If 1 watt is 1 joule per second then, honestly, what are we doing with watt-hours?
Why can’t battery capacity be described in joules? And then charge and discharge being a function of voltage and current, could be represented in joules per unit time. Instead its watt-hours for capacity, watts for flow rate.
Watt-hours… that’s joules / seconds * hours? This is cursed.
I believe it's just a matter of intuitively useful units. There's simply too many seconds in a day for people to have an immediate grasp on the quantity. If you're using a space heater or thinking about how much power your fridge uses kilowatt hours is an easy unit to intuit. If you know you have a battery backup with 5 kilowatt hours of capacity and your fridge averages 500 watts then you've got 10 hours. If you convert it all to watt seconds the mental math is harder. And realistically in day to day life most of what we're measuring for sake of our power bill, etc. is stuff that's operating on a timetable of hours or days.
"litres-of-fuel per km-driven" (Volume/Distance) is still fully reductible to an area: litres is still a volume (1 cubic decimeter) and km is still a distance (1x10⁴ dm) Maybe you meant that the other way around? Distance/Volume (as in Miles/gallon) is an Area⁻¹ (Distance⁻²), which is more difficult to imagine in space.
Now, Kg is a measure of mass (or weight, depending on who you are asking), which throws density into the equation, which is proportional to the temperature, which will vary according to where and when the driving takes place. But since the time and place, and hence the temperature is (allegedly) defined when the fuel consumption was tested, the density is a constant, and as such you can leave it out from the relation.
If you car was fueled by a fixed pipe which it travelled along, consuming all the fuel in the sections of the pipe that it moved past but no more, what would the cross section of the pipe be?
> Now, Kg is a measure of mass (or weight, depending on who you are asking), which throws density into the equation, [...]
It's the other way round: chemically how much energy you get from burning your fuel is almost completely a function of mass, not of volume. (And in fact, you aren't burning liquid fuel either, in many engines the fuel gets vaporised before you burn it, thus expanding greatly in volume but keeping the same mass.)
> [...] which throws density into the equation, which is proportional to the temperature [...]
For an ideal gas, sure. But not for liquid fuels.
> "litres-of-fuel per km-driven" (Volume/Distance) is still fully reductible to an area: litres is still a volume (1 cubic decimeter) and km is still a distance (1x10⁴ dm) Maybe you meant that the other way around? Distance/Volume (as in Miles/gallon) is an Area⁻¹ (Distance⁻²), which is more difficult to imagine in space.
I don't think that the reciprocal is a problem. No, what I mean is that you can't cancel fuel with driving. Litres-of-fuel is a different unit than distance-driven ^ 3. Similar to how torque and energy are different physical quantities that you can't cancel willy-nilly, despite their units looking similar.
You might find a physical interpretation for an adventurous cancelling, and that's fine. But that's because you are looking behind the raw unadorned units at the physics, and basing your decision on that.
Units are a very stripped down look at physics. So units working out are necessary for cancelling to make sense, but not sufficient.
Also the UK gallon is different from the US gallon. And the same applies to all the other non-metric fluid measurements such as pints and fluid ounces. Historically the UK gallon was used throughout the former British Empire (Australia, Canada, India, Ireland, Malaysia, New Zealand, South Africa, etc). By contrast, almost nobody ever officially used the US gallon except for the US (and a small handful of highly US-influenced countries such as Liberia).
I use the conversion factor so often that I know it by heart: 1 day = 86400 seconds. I punch that 5-digit integer into a calculator, not an approximation like 8.5e5 (which is the same length, haha).
I'm not sure if I would call it sarcasm, but it's a reference to a popular computer science joke format.
The first time I saw it:
>There are 10 kinds of people in the world, those who understand binary and those who don't.
The joke is that 10 is how you express 2 in base 2.
I think there is another layer to the joke, though; often in mathematics, computer science, algorithms, and software engineering, things get divided into sets, sets get broken down into two sets according to whether some property about the elements is true or false, and this joke echoes that.
It is not more cursed than km/h (1 m/s = 3600 m/h = 3.6 km/h)
Both those units are more convenient than their SI equivalent and their "cursedness" come from the hour/minute/second time division.
If we had decimal time, as it was initially proposed with the metric system, we wouldn't have this problem, but we weren't ready to let go of hours/minute/second.
Yeah. I get this is all kind of silly. I think what trips me up is that a watt doesn’t represent a timeless amount of something the way a meter does. A watt involves a unit amount of time.
Imagine if the distance between you and I was 438 kiloflerp-hours. And to get to you in one hour I have to drive at a speed of 438 kiloflerps. It works, it kinda makes sense. It just feels inconsistent with all the other units I work with.
You're right. If you really want to mess with speed and distance, just rename "nautical mile" to "knot-hour". In fact, that might be a great idea for trolling – it is fewer syllables (4 vs. 2), and aviation pilots definitely use knots for speed, so why not simplify the vocabulary and ditch the unique term "nautical mile" in favor of pairing two existing words?
