Dr-Fix-It Defines Power. The Definition of Work and PowerThe 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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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:
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:
As an example, we can now see that a Horsepower motor plugged into a 120 Volt circuit would (ideally) draw:
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!
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!
