what happens it is easier to see one bulb burn out and the other light dim. You do the experiment after the question is given.

For me and my tiny brain I need to see it in terms of what I work with. If I am in electronics then that is different.

 

[drcampbell]

... The concept is what is trying to be taught. ...

Indeed.

But in a typical classroom, there will always be that one student who brings up all the real-world variables and nuances, asks the questions you've seen posed here, and derails the lesson plan's train of thought.

It's essential that the lesson plan be bulletproof.

 

[kwired]

You guys are nuts. lol For beginners a resistor is fine but the light bulb is a visual reality to what is going on. Also the teacher may have been trying to get the students to figure out the resistance to solve the problem rather than give it to them directly.

If these are beginner level students I think the question is fine and perhaps afterward the teacher can explain that the resistance of a lightbulb changes as it gets hot.

The concept is what is trying to be taught. Now if these are EE students then perhaps this is not a great question. I am assuming it is like elec 101

I agree, and do remember similar examples being used when I learned this kind of stuff.

It maybe took a few years of experiences before I even grasped the fact that incandescent lamp resistance is different when hot vs cold.

Though it doesn't hurt to explain that this resistance isn't really fixed. We do often ignore conductor and source impedances when first learning electricity 101 as well, but they are something that can't always be ignored.

And more recent times it can get more complex if you don't give even more details about the lamp - LED lamps are not as simple of a load as the incandescent lamp.

 

[ggunn]

Indeed.

But in a typical classroom, there will always be that one student who brings up all the real-world variables and nuances, asks the questions you've seen posed here, and derails the lesson plan's train of thought.

And at that point the instructor should just say that's true but beyond the scope of the question and not the point of the calculation. Yes, you could set up the problem with X Watts going through one resistance and Y Watts going through the other with the neutral in place, but when you break the neutral X and Y would change, which would make the calculation more complex and confusing to the novice. The light bulb scenario is simply shorthand for "keep power constant and calculate the voltage". Jeez.

 

[Besoeker3]

If you wanted a beginner to physically see what happens it is easier to see one bulb burn out and the other light dim. You do the experiment after the question is given.

For me and my tiny brain I need to see it in terms of what I work with. If I am in electronics then that is different.

I don't think the intent was to have one lamp burn out. I think it was to show the two differences in voltages. Still wrong but whatever................