this post was submitted on 25 Jun 2025
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xkcd #3106: Farads (imgs.xkcd.com)
submitted 1 week ago* (last edited 1 week ago) by [email protected] to c/[email protected]
 

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/

explainxkcd for #3106

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[–] [email protected] 21 points 6 days ago (2 children)

Guys you're not gonna believe this:

50F Super capacitor

[–] einfach_orangensaft 10 points 6 days ago

pfff at 2.7V that aint much of a danger, now show me 50F @ 2kV

[–] [email protected] 2 points 5 days ago (1 children)

Yes, I do not believe it. It must be a misprint, right?

[–] [email protected] 6 points 4 days ago* (last edited 4 days ago) (1 children)

No it's real! I can't verify the exact rating since it OL's my meter, but with some circuitry it can power my Pi for a few minutes. I got them from element14, so it's unlikely to be a fake product.

[–] [email protected] 1 points 4 days ago

Amazing! Thanks for the reply.

[–] [email protected] 8 points 6 days ago (1 children)
[–] [email protected] 4 points 6 days ago

this guy gets it.

[–] [email protected] 2 points 6 days ago (2 children)

Thats literally exactly what it is. They aren't derived from the metric system like all units in the system, theyre a specialized edge case where a conversion was specifically written in because america sucks. There are no other conversions in the metric system, units are derived from constants based on a set of specific rules that the pound and yard do not adhere to.

[–] captain_aggravated 6 points 6 days ago (1 children)

So, you don't understand the joke, but you detected an excuse to say America Bad. Your employer must be pleased.

The joke of this comic is based around most units in both the metric and imperial system being fairly mundane quantities. A one meter long stick is about half the height of a man, a one pound rock is about the size of his fist, a one volt battery has the potential of a dead AA.

The Ferad is the SI unit of capacitance, equal to 1 coulomb per volt. A one ferad capacitor can hold enough energy to do a number of exotically dangerous things.

[–] [email protected] -4 points 6 days ago* (last edited 6 days ago)

Lmao what makes you think i didnt understand the joke? We're talking about units of measurement. And yeah i dont need an excuse to say America Bad. It is bro, have you been paying attention?

Edit* my comment was meant to be a reply to another comment about the metric system, not top level. Not sure how that happened aince i clicked reply right from my inbox. Stick by what i said about america

[–] [email protected] 2 points 6 days ago

And I have to say, the chemistry math that resulted from the metric system is elegant. I've forgotten most of it by now, but at some point, I was able to envision the machinery behind it all, and it was beautiful.

[–] [email protected] 139 points 1 week ago (7 children)

Haha that's a good one

Capacitors are usually in the realm of pico to micro farads

A one farad capacitor charged to a respectable voltage would feel like a doomsday device in your hand

[–] [email protected] 6 points 6 days ago (1 children)

Worked with an industrial robot one that had 700V 0.5F electrolytic capacitors on its power supply. Those things were massive.

[–] [email protected] 11 points 6 days ago* (last edited 6 days ago) (1 children)

I was in the building when when a 3F 1200V capacitor, part of a multi-rack mounted capacitor bank (powered a magnetohydrodynamic modeling experiment), failed. It ripped the rack's 30cm mounting bolts out of the floor, launched the three-tonne rack hard enough to crack the ceiling and shattered every window in the facility. I want to say that afterwards I never broke the rule about not being allowed to enter the experiment room until the banks were discharged, but I'd be lying. Undergrads are idiots, and holy cow don't fuck around with those caps...

[–] [email protected] 2 points 6 days ago
[–] [email protected] 38 points 1 week ago

You see low voltage ones for things like memory backup on hi-fi gear. I have some 3F/5v capacitors in an old Technics tiner.

[–] [email protected] 17 points 1 week ago (1 children)

Wait so this is like one mistake away from turning that stickman into a fried stickman?

[–] ricecake 39 points 1 week ago (20 children)

Depends on the voltage it's charged with, but household current would give it more energy than a shotgun has.

Realistically one would not do that unless you were dealing with something industrial. You would use them otherwise for things like dampening lower voltage systems that need a lot of current.

Closer to the danger level of someone holding two exposed wires plugged into the wall.

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[–] [email protected] 58 points 1 week ago (1 children)

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.

[–] [email protected] 28 points 1 week ago (8 children)

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.

[–] [email protected] 16 points 1 week ago (1 children)

The kilogram is just a thousand grams, so if they're tied together, they would still be tied together.

[–] [email protected] 30 points 1 week ago

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.

