0
\$\begingroup\$

Yesterday I saw a reference to Watt's law being used for the power formula P = V·I.

It's been over 40 years since I learned my basic electrical engineering fundamentals and this is the first time that I recall seeing the phrase Watt's law used to represent this formula.

Doing some searching around on the web I haven't found where this term first showed up, but I have seen plenty of people on both sides of the flag claiming this is a proper law and others saying it isn't.

I have done a fair bit of reading on Watt and his accomplishments in steam and mechanical power, but I don't recall that he ever did anything with electricity.

While the watt has since become a standardized unit of work (joules per second), I don't recall reading anything about Watt himself showing that volts times amperes equals power.

So, I'm really interested in finding out when/where this usage originated.

Updated in response to comments, here are a couple examples of where I saw "Watt's law" used: https://youtu.be/KJ5zCY5EBeQ https://www.electrical4u.com/watts-law/

One prominent stanford.edu site; https://ccrma.stanford.edu/mediawiki/index.php?title=Introduction_to_Electronics&oldid=23903 had a Wiki entry that is still showing up in Google, but the actual Wiki itself has just been updated today, possibly as a result of my email to them this weekend.

The previous text which is still shown in the Google search results is:

"Watt's Law (P = VI)

Calculating the power dissipated by a circuit element is simple. Often much of this power is converted into heat, so by thinking about the power dissipated by circuit elements, you can make sure that they don't burn up or catch on fire! (When a musical instrument catches on fire, the way in which the performer interacts with it usually changes.) Watt's Law states that: Power (in Watts) = Voltage (in Volts) x Current (in Amps) P = V I Combining with Ohm's law we get two other useful forms: P = VV / R and P = II*R Power is a measurement of the amount of work that can be done with the circuit, such as turning a motor or lighiting a light bulb. Consider a 100Watt light bulb in your home. We know the voltage applied to the bulb is normally 110V or 220V so we can calculate the current consumed as follows: I = P/V = 100W / 110V = 0.91 Amps or I = P/V = 100W / 220V = 0.45 Amps So you can see why using a 60W light buld is more economical. Your electric company normally charges you for your usage in Killo-Watt Hours (kWh). One kWh is the amount of energy necessary to do 1000 Watts of energy for one hour - in other words to keep 10 100W light bulbs shining for one hour."

\$\endgroup\$
5
  • 2
    \$\begingroup\$ A cite to where you saw it might be useful. \$\endgroup\$ Commented Jul 24, 2022 at 21:06
  • 1
    \$\begingroup\$ physics.info/electric-power \$\endgroup\$ Commented Jul 24, 2022 at 22:21
  • \$\begingroup\$ Thanks, I checked out that cite but don't see where they use "Watt's law". It does have Ohm's law but doesn't refer to P = V I as "Watt's law" \$\endgroup\$ Commented Jul 25, 2022 at 21:10
  • \$\begingroup\$ You might have better luck at history of science stack exchange. "User Math Keeps Me Busy - History of Science and Mathematics Stack Exchange" hsm.stackexchange.com \$\endgroup\$ Commented Jul 25, 2022 at 21:46
  • \$\begingroup\$ Village is a measure of energy per unit charge and current is a measure of charge per unit time, so the product is energy per unit time, that is, power. Unfortunately this sheds no light on the history behind the term Watt's Law. \$\endgroup\$ Commented Jul 25, 2022 at 23:17

1 Answer 1

Reset to default
3
\$\begingroup\$

From the link cited by Bruce Abbott.

The other unit I would suggest adding to the list is that of power. The power conveyed by a current of an ampère through the difference of potential of a volt is the unit consistent with the practical system. It might be ppropriately called a watt, in honour of that master mind in mechanical science, James Watt. He it was who first had a clear physical conception of power, and gave a rational method of measuring it. A watt, then, expresses the rate of an ampère multiplied by a volt, whilst a horsepower is 746 watts, and a cheval de vapeur 735.

Carl Wilhelm Siemens, 1882

cited from Report of the British Association for the Advancement of Science, page 6

\$\endgroup\$
3
  • 2
    \$\begingroup\$ Again as noted in the comment for Bruce's link, neither that document nor the quoted text contains a definition of P = V I being called "Watt's law". \$\endgroup\$ Commented Jul 25, 2022 at 21:41
  • 1
    \$\begingroup\$ I do not think Ohm called his formula at the outset, Ohm's Law, but this is clearly the definition. \$\endgroup\$ Commented Jul 25, 2022 at 22:35
  • \$\begingroup\$ The question is not related to Ohm's law, but rather the attribution of the term "Watt's law" to the equation P = V I. I have no concerns or questions about what we now call Ohm's law - or the fact that it is called Ohm's law. \$\endgroup\$ Commented Jul 26, 2022 at 22:13

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.