gar
Senior Member
- Location
- Ann Arbor, Michigan
- Occupation
- EE
091007-1412 EST
Mayimbe:
Comments on your three points.
Point 1.
Point 2.
Point 3.
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.
.
Mayimbe:
Comments on your three points.
Point 1.
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.
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.
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.
.
