One prediction on electric cars

[ggunn]

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

<heavy sigh> Yes, I know. We all know.

In my mind I am running a clip from the first Star Wars movie where Han Solo is talking to the security guys in the Death Star when he and Luke are in the brig rescuing Princess Leia, specifically the way that conversation ends.

 

[jumper]

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.

Don't do that darn it!!! Last time you dropped a name I ended reading about Carnot and thermodynamics for three hours!!!:rant:.........

Ah great, now I gotta that up! Curiosity is a bane of mine at times.......

 

[Besoeker]

<heavy sigh> Yes, I know. We all know.

OK. You know that
And, from your previous post, you know that a watt is an instantaneous value. And you know that if you sustain that for a period of time, you get energy. One watt, one second, one joule of energy. One watt, zero time, zero energy. All of this you know, of course.
So the qualification of "instantaneous" for the watt is superfluous, meaningless.

 

[ggunn]

OK. You know that
And, from your previous post, you know that a watt is an instantaneous value. And you know that if you sustain that for a period of time, you get energy. One watt, one second, one joule of energy. One watt, zero time, zero energy. All of this you know, of course.
So the qualification of "instantaneous" for the watt is superfluous, meaningless.

So, you are saying... what? Plot power against time (x), and the area under the curve is energy. To measure power you measure energy over time and calculate it. For a sufficiently small slice of time (dt) the area under the curve is the height of the curve. First semester calculus. Riemann intervals and all that.

Instantaneous power in AC circuits is meaningless in the real world, because no matter how much charge is moving at least 120 times a second VxI is zero. If power factor is not unity, 240 times a second it is zero (voltage and current do not cross zero at the same time). If you could measure instantaneous power, it could be anything between zero and the maximum height of the curve depending on the particular instant when you took your measurement. We measure energy for some period of time and divide the time out to get an average power over the interval. We all know (we really do) that it is not truly an instantaneous measurement of power but it is the best we can do, and it suffices for such things as peak power demand charges.

http://www.youtube.com/watch?v=D8KuH_RxUNE

 

[Besoeker]

So, you are saying... what? Plot power against time (x), and the area under the curve is energy. To measure power you measure energy over time and calculate it. For a sufficiently small slice of time (dt) the area under the curve is the height of the curve. First semester calculus. Riemann intervals and all that.

Instantaneous power in AC circuits is meaningless in the real world, because no matter how much charge is moving at least 120 times a second VxI is zero. If power factor is not unity, 240 times a second it is zero (voltage and current do not cross zero at the same time). If you could measure instantaneous power, it could be anything between zero and the maximum height of the curve depending on the particular instant when you took your measurement. We measure energy for some period of time and divide the time out to get an average power over the interval. We all know (we really do) that it is not truly an instantaneous measurement of power but it is the best we can do, and it suffices for such things as peak power demand charges.

http://www.youtube.com/watch?v=D8KuH_RxUNE

All I'm saying is that the watt is an instantaneous value, something you've already agreed.
That's it.

 

[GoldDigger]

All I'm saying is that the watt is an instantaneous value, something you've already agreed.
That's it.

Perhaps part of the disconnect we see going back and forth can be attributed to the difference between a given number being (potentially) an instantaneous value, described by an equation as a function of time, and the ability to actually make an instantaneous measurement.
What we can do with the math does not always match what we can measure in the real world, but the actual results will (hopefully) be compatible with both.

 

[mivey]

Don't do that darn it!!! Last time you dropped a name I ended reading about Carnot and thermodynamics for three hours!!!:rant:.........

Ah great, now I gotta that up! Curiosity is a bane of mine at times.......

Nerd alert!! I have a weakness for that as well. It is not restricted to engineering and I probably drive my doctors crazy. However, my eye doctor seems to enjoy delving into the fun stuff. Most of the others tend to be courteous but don't really engage.

 

[ggunn]

All I'm saying is that the watt is an instantaneous value, something you've already agreed.
That's it.

Why do you keep saying it when no one is saying otherwise? That there is a time component to Watts (there is) and that it is an instantaneous value (it is) are not mutually exclusive.

 

[Besoeker]

Why do you keep saying it when no one is saying otherwise? That there is a time component to Watts (there is) and that it is an instantaneous value (it is) are not mutually exclusive.

I tried.
I give up.

 

[ggunn]

I tried.
I give up.

You tried what? It seems (seemed) to me that you could not accept the fact that no one was disagreeing with you.

Let me try this. The Watt is a measure of power, which is an instantaneous value. If you plot power vs. time on a very granular level in an AC circuit, it is a rapidly changing waveform, but if you integrate that waveform from T0 to T1, you will get a measure of energy, which is the area under the power waveform between those two points in time. If you now divide that quantity of energy by the time interval, you will get a value for power, which is the average power over that interval. The energy transferred during that time interval is as if the power were constant throughout the interval at that level and if that were true it wouldn't make any difference when (during that interval) you took your theoretical but impossible in the real world instantaneous power measurement.