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[–] [email protected] 46 points 1 week ago (4 children)

"This magnet has one tesla"

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[–] [email protected] 44 points 1 week ago (1 children)

This capacitor will cost around one Bitcoin

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[–] [email protected] 38 points 1 week ago (2 children)

Only criticism is the use of non-metric weight units when everything else is SI-based.

[–] captain_aggravated 5 points 6 days ago

The joke wouldn't have worked as well.

A gram is actually a pretty small unit of weight, and the joke relies on the base units. It's actually a weird little abberation in the metric system that the "base" unit is considered the kilo gram. so a 1 gram rock would be a little pebble, strangely small.

[–] [email protected] 17 points 1 week ago (2 children)

Wikipedia currently says:

the international avoirdupois pound, [...] is legally* defined as exactly 0.45359237 kilograms

So, technically, a pound is a metric weight, only a niche one whose use may or may not be permitted by local regulations.

Similar is true* of the inch, which is defined as precisely 25.4 millimetres.

* The US, UK and a handful of others collectively signed this into their respective laws in 1959. You might think we don't use the pound in the UK any more but it still shows up often in informal situations. Ditto inches and feet.

[–] [email protected] 5 points 6 days ago (2 children)

That's similar to saying "Auf Wiedersehen" translated to English is "until I see you again", therefore "Auf Wiedersehen" is technically English. Just because there's a recognised translation to a thing, that doesn't make it that thing.

[–] [email protected] 4 points 6 days ago (1 children)

It's not a recognised translation- it's the definition.

[–] [email protected] 1 points 2 days ago (1 children)

If we're going to split hairs then while it's defined in terms of metric units, it doesn't scale with prefixes and factors of 10, so it can't be an S.I. unit.

[–] [email protected] 2 points 2 days ago

You're right, the imperial units are not S.I. units, but each (most?) imperial unit is defined by an S.I. unit.

[–] [email protected] 2 points 6 days ago

The other way around maybe, that is, an English word becoming technically foreign because we decide that we are going to write its definition in a different language in the dictionary.

It wouldn't make sense to do that though, which kind of breaks the analogy. Unless you count words borrowed wholesale because we didn't have that word, and those definitions were written in a foreign language first.

As it is "one pound" now translates exactly to "nought point four five three five nine two three seven kilograms" where it didn't before 1959. "kilogram" is one of those foreign borrowings.

[–] [email protected] 4 points 6 days ago (1 children)

I would argue that a legally defined conversion to a meter or another metric value does not make a unit metric in and of itself. Those units have to adhere to the system, which is clearly based on decimal values, not just some arbitrary conversion with an absurd precision that was only signed into law to minimize the inconvenience caused by non-standard units.

[–] [email protected] 2 points 6 days ago

It's not a defined conversion, it's the literal, internationally ratified definition of what those units are. Or maybe "redefinition" ought to be the word there; prior to that definition there were several very similar, roughly equal but ultimately not internationally standardised units in use. And since they were redefined in terms of SI units, they're technically SI.

This is one of those "tomatoes are technically fruit, but no-one with good sense would put them in a fruit salad" situations.

[–] [email protected] 20 points 1 week ago

However, 1 farad is really goddamn big.

Lol, explainXKCD

[–] [email protected] 19 points 1 week ago

Ah, Randall is alive! I kept thinking my bot had broken as it's so rare for him to miss an upload.

[–] [email protected] 18 points 1 week ago (2 children)

Their names are Cueball, Megan, and White Hat?

[–] captain_aggravated 19 points 1 week ago (1 children)

It is my understanding that XKCD's "characters" are somewhere between an actual character and an archetype. It isn't clear...and kind of doesn't matter, if Black Hat is the same guy in every comic or if he's a different devious schemer in each. Randall hasn't bothered to name any of them so the community has given them unofficial nicknames.

[–] [email protected] 2 points 5 days ago* (last edited 5 days ago) (1 children)

Randall hasn’t bothered to name any of them so the community has given them unofficial nicknames.

Megan is actually named in multiple comics though, ~~so is Danish (Black Hat's girlfrenemy)~~ (actually that was picked semi-arbitrarily by the community). Cueballs also have different names occasionally but they're all drawn the same.

And actually I do believe that certain non-named characters are the same comic-to-comic. Black hat and Beret guy almost certainly are.

[–] captain_aggravated 2 points 4 days ago

Yeah Black Hat called her "My dearest darling danish" after she called him "my cutie pie." Closest thing she's had to a name.

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