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Dr-Fix-It Defines Power. The Definition of Work and Power
The Definitions of Work and Power in the English and Metric Systems. Dr-Fix-It Defines the Term Power and provides practical examples as well as rules of thumb.
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Power is defined as the rate of speed which a specific work is accomplished. To understand Power, it is first necessary to define Work.

Work is done by applying a constant force to an object, thus moving it. If a Pound of force is needed to move an object one Foot, the work done is one Foot-Pound of work. Similarly, in the Metric System, A Newton-Meter (Also called a Joule) of work is performed by applying a constant force of one newton which moves a body a distance of one meter.

Time is not involved in the definition of work. The amount of work performed is the same, say, in lifting a weight a specific vertical height regardless if the action of the lifting takes a minute or an hour. However, the amount of time needed to perform that lifting work leads to the concept of POWER.

The measurement of power, then, involves a description of the work as a ratio to the time it took to perform. This is a scientific definition of what most people understand instinctively: One needs more "power" to go faster or to do more in the same amount of time.

In the British system, the unit of power is, simply, the Foot-Pound/Second (Work divided by Time). In the Metric System, a Newton-Meter/Second (Work divided by Time) is also called Joule/Second or most familiarly, a Watt.

Both the Foot-Pound/Second and the Watt are quite small units of power. Larger units of work are more commonly used. In the British system, a Horsepower is defined as 33000 Foot-Pounds of work in performed in one Minute (the same as 550 Foot-Pounds per Second) . In the Metric System, a Kilowatt is simply 1000 watts.

By just converting Pounds to Newtons (Force)and Feet To Meters(Distance) it can be easily calculated that:

1.000 Horsepower = 0.746 Kilowatt

A good rule of thumb is that 1 Horsepower is about 3/4 of a Kilowatt.

So What? It turns out to be handy when one can apply a form of Ohm's Law:

Watts = Amps x Volts

As an example, we can now see that a Horsepower motor plugged into a 120 Volt circuit would (ideally) draw:

0.746 kW= 746 W = ? Amps x 120
? Amps = 746 / 120
Amps = 6.22 Amps

In reality, the motor will draw a little more current than 6.22 Amps because of heat loss and other natural inefficiencies. Usually motors are within the 75% to 90 % efficiency range so, in the shop, the above motor would probably really pull about 6.22 amps / .75 (efficiency estimate) = 8.29 Amps.

Nameplate missing on a motor? Need to know what size it is? Measure Amps, Volts and do the math!

Oddly Enough...

"Power Companies" don't sell "power", they sell energy  (which, in this document, we are calling "Work").  Granted, "Power Company" has a better ring to it than "Work Company" but it is not exactly correct... To charge for electric consumption, Utilities use the quantity KiloWattHour which is 1 kiloWatt multiplied by 1 Hour. This is Power multiplied by time. From above:
Power x Time = ( Work / Time) x Time
Time cancels out
Power x Time = Work

KiloWattHour = 1000 Watts x 3600 Seconds
             =1000(Newton Meter / second) x 3600 seconds
              =1000 x 3600 Newton Meter
              = 3,600,000 Newton Meters (Joules)

The best name for an electric utility should be a "Joule Company" or a "Newton Meter Company" or a "Foot Pound Company"

Foot Pounds!   Get your fresh Foot Pounds here!   Foot Pounds!

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