One prediction on electric cars

[mivey]

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.

Not being argumentative or struggling with the concept. I think you fail to recognize that instantaneous is not in the definition you provided. That is something you have assumed is implied even though we use this unit for both average and instantaneous values.

I noticed you highlighted "rate". Are you are interpreting that to mean instantaneous?

 

[ggunn]

I think you (the collective you) are twisting on the point where calculus applies. For example, any a line in X-Y space has a slope for every value of x, but it cannot be determined from that point on the line alone. You can measure the average slope of a line by measuring the change in y over an interval in x, but you cannot measure the slope from a single point on the line, even though it exists. The slope of the line is the derivative of the equation describing the line at a particular value of x, which is the ratio of the infinitesimally small quantities dy and dx.

Likewise, a Watt is an instantaneous value, but it cannot be measured directly as such. Any measurement methodology of necessity involves a time interval. If the time interval is "short enough", i.e. dt, you can assume that the power over that interval is constant and therefore dividing the average value by the time interval (dP/dt) gives you the instantaneous value of power for any point in time you pick within that interval.

In the end it makes no difference whatsoever in the real world.

 

[mivey]

Bes has said you do not have an average speed

Really?
Don't believe I did.

Same premise. If the kW value used for peak demand has to be instantaneous then using the same logic would say the speed has to be instantaneous. In fact you said:

"It doesn't change the fact that the watt is an instantaneous value. Nor does whatever purpose you use it for. Like speed is an instantaneous value. Of course you can an average to work out how long a journey will take and ETA."

Perhaps you meant the readout on the speedometer which approaches an instantaneous value as we approach a constant speed but you did not clarify. The problem with the analogy is that with metering on the AC system the system is oscillating so the meaningful kW value we get is an average over a cycle at best but usually an average over a longer period like 15 minutes or more for a billing determinant.

 

[mivey]

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.

And appear consistent with IEEE:

watt: The unit of power in the International System of Units (SI). The watt is the power required to do work at the rate of one joule per second.

 

[FionaZuppa]

so, i will agree with some here that a Watt is an expression of "instantaneous", as are all rates, however, there's a very clear distinction between constant rates and changing rates. a Watt by definition is a normalized constant, same rate at any given time.

constant rates are in fact instantaneous at all times. rates that are a function of time, f(t), are also instantaneous at any given time.

and yes, power is energy, but we derive the intensity of power (energy density) by including time.
1Watt vs 1MW, both are just energy per unit of time, one more intense than the other.

 

[mivey]

I think you (the collective you) are twisting on the point where calculus applies. For example, any a line in X-Y space has a slope for every value of x, but it cannot be determined from that point on the line alone. You can measure the average slope of a line by measuring the change in y over an interval in x, but you cannot measure the slope from a single point on the line, even though it exists. The slope of the line is the derivative of the equation describing the line at a particular value of x, which is the ratio of the infinitesimally small quantities dy and dx.

Likewise, a Watt is an instantaneous value, but it cannot be measured directly as such. Any measurement methodology of necessity involves a time interval. If the time interval is "short enough", i.e. dt, you can assume that the power over that interval is constant and therefore dividing the average value by the time interval (dP/dt) gives you the instantaneous value of power for any point in time you pick within that interval.

Yes. I believe I posted a link to a page discussing that very thing as a reference for the point I made about the slope of a tangent line (for the instantaneous case) vs. the slope of a line between two points (the average case).

In the end it makes no difference whatsoever in the real world.

For most of what we do: no. It gets more interesting when you consider the mechanisms of energy exchange in a three-phase circuit. We normally consider the power to be constant and on an average basis for all three it is (we like that result and the higher efficiency of the system). When you consider the reactions in the individual conductors and surrounding fields and specifically what happens with things like losses you must consider again the oscillating power of the individual phases as not all is a balanced as it might appear.

Now whether or not that is considered the real world in the context of what we do in the world of electricians is certainly a question.

 

[Besoeker]

Not being argumentative or struggling with the concept. I think you fail to recognize that instantaneous is not in the definition you provided.

