Watt?! No Way!

Now that we are aware of a Ohm’s Law, let’s look at another common value, the Watt. The watt (W) is how we measure power, but what is power and how do we measure it?

Power is something doing work over a duration of time. In the case of electricity, that “something” is electrons and it is overcoming resistance in a circuit. The unit we attribute to power is a joule (J).

So if we wanted to express power (W) and work (J) in mathematical terms we could do so as follows:

J = W Seconds

W = J / Seconds

If we wanted to build upon our knowledge of Ohm’s Law and work watts into the equation we could do it like this:

W = V I

V = W I

I = W V

Now lets see how we can use this in a day to day situation. This example will be using AC.

Pretend we are in North America, and we are supplied with a 20 amp circuit. We have with us a 50W guitar amplifier and we would like to figure out how much current it will draw from the circuit. how could we figure this out?


Well for starters, the standard outlet in North America is 120V, and we are told that we have a guitar amplifier that draws 50W of power. We could then use the formula I=W/V to find the answer.

I = 50/ 120V

I = 0.42 (0.416 repeating)

The guitar amplifier uses about 0.42 amps of current to be used. It is important to keep track of how many amps are available for use on a single circuit because if you are not, you may pop a breaker, thus shutting off power by drawing too many amps.

So if we are using a 20 amp circuit with the 50W amplifier, we will have about 19.6 amps left on the circuit. Plenty of power for the rest of the band!

That will be it for this post, if you have any questions or comments, feel free to leave them in the comments below or send me an email at teachingmyselfblog@gmail.com



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