xkcd #3106: Farads
xkcd #3106: Farads
Title text:
'This HAZMAT container contains radioactive material with activity of one becquerel.' 'So, like, a single banana slice?'
Transcript:
[Cueball holds a stick while talking with Megan and White Hat.]
Cueball: This stick is one meter long.
Megan: Cool.
White Hat: That's a nice stick.[Cueball holds a smallish rock.]
Cueball: This rock weighs one pound.
Megan: I'd believe it.
White Hat: Looks like a normal rock.[Cueball holds a small battery.]
Cueball: This battery is one volt.
Megan: Seems fine.
White Hat: Might need a recharge.[Cueball holds a capacitor while Megan and White Hat panic.]
Cueball: This capacitor is one farad.
Megan: Aaaaa! Be careful!!
White Hat: Put it down!!
Source: https://xkcd.com/3106/
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I used to teach AP physics to kids on the weekends. One asked me why Farads were so big. I had to explain that there’s a fixed ratio between Farads, Volts, and Joules. One of them had to be crazy big or crazy small.
See also Coulombs.
Caps are especially scary because they can develop their own charge through static electricity, so large value caps are often shipped with their terminals tied together.
There's nothing in the SI system that says ratios have to be between base units. Units that involve mass are defined against the kilogram not the gram.
The kilogram is just a thousand grams, so if they're tied together, they would still be tied together.
Right. 1F = 1C/1V .. they could have just as easily said 1kF = 1C/1V. Many things use kg instead of g. You can tie together things other than the unscaled base units. Then they are still tied together but 1F is a more reasonable amount.
But kilogram is the base unit
Of the SI system. The metric system started with cgs (centimeter, gram, second) and evolved from there.
You sent me down a freaking rabbit hole, thanks! :)
From what I found is that there is the simple reason that the weird ones are distance, time and weight - the rest I looked into are based on formal non-normalized definitions (including lumen, which surprised me).
My guess is that in depends on where the unit comes from: science or day to day use.
I learned about the Siemens, the Weber and the Gray on the way.
Thanks again!
They were all done by scientists or engineers.
The meter was defined based on what they calculated as 1 millionth of the length of Paris' meridian.
The second was 1/86400 of a day, which makes sense with the angle/circle nomenclature on the clock.
The gram was initially set to be the mass of 1cm³ of water at 4°C - which is why 1l of water ≈ 1kg.
The concept of length is way older than these definitions, same for weight and so on.
The meter is an awesome example for what I mean: the 1/1000000 wasn't random. From my understanding it won over the alternatives in dezimal because of it's relative closeness to an arms length and the definition was used to remove issues in France because of the (metric) fuckton of different measurements for length.
And the second example of yours is even better describing what I meant: it's just making sense and is practical not a deep scientific reasoning.
And I won't bliebe that the foot and inch was conceived by anyone who has a scientific approach.
To be clear: you're right that basically by definition the units were done by professionals. I try to point out that for the more broader used units practical aspects were at least as important (after all it wasn't a square meter that was used for the gram but a centi of one).
It's also the length of a pendulum with a half period of 1 second.
Whaaaaaaasaaaa
I have no idea how I've missed that y today I'm over of the lucky 10,000 I assume. Thanks!