Over the years I have made tens if not hundreds of efforts to clarify the meaning of the term “power” as it applies to the capabilities of ultrasonic cleaning systems.  Despite these efforts, a functional means of rating the capability of ultrasonic cleaning systems is still undefined.  There are several conventions and declarations.  But in the end, it’s all a matter of interpretation and understanding of terms and units of measure.  With this in mind, allow me to make another attempt to put things in their place.

The foundation is a basic understanding of the terms involved.  To start with, work is the result of expending energy.  From the dictionary – – work is the “exertion or effort directed to produce or accomplish something; labor; toil.”    It takes the application of energy (work) to move an object from one place to another.  In this case the work is measured in Joules (a force applied over a distance).  The application of thermal energy (work) results in heat.  Heat is measured in BTU’s.   Joule and BTU provide a units to quantifiably measure the result an accumulated amount of work.  Neither expresses the period of time required to accomplish the work.  An object can be moved from one place to another in a minute or in a hour but the measurement of the result of the work required in Joules remains the same despite the difference in the time required.  A mass can be heated from one temperature to another (the result of applied thermal energy) in an hour or a day but the resulting total accumulated BTUs remains the same.

Work (energy) can be expressed in many different units.  The following list is from a typical conversion application –



This is an interesting list!  To better understand it, let’s first remove all of the instances where there is just a simple order of magnitude conversion involved.  The list gets a lot shorter.



Examining the remaining list we see that the units fall into

  • Changing the energy level of electrons or molecules.  This group would include BTU, calorie, therm and electron volt.
  • Moving something from one place to another or, basically, a force exerted over a distance of travel.  This would cover dyne-centimeter, erg, gram force-centimeter, joule, newton-meter, ounce force-inch, pound force-foot, pound force-inch.  Note that they are really all the same thing with different units of measure.

All of the above accumulate the amount of work expended to achieve a given result.  But there is something missing.  Specifically, that would be how long it took to do it.  This job is relegated to POWER!  Power is the rate of doing work.

Paring our list a bit further reveals that there are two units of work that depend on time.

In these two, the total work is expressed as the product of the rate of expenditure of energy times the length of time the power is applied.  The term horsepower-hour (or a conversion thereof) is usually applied in the case of mechanical work while watt-hour is more commonly used in measuring electrical work although the two are convertible.  The rate of expenditure of energy can be witnessed and quantified but may not result in total work that can be easily quantified in the real world.  To explain, let’s use light as an example.  We commonly measure the intensity of light in lumens or candlepower (convertible units).  The terms “lumens” and “candlepower” measure the rate of production of photons as the result of an expenditure of energy to produce light.  But I’m not sure how you accumulate photons to determine the total number produced in a particular time span.  The total number of photons in a given period of time must be directly related to the intensity or rate of them being produced at the source.  We can’t capture photons in a bottle but we can measure the rate at which they are produced.  From there, they dissipate on their own.  The theories on what happens to them is pretty interesting.

Note – If you have the time and interest, check out what Stephen Hawking has to say about the travels of photons.

Another, and my favorite analogy, is the road trip.  The speed you travel (miles per hour) is analogous to watts.  The distance you travel is analogous to the amount of work done.  Using watts in the context of the amount of work done is like saying that the distance from point A to point B (work) is equal to 50 (or whatever) miles per hour.  Miles per hour does not, by itself, give you the information you need to calculate the distance traveled or total work done.  You still get from point A to point B no matter if you travel at 10 miles per hour or 100 miles per hour!  But it takes a lot longer if you stop to eat lunch.

The RATE of doing work is POWER.  Power is expressed in work per unit of time and is measured in terms like horsepower and watts which have a time value attached.  The RATE of expenditure need not (and seldom is) totally consistent!  Watts, therefore, applies only at an instant in time.

One watt, for example, is equal to one Joule of work per second.  In order to know how much work was done you must first know the rate of doing work and then multiply it (if it is consistent) or integrate it (if there are variations) by the time it is applied.  Watts is a measure of instantaneous power.  If power is uniform or can be averaged over time, watts can be used as a comparison of the overall energy produced if the time is specified.  But if power is not uniform over time as in case of a resistance connected to a source of alternating current, it can not!

 – JF –


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