this post was submitted on 25 Jun 2025
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xkcd #3106: Farads (imgs.xkcd.com)
submitted 4 days ago* (last edited 3 days 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] 136 points 4 days 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] 36 points 4 days 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] 16 points 4 days ago (1 children)

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

[–] ricecake 37 points 4 days ago (4 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.

[–] [email protected] 10 points 4 days ago (16 children)

Household current pumped through a full bridge rectifier, that is.

Capacitors don't seem to do very much with AC Other than attenuate it a bit

[–] [email protected] 12 points 3 days ago

Read in electroboom's voice: FULL BRIDGE RECTIFIER!!!!

[–] ricecake 8 points 3 days ago

Technically correct. The best kind of correct. :)

I basically solved for shotgun, confirmed in was in the ~100V range and disregarded every other consideration for actually doing it.
I'm pretty sure most hand sized capacitors would just pop if you actually tried to put that much in them.

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[–] [email protected] 5 points 3 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] 10 points 3 days ago* (last edited 3 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...

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[–] [email protected] 7 points 4 days ago

That is why I like supercapacitors.

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[–] [email protected] 18 points 3 days ago (2 children)

Guys you're not gonna believe this:

50F Super capacitor

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

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

[–] [email protected] 2 points 1 day ago* (last edited 1 day 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 17 hours ago

Amazing! Thanks for the reply.

[–] einfach_orangensaft 10 points 3 days ago

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

[–] [email protected] 41 points 3 days ago (1 children)

This capacitor will cost around one Bitcoin

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[–] [email protected] 38 points 4 days ago (2 children)

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

[–] [email protected] 16 points 3 days 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 3 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] 3 points 3 days ago

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

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[–] [email protected] 4 points 3 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.

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[–] captain_aggravated 5 points 3 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] 56 points 4 days 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] 27 points 4 days 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 4 days ago (1 children)

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

[–] [email protected] 29 points 4 days 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] 45 points 4 days ago (4 children)

"This magnet has one tesla"

[–] [email protected] 13 points 4 days ago

That stucks ;-)

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[–] [email protected] 7 points 3 days ago (1 children)
[–] [email protected] 4 points 3 days ago

this guy gets it.

[–] [email protected] 18 points 3 days ago (2 children)

Their names are Cueball, Megan, and White Hat?

[–] captain_aggravated 17 points 3 days 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] 1 points 1 day ago* (last edited 1 day 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 1 points 1 day 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.

[–] [email protected] 9 points 3 days ago

I remember reading about their names in explainxkcd. I think the only one never named in the comics is Cueball.

For a while, there was a blog, but I don't think it named any character.

[–] [email protected] 20 points 4 days ago

However, 1 farad is really goddamn big.

Lol, explainXKCD

[–] [email protected] 19 points 4 days 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] 13 points 4 days ago (6 children)

But why pick one pound? The are so many fun units to choose from, only some of which are conveniently sized. How about a stick 1 mile long, or a rock that weights 1 grain?

[–] [email protected] 27 points 4 days ago

A rock that weighs one stone (14 lbs).

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[–] [email protected] 9 points 4 days ago (1 children)

I’d be the clueless guy in the room. “I’m not familiar with that unit of measurement.”

[–] [email protected] 14 points 4 days ago (3 children)

A capacitor of 1 farad at standard American 120 volts has the energy between 7.62×54 and .50 BMG, and will discharge just as violently.

[–] [email protected] 18 points 4 days ago* (last edited 4 days ago) (1 children)

Great. Now I get to be the "I’m not familiar with that unit of measurement." guy.

7.62x54

3,291 J (2,427 ft⋅lbf) to 3,400 J (2,508 ft⋅lbf)

.50 BMG

The .50 BMG round can produce between 10,000 and 15,000 foot-pounds force (14,000 and 20,000 J), depending on its powder and bullet type, as well as the weapon it is fired from

[–] [email protected] 18 points 4 days ago (3 children)

foot-pounds

Oh, you Americans and your silly made-up units.

[–] [email protected] 10 points 4 days ago (1 children)

All units are made up.

I totally agree that imperial units are silly, though. Base 10 is the way things should be.

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