this post was submitted on 25 Jun 2025

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xkcd #3106: Farads (imgs.xkcd.com)

submitted 3 days ago* (last edited 2 days ago) by [email protected] to c/[email protected]

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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

(page 2) 39 comments

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[–] [email protected] 38 points 2 days ago (7 children)

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

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[–] [email protected] 44 points 3 days ago (3 children)

"This magnet has one tesla"

[–] [email protected] 13 points 2 days ago

That stucks ;-)

[–] [email protected] 7 points 2 days ago* (last edited 2 days ago) (1 children)

It is not that much though. You could easily make an electromagnet with magnetic flux density of multiple tesla in it's core.

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[–] [email protected] 20 points 3 days ago

However, 1 farad is really goddamn big.

Lol, explainXKCD

[–] [email protected] 9 points 3 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 3 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 3 days ago* (last edited 3 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 2 days ago (3 children)

foot-pounds

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

[–] [email protected] 10 points 2 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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[–] [email protected] 54 points 3 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] 26 points 3 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 3 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 2 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] 13 points 3 days ago (3 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] 2 points 3 days ago (1 children)

I bet it's kind of going off of the original SI representation where, like, a foot was the length of the king's foot, and that was what they had to measure against to make sure everything was accurate.

[–] [email protected] 3 points 2 days ago

https://en.wikipedia.org/wiki/List_of_human-based_units_of_measurement

[–] [email protected] 26 points 3 days ago

A rock that weighs one stone (14 lbs).

[–] [email protected] 8 points 3 days ago (2 children)

Or a barleycorn that's one barleycorn long? Or a really large foot that's a foot long. Or a chain that's a chain long?

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[–] [email protected] 19 points 3 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] 135 points 3 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] 7 points 2 days ago

That is why I like supercapacitors.

[–] [email protected] 16 points 3 days ago (1 children)

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

[–] [email protected] 37 points 3 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 3 days ago (5 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] 4 points 2 days ago* (last edited 2 days ago) (1 children)

Actually, they act like a short circuit to high-frequency AC, so it is more like "blow up" (in general case).

[–] [email protected] 3 points 2 days ago

Fun thing. Cheap aluminium capacitors do blow up, when you plug them in the wrong way.

https://www.youtube.com/shorts/53kl5kNJ728

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

Well by attenuate it a bit you mean they pretty much filter ac out if you have the right capacitance and resistance values as capacitors act like low pass filters.

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

Capacitors can be used to remove ripple from a DC current. Ripple is basically alternating current that is running along a DC current. So, attenuation, I believe, is the correct terminology.

They generally don't completely get rid of AC, and they don't perfectly filter it out unless they are perfectly matched for the AC, and even then, I don't know of any capacitors that are used in lieu of a full-bridge rectifier or half-bridge rectifier to convert AC to DC.

I could very well be wrong. I am far from an electronics expert, But this is what my understanding tells me.

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

Well youre not far off. They are used to filter ac, not convert it. They act as low-pass filters which means if you have a setup which is a 100khz low-pass filter it means it only lets through frequencies that are under 100khz. There are of course more accurate but complicated ways of explaining this but that is out of scope for this comment. Also nothing is perfect in the real world but you can calculate how much of the signal it lets through.

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

but you can calculate how much of the signal it lets through.

Shouldn't that be "noise" instead of "signal"?

[–] [email protected] 3 points 2 days ago (1 children)

I mean depends on the context, in the specific case which i was thinking of it is signal but usually its noise. I have mainly worked with ac and dc signal coupling so thats why both sre signals for me when i think if it.

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

That makes sense. In distortions for music instruments all of it is signal for instance. It's just that sometimes you don't want all of it because you want a specific sound to be produced.

I'm most used to working with digital electronics with a crystal supplying the frequency, so for me anything that is above that frequency is noise.

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

AC units have beefy capacitors, right? Do you know in what range, for comparison?

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

Still tens to maybe low hundreds of microfarads.

[–] [email protected] 3 points 2 days ago (1 children)

Oh. I thought it would be more impressive, but that's still orders of magnitude away. Thanks!

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

And when they are used for air-conditioning units, they are typically boost capacitors, which means they store up a nice amount of juice for when the compressor powers on and needs a sudden rush of energy, but that's only a very small amount, like you couldn't crank a car with the amount of energy in these capacitors.

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

Operation Sundial 2.0, electric boogaloo.

[–] [email protected] 36 points 3 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.

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