 

[readydave8]

electric cars

electric cars

This thread is about Watt?

 

[GoldDigger]

This thread is about Watt?

Who?

 

[readydave8]

Who?

What?

 

[FionaZuppa]

<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!

its only a not polygon in the math. i know of nothing in our uverse that is observable to match the math. do you? an omni antenna makes a donut with the rf, but does any slice match the math of a circle?
i might venture into bending of light near black holes, but still nothing we have observed.

 

[mivey]

You tried what? It seems (seemed) to me that you could not accept the fact that no one was disagreeing with you.

I had a disagreement over the continued emphasis of the word "instantaneous" and the assumptions made:
I disagreed with Besoeker's insistence that a watt is an instantaneous value and that time is irrelevant. Bes thinks the instantaneous label is irrelevant because evidently he assumes "rate" in the definition means a watt is an instantaneous value. Instantaneous is not used in the definition. However, I think it is irrelevant because watt is in use for both instantaneous and average values.


A unit should be neutral:
A unit is a generic reference quantity for the quantity you are currently measuring. If the quantity you are measuring is instantaneous then the unit better match that and if the quantity being measured is average then it better match that. Since we use it for both it is probably more appropriate to say the unit in general is neutral and can match either type measurement.


What the watt definition specifically states:
Consider the watt definition. The word "instantaneous" is not used so there is no valid reason to think it is implied. If we want to be picky and look closely at the definition, we will notice it is actually a unit value so we have a unit of work over a unit of time. We have one Joule of work expended/converted/exchanged over one second of time (J/s).

One second is hardly instantaneous. To specify an instantaneous value, we would take the limit as time approaches zero and specify the instantaneous change in work being done (the time derivative dJ/dt).

This is how average power and instantaneous power are defined. If we want to just look at the watt unit definition we would say it is anything but instantaneous. If you look strictly at the definition it is work over a specified time interval or average work.

I do not think it is important to be so restrictive since it is a generic reference quantity for the quantity you are currently measuring. We could easily specify a reference J/s quantity and measure it by any number of methods if we want a reference quantity to match our desired measurement method.


Average measurements for fixed equivalent vs. error reduction:
In our case, the power oscillates and we are constrained to use the average watt value to get an equivalent fixed value. This average value is used to measure an oscillating quantity. This single-cycle average is not the same as taking an average of many readings of a fixed quantity to reduce measurement error. However, for the oscillating quantity we can take the average over several cycles to reduce the measuring error.

 

[mivey]

IEEE perspective

IEEE perspective

If we want to look at definitions, the watt definition shown before does not support "instantaneous" only. If anything it supports "average".

For another perspective, let's look at three terms in the IEEE dictionary:

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.

rate: The change in a value over a specified period of time. Note: Instantaneous rate is the derivative of the value with respect to time and cannot generally be measured. The measured rate approaches the instantaneous rate as the specified period of time approaches zero.

units: The units of a measured value of a physical variable define the standard quantity of the measure of that variable used to express the value.


So by definition a watt is one Joule over one second, not a limit or derivative to get an instantaneous value.

Notice that "rate" does not mean instantaneous only. An instantaneous rate is a time derivative. We can have instantaneous and average rates.

Also notice that the units are a standard quantity of the value we are measuring so the units should agree with what we are measuring. There is no reason to restrict the watt unit to a particular measurement type and wind up with a mismatch.

 

[mivey]

average watts

average watts

There are mountains of these but here are a few cut and pastes from the internet where we use the concept of average watt values:

http://hyperphysics.phy-astr.gsu.edu/hbase/electric/powerac.html:
"Almost always the desired power in an AC circuit is the average power...Pavg = VI cos(phi) = ___ watts"

http://www.smps.us/power.html: "Average power (9) is called active or real power, or simply watts."

Prof Ganz from the Journal of the Franklin Institute:
"With alternating currents the electromotive force and current strength are constantly changing in accordance with some periodic law. It is still true that the power is at any instant equal to the product of the volts and amperes at that instant. Alternating current voltmeters and ammeters do not indicate instantaneous values, but the square root of the average squares of these instantaneous values, which are called in practice the effective values. The average power is equal to the average of the products of the instantaneous volts and amperes, and this average power in watts is in general less than the product of the effective volts and amperes. The factor by which this product of volts and amperes must be multiplied in order to obtain the true average watts is called the Power Factor."

The Fundamentals of Three-Phase Power Measurements-Tektronix:
"Power is measured in ac systems using wattmeters. A modern digital sampling wattmeter, such as any of the Tektronix power analyzers, multiplies instantaneous samples of voltage and current together to calculate instantaneous watts and then takes an average of the instantaneous watts over one cycle to display the true power. A wattmeter will provide accurate measurements of true power, apparent power, volt-amperes reactive, power factor, harmonics and many others over a broad range of wave shapes, frequencies and power factor."

