THE PHYSICS OF... POWER

[wwhitney]

right, so the diff bewteen PF's means you needed to do more work on the gen side so you can deliver same P_watts of real power to the load. so in reality, the energy associated with X_L is wasted power. bad PF simply means more work to take a "kW package" from src to load.

Correct. But the discrepancy is a couple orders of magnitude different from the nominal value of kVAr. In the example, it was 8.7W to deliver 1120 VAr.

Some of the comments made earlier in the thread would make one think that delivering 1120 VAr requires wasting 1120W, but in this example it was only 8.7W. Less than 1% of the nominal VAr value, or of the load active power.

Cheers, Wayne

 

[winnie]

right, so the diff bewteen PF's means you needed to do more work on the gen side so you can deliver same P_watts of real power to the load. so in reality, the energy associated with X_L is wasted power. bad PF simply means more work to take a "kW package" from src to load.

Agreed: the difference in PF means you need to do more work on the gen side to deliver the same watts to the load.

But take a look at the numbers in the example that Wayne came up with.

The PF of 0.667 means that to deliver 1000W to the load, an additional 9W of losses had to be carried versus the PF 1.0 case.

But in the PF 0.667 case, your load is 1500 VA! To move the extra 500VA the generator needed to do 9W of additional work.

-Jon

 

[Sahib]

With a 1VA (edit: purely reactive) load, in the first quarter cycle, 0.00265 joules flows towards the load. In the second quarter cycle, 0.00265 joules flows away from the load. Etc.

Agree.

 

[FionaZuppa]

i am building a more realistic example of how impedance characteristics affect the delivery or real power on the load side. half of the example is done, i need to complete the other half. once complete i shall post it.

 

[Ingenieur]

Power is power.
It's a black and white issue.

The police love him.

View attachment 15670

yes, power is power
it has many forms
active
reactive
mechanical
chemical
etc

an engineer would understand this lol

 

[Ingenieur]

Both of the references in your quoted post describe reactive power as 'the instantaneous power absorbed by the reactive part of the load,... is a double frequency sinusoid with zero average value'...

I agree: 'reactive power' is associated with instantaneous power.

The 'zero average value' is the point that we are all circling around from opposite sides. The 'zero average value' means (other than losses) that no net power is consumed at the generator, and no net power is delivered to the load.

Zero average value means that 'reactive power' does not on average deliver energy to a load.

-Jon

voltage source, reactor L, capacitor in parallel C
charge the system
isolate source
what happens between L and C?

 

[Ingenieur]

We need to have only one definition.

Those aren't definitions. Those are physics theorems or results.

As I've pointed out, Q is not the power over a 1/4 cycle. For a purely reactive power Q with sinusoidal current and voltage, the average power transfer over a standard 1/4 cycle is 2Q/pi. Which is one good reason not to call Q power, since it is off by a factor of 2/pi (in the purely reactive case, for a standard 1/4 cycle).

I take 'rate' to mean a signed sum. If you prefer, we can change the definition of power to 'the net time rate of transfer of energy' to make that more explicit.

Cheers, Wayne

Not when describing multiple phenomena

no, they are mathematical definitions
your 'rate of energy transfer' is not valid
unless the ebergy expends work P = work/time
an LC ckt transfers energy but does no work
it is reactive power which some belive is not a 'power'

you did not define it as such
you said time
gavebno interval or mandate 1 cycle
be clear and precise when fishing and posturing for the answer you want
even though it is wrong lol

every math construct has 'sign'
but you want a word definition
please pick one
again, not energy but work
rate = qty of something / unit time
if a volley ball is hit over a net an even amount of times does work= 0 since it ends up where it started, ie, no net displacement
how about a car driven in a circle?
it does 1 cycle, no net displacement, hence no work?
Lol

In the context of a mechanical system or electrical power system?

 

[wwhitney]

again, not energy but work

Your proposed definition of "power = work/time" fails for something as simple as an electric resistance heater. By that definition, the heater is using no power, as it is doing no work.

A definition in terms of energy transfer or conversion is broader and more appropriate for electrical engineering.

Cheers, Wayne

 

[Ingenieur]

Your proposed definition of "power = work/time" fails for something as simple as an electric resistance heater. By that definition, the heater is using no power, as it is doing no work.

A definition in terms of energy transfer or conversion is broader and more appropriate for electrical engineering.

Cheers, Wayne

Wrong
heating the air x deg in t sec is work/time
moving it from one state to another takes work and time
whether I^2R or friction like rubbing hands or a brake disc

energy transfer is just plain wrong

 

[wwhitney]

heating the air x deg in t sec is work/time

I'm not aware of a definition of work that includes heating. What is your definition of work, and how does it differ from energy?

Cheers, Wayne

 

[Ingenieur]

I'm not aware of a definition of work that includes heating. What is your definition of work, and how does it differ from energy?