Another place where the cursed unit of hour crops up is describing the amount of electric charge that you can pull out of a battery (especially rechargeable ones) in terms of millamp-hours (mA⋅h). Note that in actual SI, 1 mA⋅h = 3.6 C (coulombs). Even more cursed is high-capacity lithium-ion USB power banks that are advertised like 10,000 mAh (or even "10K mAh"), which should at least be simplified to 10 A⋅h (ampere-hours). But mA⋅h isn't a good way to describe batteries because you also need to multiply by voltage (3.7 V for Li-ion, I think 1.2 V for NiMH) to figure out the energy (usually expressed in W⋅h).
One more fun fact - photographic flash units are advertised in watt-seconds (W⋅s) for the maximum amount of energy delivered in a flash pulse of light. But that just simplifies to joules, which is a shorter and less confusing unit name. People really need to stop multiplying watts with time and use joules as designed in the SI.
For me, one of the most cursed unit, but not because it is ill-conceived is the Nm (the unit of torque).
It is analogous to the Joule, but it doesn't mean the same thing. "This car has a 250 million ft.lb battery and 0.1 Wh of torque" passes dimensional analysis.
A watt of power multiplied by a second of time has an agreed upon name called joule, but a watt second is also a perfectly valid SI name.
A watt is a joule of energy divided by a second of time, this is a rate, joule per second is also a valid name similar to nautical mile per hour and knot being the same unit.
Multiplication vs division, quantity vs rate, see the relationship? Units may have different names but are equivalent, both the proper name and compound name are acceptable.
A watt hour is 3600 joules, it’s more convenient to use and matches more closely with how electrical energy is typically consumed. Kilowatt hour is again more directly relatable than 3.6 megajoules.
Newton meter and Coulomb volt are other names for the joule. In pure base units it is a kilogram-meter squared per second squared.
So when I torque all 20 of my car's lug bolts to 120 n-M, I've exerted 2/3 of a W-h? So if it takes me 4 minutes, I'm averaging 10 watts? That's neat. I wonder what the peak wattage (right as the torque wrench clicks) would be; it must depend on angular velocity.
Newton meter as a unit of energy is not the same as the newton meter unit of force for torque.
The energy unit meter is distance moved, while the force unit meter is the length of the moment arm.
This is confusing even though valid, so the energy unit version is rarely used.
You can exert newton meters of force while using no energy, say by standing on a lug nut wrench allowing gravity to exert the force indefinitely unless the nut breaks loose.
Ah! I guess that explains the "f" for "force" in the imperial abbreviation "ft-lbf", to distinguish it from work. I wonder if there's ever been an analogous variant for metric such as "Nmf"...
It seems the common thread is that the f means to introduce G, but not exactly. In my own research, the AI summaries are about as sloppy as I've ever seen, due to the vague and often regional differences (with the difference between ft-lb and lb-ft sometimes being described as relevant, as well).
Of course it can be. Nobody does it in practice because it's inconvenient.
Watts = volts * amps and the people working with batteries are already thinking in terms of voltage and amperage. It'd be painful to introduce a totally new unit and remember 1 watt for an hour is 3.6kj instead of... 1 watt-hour.
What people care about when talking about EVs and consumption is generally how much distance they can cover. If you take away the distance factor and just report power, it becomes meaningless/almost useless.
Many people think of driving in time rather than distance. I'd say it's actually more common to say a city is 3 hours away rather than 200 miles.
What makes kW less useful is really just that most EVs don't advertise their capacity very prominently. But if you knew you had an 80 kWh battery and the car uses 20 kW at freeway speeds, then it's easy to see that it'll drive for 4 hours.
The problem with this is that destinations are a fixed distance away, whereas their time distance is not fixed. In most journeys people want to reach a specific place rather than drive for a given amount of time.
I understand all this but the most important question for me is definitely still "how much distance can I cover on a charge"? That's why I prefer kWh/100km.
Directly reporting required power is still comparable among vehicles: 55kW vs 49kW, eg
Which is definitely less intuitive because it hasn't been introduced to the public, but is interchangeable in the same quirky way we already compare MPG (Distance/Volume) with lt/100KM (Volume/Distance)
Heh. To borrow an idea from xkcd (measuring gas consumption as area): The kWh measures energy, right? And energy is force times distance. So energy divided by distance is force!
Let’s all start measuring EV consumption in newtons, folks.
It even makes intuitive sense: It correlates well with how hard you need to push the car to get it going at the usual travel speed. But it sucks if you need to figure out how far you can travel on a given charge.
Yeah, if only we would define seconds to be 13.4% shorter than that are, we could have 100ks days. Also, ksecs would be a really convenient unit for planning one's day: a ~15 minute resolution is just right for just about any type of appointment.
Oh, and 1Ms weeks, consisting of 7 working days and 3 off days sound nice too.
Much better to make seconds slightly larger than 2 seconds, and move to a dozenal system throughout. One hour is (1000)_12 novoseconds. A semi-day is (10000)_12.
Oh, we should switch our standard counting system to dozenal a well.
Why can’t battery capacity be described in joules? And then charge and discharge being a function of voltage and current, could be represented in joules per unit time. Instead its watt-hours for capacity, watts for flow rate.
Watt-hours… that’s joules / seconds * hours? This is cursed.