this post was submitted on 13 Mar 2024
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Because in^2 is generally said "square inches."
So it's "pounds per square inch."
Sometimes "per" will get its own letter, like in PPM - parts per million - and sometimes it's left off, as in PSI.
Thanks, friend :)
I know how it comes to be, I just think it's stupid.
For example, kW times h is not the same as kW per hour. That's why kWh means kilowatt times hour.
If I wrote ms to denote meters per second that would create massive confusion.
That is an excellent point. Yeah, PSI would totally read as pounds times square inches which would be something else entirely. Adding in the extra P would fix it, too. PPSI. Suppose it's another thing that people just have to get used to, haha.
I would even say, it reads as pounds times seconds times inch
Or pikoseconds times inch
Or pikoseconds times square root of -1 but now I'm being silly
Well, I don't have to get used to it, but some people seam to handle it well.
Eh, it’s pretty unambiguous. kW/hour is a pretty useless unit. Power surges may be measured in kW/s or something, but they don’t really have any impact over a span of more than a couple seconds.
Likewise, pounds times square inches is equivalent go kg*m^3/s^2 in SI units - which also seems pretty meaningless. Maybe there is a use for it?
What really grinds my gears is that pounds are a unit of mass, not force. The “pounds” in “pounds per square inch” is short for “pounds-force“. It’s the force of one pound of mass accelerating at 1g. Preposterous.
Wait wait Wait, can you give me more on this kWh thing? I thought I understood this already.
A single kW is a unit of power, literally 1000 watts.
A kWh is a unit of energy, as in stored or delivered. Draw 500 watts for 2 hours? That's a kWh. Or have a battery that can hold 1 kWh, then assuming 100% efficiency you could draw 1000 watts from it for an hour before it was empty.
All of this is kW times hour, I would say? But in my mind I would interchangeably say per hour as well, they feel the same.
Obviously I'm wrong, but I'd like to know why lol
If you use exactly 20 kW for an hour, it will translate to 20 kWh. But if your power usage varies over time, you can't keep track of it so simple. It's just how it is.
The unit is really watt [W] and the Greek prefix kilo (k) for 1000. This way it's fast and easy to convert to different scales (like Mega, Giga etc) for comparing numbers
A watt is a derived unit for a rate of change, an amount of energy used in a unit of time, so P = E / t. A kW per hour would be a rate divided by time, or E / t^2, resulting in another rate.
More colloquially, think of watts/power by analogy to another rate, that of speed. Moving at a speed of 100kph for 3 hours results in 300 speed-hours of distance. Saying 100 kilometers per hour per 3 hours sounds awkward, but is actually a weird way to say acceleration, a rate of change of speed. (And probably a hint to get your car serviced.)
Anyway, the key is to think of a kilowatt as a rate, not a quantity.
Thanks, I guess I still don't understand though.
I see now that watts and therefore kW are rates. So it's silly to add another rate to the end by appending "per hour". But what is the time component of the watt calculation? To me it's essentially instantaneous, even if that's wrong. Even if that breaks the math, it's still essentially true on a macro scale. And if it's instantaneous, or even just close like microseconds, then it doesn't hurt to apend another rate to the end, does it?
So why not use it? Batteries come with capacities rated in Wh and kWh, and it weirdly still makes sense to me because of my usage rate per hour example in my last comment.
And if we shouldn't use it, then what should we use?
Is this problem we're discussing, one that only occurs if you try to get really accurate with the numbers and times? Because for my uses it's always seemed to work well enough.
Not being argumentative, just trying to learn, thanks
Oh, hey, Jerboa is not so good about updating the Inbox tally...
I was responding to your question about kW per hour, and I was going for the intuitive sense of why that's not right. The more "it's just so" reason is that the math just doesn't work, since the word "per" signifies division. So if we discharge a battery at a rate of 100 watts for 3 hours, that's 100W * 3 hours, or 300 Wh used. If we say 100 watts per hour for three hours, that's 100W / 1 hour * 3 hours. The hours cancel, and the result is 300 watts, which is a rate.
It's totally confusing, I know, because people often use "watts" and "watt-hours" interchangeably, but they're as different as speed and position.
Anyway, the watt is a derived unit in SI, and it's equivalent to kg·m^2^ / s^3^. The per-unit-time is hidden when you write it as a watt, but clearly there when you write it in terms of base units. Of course, the joule is kg·m^2^ / s^2^, so energy also has time in the denominator, and I guess could technically also be a rate, but understanding that is way above my pay grade. 😀
And why?
Because we said so.