ohms law

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gar

Senior Member
091007-1412 EST

Mayimbe:

Comments on your three points.

Point 1.
Did you know that ohms law is based on aproximation of a graphic where is ploted the current vs the voltage meassured of a certain material?
Ohm's law is a mathematical model to provide a tool to analyze real world problems. As a model it is precise. As a predictor of the real world it is an approximation. The degree of approximation is a function of many different factors. In a reference book the "International annealed copper standard" at 20 deg C is listed as 1.7241 microhm-centimeters. To describe this value to this accuracy implies measurement repeatability better than 1 part in 20,000. No mention of voltage sensitivity. Ohm's law says that for a constant resistance that voltage across the resistance is proportional to the current thru the resistance.

Point 2.
Did you know that the every meassured has an error associated?
True, but you need to define the relative magnitude of the error to the value being measured. This comment has no bearing on the question unless you add some quantitative values. How stable is the resistance? How accurate are the instruments being used? What is the phase of the moon? What is the atmospheric pressure? What is the ambient temperature? Etc.

Point 3.
Did you know that if you meassure the resistance of lamp, and I do it and the same time with a similar lamp, theres high probability that we dont get the same values??
If you are trying to evaluate the characteristics of a device, in this case how the resistance of a tungsten filament lamp varies with voltage, then you test one single device for its response to a voltage variation. From this you create a model. Next you test another device. Does it fit the model? And so on until a useful model is obtained. Then the final model should be able to predict the performance of any similar device for a parameter of the model. In this case variation of the resistance of a tungsten filament lamp with applied voltage.

Because I sample two different parts from a production run and they have different resistances at nominal voltage does not mean that the model for change of resistance vs voltage can not predict the resistance each has at some different voltage. The accuracy of the prediction may be quite good.


Why should an ordinary electrician know that the resistance of a standard light bulb changes with applied voltage? To have an understanding of why there is likely a very high inrush current when a bulb is turned on. To have a better understanding of the products with which they work. To understand that when they run the experiment of measuring the current to an incandescent lamp at 100 V that using ohms law without additional information will badly predict the current at 130 V.

If I repeat measurements on a specific bulb my error is in the range of 0 to 0.2 %. Could be better with a more stable voltage source. The primary error is not from the bulb.

The question of the original post was very specific to a 500 W quartz lamp. Why pick on a quartz lamp and the voltage range specified instead of just a 500 W fixed resistor?

broadgage's suggestion of an electric water heater would have been a much better choice for an approximately constant load resistance. My test load for experiments, a 1500 W portable heater, changes from 8.6 ohms at room temperature with no voltage applied to 10.6 ohms with 118 V applied. This is a 2 ohm change for a 1313 W change. 115 to 120 would be about a 14 W change, and in turn about a 1% change in 2 ohms, or 0.02 ohms as an estimate. About 0.2% change of total resistance of the heater. This compares with about a 1.8% change for a 120 V Quartz lamp from 115 to 120 V. The heater is about 9 times less sensitive compared to the lamp at these voltage levels.

An ordinary power resistor, Ohmite for example, probably has less change with voltage from self heating than my electric heater.

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LarryFine

Master Electrician Electric Contractor Richmond VA
Location
Henrico County, VA
Occupation
Electrical Contractor
You got an ave of .666 :D
Ooh! The mark of the devil!
sym56.gif
 

steve66

Senior Member
Location
Illinois
Occupation
Engineer
Ok here is the full story. It was actually a 3 part question on a liscense exam that I took last night.
1) What is the resistance of a 500 watt quartz lamp rated at 115 volts.
answer : R= E x E / P 115 x 115 / 500w =26.45

If we assume the resistance of the lamp stays the same (it doesn't vary by much, since the change in voltage is only about 10%):

You are correct for #1. You could also do this in two steps: P/V = I which gives 500/115 = 4.34 amps. Then V/I = R which gives 115/4.34 = 26.45 ohms.

2) What is the amps at 120 volts?
answer: I = 500/120 = 4.16

You're only mistake is using the 500 watts. The wattage will change since you have a new voltage. You have to decide what number is going to stay constant when you change the voltage. We can only assume they want you to assume the resistance stays constant. (If you had compared your last answer of 544 watts to the 500 watts you used in this step, I'll bet you would have realized your mistake.)

So you have V/R = I or 120 / 26.45 = 4.53 amps.

3) what is the wattage at 120 volts?
answer: P = E x E / P 120 x 120 / 26.45 = 544 watts

Again, this is correct. You also could of used P = V*I using the current found in step #2. 120V * 4.53 amps = 543.6 watts.

You could also use Rattus's formula using the squares of the voltages. With simple ohms law problems, I try to avoid all the forumlas with squares. You can usually get by without those by using a two step process like I listed in problem #1.

Steve
 
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Mayimbe

Senior Member
Location
Horsham, UK
It is a forum, not a classroom. If it were strictly facts it would be boring.

:grin: Agree. In the "Am I allowed to post here", I was having a nice interchange of experiences and ideas, relating my experience of life, giving my heart in every post and then the teacher/moderator closed the thread. And heaven knows and miserable now...
:)

They should open a thread for the cooldown.
 

mivey

Senior Member
:grin: Agree. In the "Am I allowed to post here", I was having a nice interchange of experiences and ideas, relating my experience of life, giving my heart in every post and then the teacher/moderator closed the thread. And heaven knows and miserable now...
:)

They should open a thread for the cooldown.
If we had a therapy forum, the line would stretch to Venezuela and we could just talk face-to-face. :grin:
 

Mayimbe

Senior Member
Location
Horsham, UK
If we had a therapy forum, the line would stretch to Venezuela and we could just talk face-to-face. :grin:

Quite disturbing for me, I must say. Thank good this isn't a therapy forum.

we should get to the ohms law theme again. Or the moderator will close the thread.
 
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