One prediction on electric cars
- Thread starter gar
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Yes thanks. I have that in my "favorites" list too; it's just the list is alphabetical and "BIPM" comes before "NIST". Of course, both are fully authoritative for the terms under discussion.
The instantaneous qualifier is irrelevant.
As I posted earlier we sometime use words with an understanding that the context of their use fills in more meaning than the word itself.
I mentioned pen. For most, that would mean a ball-point pen.
I happen to write with a fountain pen from time to time. But it will always be seen a a fountain pen.
I don't how to describe it any better. A watt is just a watt.
ggunn
PE (Electrical), NABCEP certified
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Or a bad smell.And then there is the federal pen.
Cockney rhyming slang from "pen and ink" meaning stink.
mivey
Senior Member
With what has been presented, I must say that your statement ""kW is an instantaneous value. Time or duration doesn't come into it." was irrelevant and unrevealing. While it led to an interesting review of the topic, and gave me an excuse to search through my library, I find little more than opinion in your statement.
Your position that watt value are simply watts and that they are instantaneous values simply does not hold water. Watts are the units for both instantaneous and average values and the same is presented in many references. I have posted evidence supporting the use of both instantaneous and average values of watts and you have presented no evidence to the contrary. There is simply a mountain of reference material contradictory to your position.
The simplest of arguments against your opinion is that the AC function for instantaneous power has both a constant and time dependent component. If we were to evaluate the function at a single instant in time, say t1, then we get an instantaneous value of watts. If we take the average over a cycle (the smallest interval we normally use for AC signals) we get an average value of watts.
This theme is repeated throughout the other disciplines in one form or another with instantaneous and average power, rates, etc.
Thanks for the exercise but due to lack of supporting evidence I must conclude your position is more opinion than fact.
mivey
Senior Member
From what I have seen, you are in line with the majority of the engineering and scientific community, if that's any comfort (not that it always is).
What you mostly find in physics texts is that they start with work or energy, then talk about the work done or energy transfer over time to get an average rate of power. They also further develop the limiting case to get instantaneous power, usually presented as the dot product of a constant force and constant velocity.
It's a simple statement the watt is an instantaneous value.
That is not a matter of opinion.
I had a quick check back through the thread and found these comments:
Rate is an instantaneous measure as in miles per hour or RPM
gar made some further comments to elaborate.
Perhaps you should just accept and move on.
mivey
Senior Member
Not only is it opinion, it is unsubstantiated opinion. I have given you examples of the use of instantaneous and average watts. Many more to be found with a simple search.
Both average and instantaneous rates exist. I have a library full of books going back to the turn of the century showing that and there are even more references to be found on the internet. Mountains of it.
I'm not in the habit of accepting opinion that is in conflict with mountains of contradictory evidence.
Your statement that "Rate is an instantaneous measure " is a misapplication of concept in this case and tries to apply the concept of what some might call a "true rate" to a value that simply does not fit. As I have stated, there are both average and instantaneous rates (really, just look it up).
To summarize what you will find:
When we have an instantaneous measure we try to reach the theoretical point where the time delta approaches zero (i.e., the individual points along the sinusoidal power waveform). Trying to make the concept of a point at a specific time limit stick to a value that is the average of those points over a time period is a misapplication.
You simply can't throw out the time dependency of the average value because there also exists a different application that uses the concept of instantaneous change to get an instantaneous value, even if the two use the same units of power.
This is further emphasized by the fact that the power in an AC system is varying. The instantaneous change is not usually the value of interest but it is the real or average value that is of interest.
In addition to the time dependence or real watts and that we have to consider times in cycle multiples, as we shorten the time interval from periods like hours, minutes, seconds and on down to one cycle (the smallest interval we normally consider) we will find that the maximum kW value increases. An obvious time dependence for that watt measurement as well.
A value obtained by averaging instantaneous values does not transform the resulting value into an instantaneous value. It is still an average value.
Another way to clarify it to recognize that a set function does not become a point function just because they have the same units for the resulting values.
