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xkcd #3106: Farads (imgs.xkcd.com)
submitted 1 year ago by [B] to c/xkcd@lemmy.world
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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

'This HAZMAT container contains radioactive material with activity of one becquerel.' 'So, like, a single banana slice?'
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[–] 134 points 1 year ago (25 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

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  • [–] 19 points 1 year ago (18 children)

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

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  • [–] 39 points 1 year ago (17 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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  • [–] 12 points 1 year ago (15 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

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  • [–] 11 points 1 year 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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  • [–] 2 points 1 year ago (5 children)

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

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  • [–] 2 points 1 year ago (3 children)

    Still tens to maybe low hundreds of microfarads.

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  • [–] 3 points 1 year ago (2 children)

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

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  • [–] 2 points 1 year ago (1 child)

    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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  • [–] 1 point 1 year ago

    No. They provide phase shift to give the single-phase induction motors a rotating rather than oscillating magnetic field. They charge and discharge 100/120 times per second depending on grid frequency.

    They do not cover inrush current, and would need to be orders of magnitude bigger and a different topology to do so.

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  • [–] 2 points 1 year ago (5 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.

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  • [–] 1 point 1 year ago (4 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.

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  • [–] 1 point 1 year ago (3 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.

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  • [–] 1 point 1 year ago (2 children)

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

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

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  • [–] 3 points 1 year ago (1 child)

    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.

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  • [–] 2 points 1 year 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.

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  • [–] 6 points 1 year ago

    As long as the voltage is high enough, it does not need a whole Farad to wreck havoc. One of the first pranks they played on me in the lab was the "hey, catch" thing with a large, charged capacitor. Yes, I caught it. And I regretted it soon afterwards.

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  • [–] 6 points 1 year ago (2 children)

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

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  • [–] 11 points 1 year ago* (1 child)

    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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