# Thread: Spin down harddrives. Worth it?

1. EDIT: The P=I*V equation is only true instantaneously. For time-varying potentials or currents, the relationship must be treated as a differential one, and integrated over time.
It happens to work for alternating currents, too, if you take the RMS of the voltage.

2. Originally Posted by cyberfish
Well, SI uses V. Wikipedia page has no mention of "U", either.
This is confusing, because I found this (in local language):
Appendix:SI-enheter - Wiktionary
Clearly, it says the symbol is U, and the unit is V.
But if you look at any USA page, it says the symbol is V.

U usually represents potential energy (unit J), and voltage the potential energy per unit charge (V = J/C), so you might have them confused.
I know this: here voltage's symbol is U. And USA's symbol is V.
This is how we are taught.

Watts is a unit of power (rate of energy transfer, J/s).

Energy is measured in joules.
I remember this part from the physics

3. The way we learned it, both the symbol AND the unit are V.

4. I think Elysia has got this one right. Obviously you'd have to sit in on a few physics classes in a few foreign countries to be sure, but...

First, I learned it as P=I*V and V=I*R. Not sure why you would want to use another symbol for Volts, but whatever. (What does the U stand for, Elysia?) What Elysia seems to be talking about is not the SI unit for Volt, which everybody uses V for, but rather the symbol representing it in the equation. We usually represent mass with a m, for example, but the unit is kg. F and N for force (Newtons) is the same idea.

Go look at the Wikipedia pages for Joule's Laws, in several languages.
The Italian, Spanish, Chinese pages seem to use V, whereas the Polish, French, German and Portugese pages use U. (The Ukranian page seems to use altogether different letters, but it might just be me having trouble reading it...) There also seems to be some use of the letter Q for power as well.

5. Yes, we agree to disagree .

SI only standardizes units, so people can still use whatever symbols they want in equations.

6. Originally Posted by Cactus_Hugger
(What does the U stand for, Elysia?)
U is U, the symbol for voltage.

7. I learned it as V = IR when I took a class to become an electronic technician for the electrical systems in CAT mining and construction equipment. But as has been said this is an instantaneous formula and cannot readily be applied to all situations. In situations where conditions are consistent (such as voltage, resistance, and amperage) then it is a good representation of what is going on in the circuit or at least what should be going on in the circuit.

8. Originally Posted by cyberfish
We use V and A for units, V and I for constant voltages and currents, v and i (lowercase) for time-varying voltages and currents (first year university physics).

Maybe they use different things on the east coast (French influence?)
Maybe the difference is between the Physics textbooks and the Electronics textbooks then? Now that I think about it, I think I do remember P = I*V in my physics class; but the mnemonic that my electronics teacher used was "The Eagle flies above the Indian leaning against the Rock" i.e. E = I*R or I = E/R ...

9. Hmm I have never heard of that, but I have never taken any electronics course, either. My first circuit analysis course is in 3 months .

10. Originally Posted by cpjust
Maybe the difference is between the Physics textbooks and the Electronics textbooks then? Now that I think about it, I think I do remember P = I*V in my physics class; but the mnemonic that my electronics teacher used was "The Eagle flies above the Indian leaning against the Rock" i.e. E = I*R or I = E/R ...
In electrical engineering, you have to distinguish between voltage and electromotive "force" although they are both measured in volts.

Imagine a closed loop of wire with an increasing magnetic field through the center. This will cause a current to flow in the loop. But you can't talk about the voltage change around the loop, since by definition, this is zero (one point cannot have two voltages). And yet, current is flowing. So we have this weird term "electromotive force" to talk about the voltage that seems to be present, but isn't actually there.

So I think that's why they use E instead of V.