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I'm revising some electrical curriculum for a technical training program. In the curriculum students have to calculate values using Ohm's law and the equation Power = Current * Voltage (or P = IV).

Some of my coworkers, who do not have science backgrounds, have started calling the equation P = IV "Watt's Law." When I told my co-worker it was appropriate to call P = IV the power equation she told me I was crazy and "everyone is calling it Watt's Law" according to her internet research.

Am I going crazy? I've only every heard P= IV referred to as the power equation (as it applies to circuits). I've never used the term "Watts Law" in the 10+ years I've been studying and teaching physics. An if I were to call something Watts law it would be in reference to content in an energy unit not an electricity unit.

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2 Answers 2

I would stick with the power equation. "Watt's Law", while that may be what "everyone is calling it", is not the proper term.

From wiki:

A scientific law is a statement based on repeated experimental observation that describes some aspect of the world. A scientific law always applies under the same conditions, and implies that there is a causal relationship involving its elements.

While the equation may use Watts as the unit of power, it includes electrical and thermal work - James Watt was a scientist who aided the steam engine, not electricity.

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While I agree in essence, I don't see how your Wikipedia(?) quote is relevant here... – David Z Jul 25 '12 at 22:14

Historically speaking, calling the electrical power formula "Watt's law" is wrong. Again from Wikipedia, the origins of the unit of power now known as a Watt were due to James Watt's work in the field of mechanical power (specifically steam engines), predating most work on quantifying electrical energy.

The formal definition of a watt is in mechanical terms, and is a rate of energy expenditure (power) equal to one Joule per second, thus to one Newton-meter per second and thus to the power required to maintain acceleration of a 1kg mass of 1m/s2 (or to maintain velocity of 1m/s of any mass given an opposing force of 1N). Watt's actual work on power was done in foot-pounds per minute, developing the concept of "horsepower" by empirical analysis of a horse turning a mill wheel. 1hp ~= 33,000 foot-pounds per minute, which when all units are converted to SI is approximately 746 watts.

In fact, the electrical units of measure are defined in terms of mechanical units, not the other way around (because those concepts were known and defined before electricity was well-understood). The ampere is defined as the current (rate of electron flow) which, when passing through two parallel conductors of arbitrary length placed 1 meter apart in a vacuum, induces a force of 2 ten-millionths (2*10-7) of a Newton of electromagnetic force between them, thus equating force and current. A volt is then formally defined as the potential inherent in a circuit carrying one ampere of current exerting one watt of power. So, the original form of the relationship is V=W/A.

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