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

  • ryannathans@aussie.zone
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    15 hours ago

    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

    • HakFoo@lemmy.sdf.org
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      14 hours 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.

    • Sal@lemmy.world
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      14 hours ago

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

      • ricecake@sh.itjust.works
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        13 hours ago

        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.

          • ricecake@sh.itjust.works
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            3 hours ago

            If we’re getting into practical realities it would probably pop and smolder long before it got fully charged. Capacitance is how much charge something will hold per volt. Doesn’t say anything about how much charge it holds before catching on fire. :)

        • Treczoks@lemmy.world
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          8 hours 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.

        • bizarroland@fedia.io
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          13 hours ago

          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

          • ricecake@sh.itjust.works
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            6 hours 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.

          • kaidezee@lemmy.ml
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            9 hours ago

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

          • AItoothbrush@lemmy.zip
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            10 hours ago

            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.

            • bizarroland@fedia.io
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              10 hours ago

              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.

              • AItoothbrush@lemmy.zip
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                10 hours ago

                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.

                • Ragnor@feddit.dk
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                  9 hours ago

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

                  Shouldn’t that be “noise” instead of “signal”?

                  • AItoothbrush@lemmy.zip
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                    9 hours ago

                    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.

            • clif@lemmy.world
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              6 hours ago

              If by AC you mean air conditioner, I just replaced mine with a 50+5uF dual cap @ 370/440 VAC

              • ByteJunk@lemmy.world
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                10 hours ago

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

                • bizarroland@fedia.io
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                  10 hours ago

                  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.