An one more way to look at it is that the slope of a line tangent to a curve at a particular point, and the slope of a line between two different points are not the same thing.
Fine. Cite any substantiated evidence that proves that the watt is not an instantaneous unit.
mivey
Senior Member
The watt is used for both instantaneous values and average values. The watt can be used for both an average rate and an instantaneous rate. You are trying to say it's use with a point function evaluation that results in an instantaneous value means we are constrained say we have an instantaneous value when we evaluate a set function. The truth is that one produces an instantaneous value and one produces an average value but they both use watts.
Fine.
Cite any substantiated source that proves that the watt is not an instantaneous unit.
GeorgeB
ElectroHydraulics engineer (retired)
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Mivey, I believe you are overthinking it here. 1 watt is (definition) one joule/second, and can be identically identified as one volt-ampere. (For you real and apparent power folks, that is true ... it is instantaneous voltage times the instantaneous current. Our integrating meters don't show instantaneous.)
If a circuit consumed 1 watt for 1 second, then 3 watts for one second, that circuit has consumed 4 watt seconds in 2 seconds, or an average of 2 watts over a time of 2 seconds. Note that at no time did it consume the average.
I can drive 6 miles at 60 mph (an instantaneous value) which would take 0.1 hours. I can sit for 0.9 hours, and my average speed is then 6 mph.
I don't get where you are coming from with your arguments.
If a circuit consumed 1 watt for 1 second, then 3 watts for one second, that circuit has consumed 4 watt seconds in 2 seconds, or an average of 2 watts over a time of 2 seconds. Note that at no time did it consume the average.
I can drive 6 miles at 60 mph (an instantaneous value) which would take 0.1 hours. I can sit for 0.9 hours, and my average speed is then 6 mph.
I don't get where you are coming from with your arguments.
mivey
Senior Member
I have said it can be used for both. You have said it is only for instantaneous value. I have already provided a source showing my argument, so how about you provide a source proving that all watt values are instantaneous.
mivey
Senior Member
Bes has said you do not have an average speed but that your 6 mph is an instantaneous value. He also has said that time or duration had nothing to do with your 6 mph value. At the time, we were discussing an average kW value which is equivalent to your discussion of an average speed.
All watt values?
There aren't all watt values.
It's a single derived SI unit.
From Wikipedia:
The watt (symbol: W) is a derived unit of power in the International System of Units (SI), named after the Scottish engineer James Watt (1736?1819). The unit is defined as joule per second[1] and can be used to express the rate of energy conversion or transfer with respect to time. It has dimensions of L2MT-3.
You seem to to struggle with that concept. Or maybe you are just being argumentative.
Really?
Don't believe I did.
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I don't usually care for Wikipedia as an authoritative source unless I can also determine its original source. Since it cites the BIPM as the source for defining watt, it would seem to be ok.
A bit more "grist for the mill."
The BIPM's SI brochure has some interesting discussions about "quantities and units", especially Section 1.1. It seems arcane IMO and I think the NIST's discussion is a bit easier to follow, but they would appear consistent from a careful read of both.
A bit more "grist for the mill."
The BIPM's SI brochure has some interesting discussions about "quantities and units", especially Section 1.1. It seems arcane IMO and I think the NIST's discussion is a bit easier to follow, but they would appear consistent from a careful read of both.
And can save a bit of typing.......
mivey
Senior Member
Is that really necessary? You know this discussion started with ggunn's bringing up metered values for peak demand charges. You tried to say they were instantaneous values when we know the meter reports average values. Now you want to pretend we are strictly talking about a single defined unit instead of the metered values? When you ask someone for the amp readings on a piece of equipment what would your response be if they told you there is only a single amp value and then quoted you the unit amp definition? This dancing around doesn't help.
ggunn was talking about a peak demand charge: "Actually, in many places you get charged for both. In addition to the kWh charge for energy used there is a kW surcharge based on your peak demand."
You wanted to call that an instantaneous value and said that your billing was based on a value over a period rather than an instantaneous value. Gunn pointed out that he was talking about a kW charge in addition to a kWh charge.
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