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#15
August 28th 03, 01:35 PM
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W5DXP wrote:
wrote: So when I use my voltmeter to measure a voltage, I actually have two voltages present which sum to the reading. Yes, that's why you need a directional voltmeter or directional wattmeter to separate the forward wave from the reflected wave when reflections exist. When I do this on a D-cell, what are these two voltages? Reflected voltage is zero during DC steady-state. But how did you know this little cylinder with a bump on the end produced DC? And why can't DC circuits have net voltages derived from superposition? Seems that in your model you need extra information before you know what your voltmeter is telling you. I observe that you did not provide an answer for the other examples. What about that power plug on the wall? Too difficult? Feel free to assume that you own an abstract voltmeter which will tell you the voltage as a function of time (an oscilloscope maybe?). It would seem that this belief will lead to some serious decidability issues. If standing waves exist, there is one forward wave component and one reflected wave component. If standing waves do not exist, there is no reflected wave component. Shirley, you can tell whether standing waves exist or not so you will know whether you are dealing with one or two waves. And don't ever trust your voltmeter again. Do you use a DC voltmeter to measure AC? Do you use a 60HZ AC voltmeter to measure 1 GHz RF voltages? Do you use a 100MHz o'scope to look at a light wave? Since we are discussing the world of the ideal, we can assume the existence of ideal voltmeters. If reflections exist and you are using a non-directional voltmeter, you will read the NET voltage. USE THE APPROPRIATE MEASURING INSTRUMENTS! So I will rephrase my question for your benefit: When we measure a voltage using an ideal voltmeter, how do we know if this voltage is composed of components (I generalize, since similar difficulties arise in situations without reflections) so that we can do extra measurements and computations to learn the 'real' voltages and 'powers'? If you wish, the wall outlet provides an excellent opportunity for explanation by example. ....Keith |
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