Stupid question about electrical "power"

On 4 Oct 2006 22:41:41 -0700 string snipped-for-privacy@hotmail.com wrote: | Hi everybody. I have a question about "power" | | What is the significance of "power" in electronics? I know that its | the voltage x the current, but how is that significant in any way? | | For example, when I buy a 30 watt light or my speaker is 15 watts or | something, what does that really mean and is that wattage for my | speaker electrical wattage?

Power over a given period of time is energy. That's work done. So power is effectively a measure of the rate of work. More power means working faster.

Physics has a lot of formulas where things get squared. Power is the result of one of those. You can think of current as the rate of electrons passing a given point (back and forth for AC). It might seen intuitive to think that if the number of electrons is twice as much, you'd have twice the power. But that is not true. When you have twice the current, you actually have 4 times the power. Power is in proportion to the square of the current.

| Also, I read that circuits are designed for "maxiumun power" and I've | heard of power losses (that turn to heat or something). Why are they | designed for maximum power? also what is a power loss in this meaning | (its not voltage x current loss, right?)?

This varies. Commonly heat is a limiting factor. But in many cases a higehr power results in a higher voltage on some part and that damages it.

You really need to read and understand an elementary physics book in order for your questions to be answered meaningfully in the limited space available in a post like this. You must understand the conservation of energy (and matter). That is difficult if you do not know what energy is.

Even in these days of $3 gasoline, energy is usually cheap compared to the other costs of running electrical equipment. To minimize cost, circuits are designed to give maximum possible power for the size of the components.

Bill

-- Fermez le Bush

without power no work gets done, this is true in electronics as well as any other endeavor.

in terms of speakers, not much. the speaker rating issues can (and are) hotly debated with no resolution in sight. in terms of lamps, it approximately how much power will be consumed when operated at the rated voltage. in addition its a relative indication of how much light will be produced for a give type of lamp. (i.e. tungsten incandescent)

all wattage is electrical by definition :)

all circuits will have a maximum power handling ability. this is the point just before smoke and or flames appear and/or the circuit ceases to function.

it would be better to say: a prudent engineer designs circuits with an adequate safety margin; a production engineer finds ways to reduce the safety margin until the product is cost effective to produce and market.

loss the amount of power that dosent end up doing useful work.

does you high school offer electricity as a subject? mine did.

The number printed on a speaker box with "watts" after it is related to the price you will pay for the box. This may have an incidental effect on the actual power-handling capability of the speaker, but the primary purpose of printing a bigger number on the box is to get you to pay more money for the speaker. Similarly, any number seen allegedly rating to "power", "watts" or "horsepower" on a consumer product such as power tools, air compressors, etc. is a fictional value dreamed up by the marketing department, on the theory that you will pay more for a product that has a bigger number printed on it. There's a technical term in electrical engineering called "imaginary power" but this is not related.

Bill

Another example of bogus power claims would be in the marketing promotions for vacuum cleaners and treadmills. The weasel word that is used is "peak power" or "peak horsepower". This is often vastly higher than the actual rated average power draw of the motor under normal conditions.

I have a treadmill that claims it has a 3.5 peak horsepower motor inside it. Sure maybe if it is under locked rotor conditions with smoke coming out of it.

Beachcomber

On Sat, 07 Oct 2006 19:52:45 GMT, snipped-for-privacy@notreal.none (Beachcomber) Gave us:

More likely during a specific non-zero RPM window.

Under locked rotor, the power mechanical power produced is less than the electrical power going. That is, you now have a ZERO horsepower motor.

Bill

-- Fermez le Bush

Well, not locked rotor, but definitely some sort of short-term rating. The other bogus claim used a lot is "xx amp motor". Sounds impressive to the uninitiated to have a "12 amp motor" for your vacuum cleaner, but it doesn't really mean very much in the way of motor power rating (or cleaning ability). Just a lot of hype.

daestrom

On Sat, 07 Oct 2006 21:46:14 GMT, "daestrom" Gave us:

Not true. When those product maker declare something like this they assume that the consumer knows what his locality's standard line voltage is.

To put it simply more IS more, and they do have a lot of regulations regarding what they can declare, and what, if any standards such declarations must conform to.

Bill was a bit overzealous, if one asks me.

I know that a 1/2" impact wrench can perform a higher final torque level than a 3/8" impact wrench or a 1/4" version.

I would almost bet that ANY 24V battery operated drill will torque (and yes that's all that matters) higher than a 12Volt version will.

Air compressors are usually designated with proper terms as well. They have a canister size, and a rating on the compressor itself, which will be directly related to how fast it can bring the surge tank up to final fill pressure, as well as what that top pressure can be.

| The number printed on a speaker box with "watts" after it is related to | the price you will pay for the box. This may have an incidental effect | on the actual power-handling capability of the speaker, but the primary | purpose of printing a bigger number on the box is to get you to pay more | money for the speaker. Similarly, any number seen allegedly rating to | "power", "watts" or "horsepower" on a consumer product such as power | tools, air compressors, etc. is a fictional value dreamed up by the | marketing department, on the theory that you will pay more for a product | that has a bigger number printed on it. There's a technical term in | electrical engineering called "imaginary power" but this is not related.

I think they call it "power lunch".

I used think that the "rated" HP that is shown on the box, is the product of the locked rotor torque and the no-load speed. This makes little sense but, hey, it is advertising. More likely it is the power corresponding to the peak torque point- which generally not where one wants a motor to be running unless there is a wish to blow out magic smoke. The fact that one can pass through but not operate at this point is not of importance.

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Depends on the design. Torque depends on current while speed is voltage dependent. This is fundamental. If you have a 24V motor you can get a given torque at a higher speed - or get longer battery life at a given speed and torque.

Please don't tell me that Ohms law applies to a motor because it doesn't.

| On 10/7/06 12:52 PM, in article snipped-for-privacy@news.verizon.net, | "Beachcomber" wrote: | |> I have a treadmill that claims it has a 3.5 peak horsepower motor |> inside it. Sure maybe if it is under locked rotor conditions with |> smoke coming out of it. | | Under locked rotor, the power mechanical power produced is less than the | electrical power going. That is, you now have a ZERO horsepower motor.

However, some power is still taken and dissipated as heat.

MPF marketing power fugure.

here is one that band asked me for help setting up a crossover. look up Peavey QW 4. it claims: "Power handling: 2800 watts program, 5600 watts peak"

it features "exclusive Quadratic Throat WaveguideT horn, 44XTT titanium diaphragm compression driver "

On 8 Oct 2006 01:10:16 GMT, snipped-for-privacy@ipal.net Gave us:

Yes, on the power SOURCE side, but NONE is provided on the WORK side.