It's irrelevant and thus superfluous.

A bit like saying "a round circle".
It's tautological.

So yes, with respect, you do seem to be struggling with the concept.

 

[Besoeker]

Perhaps you meant the readout on the speedometer which approaches an instantaneous value as we approach a constant speed but you did not clarify.

No approaching about it.
Speed is an instantaneous value.
The blue car I had, the one in the avatar, could get from 0-60mph is six seconds.
Graph speed against time and it would be an ever changing instantaneous value ranging from 0 to 60, never the same from one instant to the next.

 

[mivey]

It's irrelevant and thus superfluous.

A bit like saying "a round circle".
It's tautological.

So yes, with respect, you do seem to be struggling with the concept.

Tautological redundancy?

So you are interpreting "rate" to mean instantaneous?

 

[mivey]

No approaching about it.
Speed is an instantaneous value.
The blue car I had, the one in the avatar, could get from 0-60mph is six seconds.
Graph speed against time and it would be an ever changing instantaneous value ranging from 0 to 60, never the same from one instant to the next.

To get an instantaneous readout you must have one heck of a speedometer. Instantaneous eddy current production and all that.

Ever have to slam on the brakes? Ever notice what happens to the readout or were you otherwise engaged?

I know you have been exposed to a D'Arsonval meter movement (you may even have known him!) and would recognize a delayed response, even a faster response.

 

[Besoeker]

Tautological redundancy?

So you are interpreting "rate" to mean instantaneous?

No interpretation required.

 

[Besoeker]

To get an instantaneous readout you must have one heck of a speedometer..

I made no claim that the speedometer could do so.

 

[FionaZuppa]

It's irrelevant and thus superfluous.

A bit like saying "a round circle".
It's tautological.

So yes, with respect, you do seem to be struggling with the concept.

no such thing as a round circle. a circle is a closed loop made up of infinite amount of infinitely small segments of which every segment is located same distance from a single point.

and there are no "squares", a "square" is a special type of rectangle, of which is a special type of polygon, just like the "circle".



ah..... i think this debate has gone on long enough. an ice cold one........ cheers.

 

[GoldDigger]

<lecture mode> Actually, a circle is not a polygon, it is just the limiting conditions of a series of polygons as the number of sides approached infinity.
Quite often the limit of a series does not belong to the same underlying group as the members of the series.
</lecture mode>

Prost!

 

[mivey]

No interpretation required.

Since they are not the same word, and since they do not show up as synonyms, and they are not in the definition for each other, it is kind of a requirement.

"But everybody knows..." you might begin to say. Do you ever stop to ask yourself when you hear that: Do they? If they do then why? The same goes for "we have always used it that way" or "some well respected individual told me that years ago" or similar phrases.

You can find out a lot of things by double-checking what you think you know or have accepted for a long time. You might remember a long discussion from a while back: There were some that held firmly to past understandings that we might consider limited and could not bring themselves to see the broader picture that folks like you and I can plainly see. Do you want to be that guy?

Think about why you tie the words "rate" and "instantaneous" together. Have you considered that there may be a little more to it than that?

 

[mivey]

<lecture mode> Actually, a circle is not a polygon, it is just the limiting conditions of a series of polygons as the number of sides approached infinity.
Quite often the limit of a series does not belong to the same underlying group as the members of the series.
</lecture mode>

Prost!

How strikingly similar to the discussion at hand!

 

[mivey]

I made no claim that the speedometer could do so.

No you didn't but it did add a bit of flair don't you think? Overcooked a little?

 

[Besoeker]

Do you want to be that guy?

The watt is an instantaneous value.

 

[ggunn]

The watt is an instantaneous value.

Of course it is; I don't believe anyone has said otherwise. Measuring that Watt in the real world involves averaging some quantity of energy over a time interval, though.

 

[Besoeker]

Of course it is; I don't believe anyone has said otherwise. Measuring that Watt in the real world involves averaging some quantity of energy over a time interval, though.

If you want energy, you need a time period.
Then you get joules, ws, kWh.........not watts.