Cheers, Wayne

First Law of Thermodynamics:

Esys = q + w

delta E system change in energy
q heat added/removed
work performed/absorbed

When heat enters a system it can increase the temperature of the system or it can do work.

q =

E_sys - w

​

a book on a shelf possesses energy but does not work
remove the shelf and the book performs work

 

[GoldDigger]

First Law of Thermodynamics:

Esys = q + w

delta E system change in energy
q heat added/removed
work performed/absorbed

When heat enters a system it can increase the temperature of the system or it can do work.

q =

E_sys - w

​

a book on a shelf possesses energy but does not work
remove the shelf and the book performs work

That work done by heat entering a system is typically expansion of a gas against a piston, which is real mechanical work. If you add heat to a closed system kept at constant volume the heat can only raise the temperature of the working fluid (gas).
When a resistor heats air or water using electric power, typically no mechanical work is done, and the textbooks do not consider the increase in temperature as work. It is an increase in the thermal energy of the destination system. Note also that most thermodynamic texts choose, IMHO wisely based on their definitions, not to call a rate of transfer of heat energy "power".

Working with electricity and both heaters and motors to perform useful tasks (not necessarily useful work) seems to be easier to describe using a more flexible definition of power.

 

[Ingenieur]

That work done by heat entering a system is typically expansion of a gas against a piston, which is real mechanical work. If you add heat to a closed system kept at constant volume the heat can only raise the temperature of the working fluid (gas).
When a resistor heats air or water using electric power, typically no mechanical work is done, and the textbooks do not consider the increase in temperature as work. It is an increase in the thermal energy of the destination system. Note also that most thermodynamic texts choose, IMHO wisely based on their definitions, not to call a rate of transfer of heat energy "power".

Working with electricity and both heaters and motors to perform useful tasks (not necessarily useful work) seems to be easier to describe using a more flexible definition of power.

we are discussing a closed system
a combination will be done, q and w
temp will rise and pressure will be exerted to expand a volume
the ratio of q/w is not of consequence
but if it could be measured the sum would = energy added
if time is considered then the 'power'

 

[GoldDigger]

...
the ratio of q/w is not of consequence....

So you are saying that if I provide electrical energy at the rate of 1kWh per hour and all of it becomes heat, with .9 of it raising temperature while .1 of it does mechanical work somewhere, the entire amount of energy is counted as power because some of it does work?
But if I provide the same amount of energy and all it does is raise the temperature of the target system I am not providing power because it is not doing work?

What I am trying to avoid is the IMHO unnecessary and very inconvenient restrictive definition of the word "power" as the rate of doing work and instead expanding it to include the rate of transferring energy in any form across a boundary.
Do you disapprove of such an expanded definition?

If you keep the strict definition, you will have problems with power used to store energy in, for example, a magnetic field. There is without a doubt energy transfer. But very few people would call the creation of EM potential energy by creating a magnetic field "work". And electric heaters will not use any power no matter what I*V*cos(theta) might be.

 

[Phil Corso]

a book on a shelf possesses energy but does not work
remove the shelf and the book performs work

The book doen't do the work. The human/animal/device that moved the book-mover does!

Phil

 

[GoldDigger]

The book doen't do the work. The human/animal/device that moved the book-mover does!

Phil

:thumbsup:
Gravity does work on the book. While the book is free falling, the book is not doing any work. When the book finally lands, it may do work. And at the same time create heat via an inelastic collision.

 

[Ingenieur]

So you are saying that if I provide electrical energy at the rate of 1kWh per hour and all of it becomes heat, with .9 of it raising temperature while .1 of it does mechanical work somewhere, the entire amount of energy is counted as power because some of it does work?
But if I provide the same amount of energy and all it does is raise the temperature of the target system I am not providing power because it is not doing work?

What I am trying to avoid is the IMHO unnecessary and very inconvenient restrictive definition of the word "power" as the rate of doing work and instead expanding it to include the rate of transferring energy in any form across a boundary.
Do you disapprove of such an expanded definition?

If you keep the strict definition, you will have problems with power used to store energy in, for example, a magnetic field. There is without a doubt energy transfer. But very few people would call the creation of EM potential energy by creating a magnetic field "work". And electric heaters will not use any power no matter what I*V*cos(theta) might be.

Now you are introducing eff into the mix
'waste' heat vs work

an energy transfer is not always work

in the case of R heating raising the air temp IS the work

 

[Ingenieur]

The book doen't do the work. The human/animal/device that moved the book-mover does!

Phil

Wrong

as the book falls it generates a force = mg
that force (gravity) moves the book x dist
F x d = W
if it lands on a spring it will compress it, ie, do work
the book possesses potential then kinetic energy

 

[GoldDigger]

Now you are introducing eff into the mix
'waste' heat vs work

an energy transfer is not always work

in the case of R heating raising the air temp IS the work

1. An energy transfer is not always work, but some are arguing that unless an energy transfer does work, there cannot be "power". That I take issue with.
2. NO, NO, NO. Raising the temperature of a gas at constant volume is not work. It is something useful, rather than waste heat, but that does not make it work.

 

[Ingenieur]

1. An energy transfer is not always work, but some are arguing that unless an energy transfer does work, there cannot be "power". That I take issue with.

2. NO, NO, NO. Raising the temperature of a gas at constant volume is not work. It is something useful, rather than waste heat, but that does not make it work.

1 I agree with you, ie, reactive power IS power, just not active power

2 yes, yes, yes, it exerts a pressure which induces stress in the walls
the walls will 'give' or move, I never said constant volume
in a constant vol all q no w