Nexus mpower meter manual:
"Average (Power): When applied to power values (Watts, VARs, VA), the average is a calculated value that corresponds to the thermal average over a specified time interval. The interval is specified by the user in the meter profile. The interval is typically 15 minutes. So, the Average Watts is the thermal average of Watts over the previous 15-minute interval. The thermal average rises to 90% of the actual value in each time interval. For example, if a constant 100kW load is applied, the thermal average will indicate 90kW after one time interval, 99kW after two time intervals and 99.9kW after three time intervals."

IEEE paper on HV resistance current measurements using a watts loss technique:
"The watts loss calculation is based on the general formula for average watts so that the resistive current can be measured accurately with some harmonics of the fundamental present in both the test current and the supply voltage."

Standard Handbook for Electrical Engineers:
"Alternating Current Power...If the product of the instantaneous values of current and potential are plotted, the curve P is obtained. The average value of this curve {the plot of the instantaneous power} is the power equivalent of a continuous current producing the same effect. Also, W = EI where W = average watts, E = mean effective volts and I = mean effective amperes."

 

[GoldDigger]

By definition a watt is NOT one Joule over one second. It is the rate which if continued for one second will produce one Joule of work.
That same power is still one watt if it only produces one half Joule over one half seconds and then stops.
The definition of any derived unit does not tell us what it "is", just how to measure it.

 

[mivey]

There are also mountains of these but here are a few examples of a rate being average or instantaneous:

http://facultypages.morris.umn.edu/.../Precalculus/Lectures/AverageRateofChange.pdf
A document with graphs explaining average and instantaneous rates of change


http://www.mathwords.com/a/average_rate_change.htm:
"Average Rate of Change: The change in the value of a quantity divided by the elapsed time."


http://www.chem.purdue.edu/gchelp/howtosolveit/Kinetics/CalculatingRates.html:
"We calculate the average rate of a reaction over a time interval by dividing the change in concentration over that time period by the time interval
...
An instantaneous rate is the rate at some instant in time. An instantaneous rate is a differential rate"


http://www.sparknotes.com/math/calcab/applicationsofthederivative/section1.rhtml:
"The average rate of change is equal to the total change in position divided by the total change in time
...
The instantaneous rate of change measures the rate of change, or slope, of a curve at a certain instant. Thus, the instantaneous rate of change is given by the derivative"


http://www.mast.queensu.ca/~peter/grade12/MHF4U-5/51.pdf:
A nice document with graphs explaining average and instantaneous rates of change


http://www.brightstorm.com/math/calculus/the-derivative/instantaneous-rate-of-change/:
"The instantaneous rate of change is the rate of change of a function at a certain time. If given the function values before, during, and after the required time, the instantaneous rate of change can be estimated. While estimates of the instantaneous rate of change can be found using values and times, an exact calculation requires using the derivative function. This rate of change is not the same as the average rate of change. "


http://chemwiki.ucdavis.edu/Physica...eaction_Rate/The_Rate_of_a_Chemical_Reaction:
"Reaction rates have the general form of (change of concentration / change of time). There are two types of reaction rates. One is called the average rate of reaction, often denoted by (delta[conc.] / delta[t]), while the other is referred to as the instantaneous rate of reaction, denoted as either:

limit delta[t]->0 delta[concentration]/delta[t]

which is the definition of the derivative

d[concentration]/dt

The average rate of reaction, as the name suggests, is an average rate, obtained by taking the change in concentration over a time period, for example: -0.3 M / 15 minutes. This is an approximation of the reaction rate in the interval; it does not necessarily mean that the reaction has this specific rate throughout the time interval or even at any instant during that time. The instantaneous rate of reaction, on the other hand, depicts a more accurate value. The instantaneous rate of reaction is defined as the change in concentration of an infinitely small time interval, expressed as the limit or derivative expression above. Instantaneous rate can be obtained from the experimental data by first graphing the concentration of a system as function of time, and then finding the slope of the tangent line at a specific point which corresponds to a time of interest. Alternatively, experimenters can measure the change in concentration over a very small time period two or more times to get an average rate close to that of the instantaneous rate. The reaction rate for that time is determined from the slope of the tangent lines"

 

[mivey]

By definition a watt is NOT one Joule over one second. It is the rate which if continued for one second will produce one Joule of work.

Trying to be excessively literal, similar to trying to say the word "rate" has to mean "instantaneous rate".

That same power is still one watt if it only produces one half Joule over one half seconds and then stops.

No doubt.

The definition of any derived unit does not tell us what it "is", just how to measure it.

My point is we can get a Joule per second with an instantaneous measurement or an average measurement. We do not have to restrict the nature of the unit such that it is not a standard quantity for our measured quantity.