explain xkcd - User contributions [en]

Talk:3106: Farads

2025-06-26T16:19:45Z

2602:FF4D:128:D56:8114:9FE5:5A4D:499F:

Who wrote this description? It's complete nonsense. A capacitor can't throw a stone. A 1 F capacitor is also not remotely dangerous unless it's charged to a high voltage — except that a 1 F capacitor and a 0.01 F capacitor can be charged to essentially the same maximum voltage!

Unlike other units of measure where a single unit is non-extreme, "The capacitance of the Earth's ionosphere with respect to the ground is calculated to be about 1 F." [https://en.wikipedia.org/wiki/Farad] Most capacitors in practical use are measured in pico, nano, or micro farads. 03:04, 25 June 2025 (UTC)

Please note that the pound, shown in panel 2, is not an SI unit. The corresponding SI unit is the kilogram; an item with a mass of one kilogram is still commonplace. [[User:Troy0|Troy0]] ([[User talk:Troy0|talk]]) 03:11, 25 June 2025 (UTC)
:I added Trivia to mention that (and another thing), sorry that I didn't read here first but I think I've covered your thoughts on the subject. [[Special:Contributions/82.132.246.160|82.132.246.160]] 13:07, 25 June 2025 (UTC)

When my father was a young engineer, the old guys would haze the new kids by asking them to fetch a "one farad capacitor". But everybody in the lab said "Sorry, I ran out, go ask Fred on the top floor", "Go ask Tom in the basement", "Try Peter's Parts on Vine St", etc--- give the kid a run-around. The joke was: at the time, 1F was likely large than a large garbage can and many hundred (non-SI) pounds. But the world changed, and in recent years you can easily buy 1F @ 16V, about the size of a soup can, to smooth car sound power feeds. --[[User:PRR|PRR]] ([[User talk:PRR|talk]]) 03:27, 25 June 2025 (UTC)

This explanation would benefit from some elaboration on how and why supercapacitors are dangerous. [[Special:Contributions/195.252.226.234|195.252.226.234]] 04:41, 25 June 2025 (UTC)

Funnily enough, the wikipedia page for "Farad" (https://en.wikipedia.org/wiki/Farad) currently has a 1 farad supercapacitator as the title image. It looks pretty unassuming. [[User:Mouse|Mouse]] 08:54, 25 June 2025 (UTC)

Top of the page says June 23 even though it looks like this came out on June 25. Should it be changed? [[Special:Contributions/85.76.9.43|85.76.9.43]] 05:15, 25 June 2025 (UTC)
:Randall-time, it was 24/Jun (or Jun/24, being leftpondian with potentily mixedendian dates). It's not unnown for it to be an early-hours-of-day-after (EST) release, though late-hours-of-day-after is rather unusual. I'm guessing awkward commitments took over, perhaps even the prescheduled timer (if left 'in charge', not having even been put to the test in a while) didn't work when/how it should have.
:We've also had surpisingly ''early'' releases (noon or earlier, UTC, making it very-early-on-day-of-release), but I haven't any specific memory of it being so early that it ended up ''preceding'' the scheduled day (off-schedule additions don't count), other than perhaps when he was currently on a book-tour and (e.g.) in Europe so probably doing his prefered time-of-day (or when it was most convenient for his schedule) in UTC/UTC+1/UTC+2 'mode', though it was still "yesterday" back home.
:Best suggestion is to see when [[3107]] comes out. If it's a Wednesday(ish)-compatible time, this was just overdue for ...reasons. But if it's Friday(ish), then we can re-examine its true position (with much arguing, I suspect) in the schedule.
:It ''could'' also be an attempt to subtly shift what number pops up when (I think a past "whole week series", or two, were conjectured to alter the numbers to reasonably engineer the landing of [[404]] upon April 1st), but that's probably beyond speculation until we at least can assess what has happened by the end of this week. (''Then'' start looking for what numbers land (near) where, up to arbitrary points in the future.)
:Also something to add to Trivia, ''when'' we can rule out some of the possibilities (or be prepared to be wrong/overly-comprehensive, like here, and remove the wronger bits later). [[Special:Contributions/82.132.246.160|82.132.246.160]] 13:07, 25 June 2025 (UTC)
::The official archive at https://xkcd.com/archive/ lists it as released at 2025-6-23. We should follow that listing, as we have done before at (vary rarely) delayed comics. [[Special:Contributions/2001:16B8:C731:2E00:9AC:BBD8:8775:315D|2001:16B8:C731:2E00:9AC:BBD8:8775:315D]] 15:34, 25 June 2025 (UTC)

I agree with [[User:Troy0|Troy0]] that having a non-SI unit in there (1 pound) is incongruous, and it should instead be a sugar crystal weighing 1 gram. [[Special:Contributions/121.98.227.79|121.98.227.79]] 06:52, 25 June 2025 (UTC)

"most consumer electronics use at most a nanofarad" -> nah... Several hundreds of microfarads are quite common. But so are tens-of-picofarad, mostly in HF/RF filters etc. Calculating an average over all capacitors in all consumer electronics makes no sense anyway... But I'd say "Most consumer electronics use capacitors in the picofarad to milifarad range".
"To prevent static electricity from building lethal charge, unused supercapacitors are usually stored and transported with a "keeper", a steel or aluminum bar shorting the terminals." -> Static charge won't change the voltage of a 1 F capacitor much... V=q/C with small q and large C... The shorting is for high voltage capacitors that 'recharge' themselves trough {{w|Dielectric_absorption|dielectric absorption}}... Interesting, but completely different. -- [[User:Gautee|Gautee]] ([[User talk:Gautee|talk]]) 07:52, 25 June 2025 (UTC)
:Even a supercapacitor is not necessarily lethal. It depends on the voltage. A project I'm working on has a 6v supercapacitor (to keep the clock running for a few days when power is disconnected). And they're not even expensive parts. For example [https://www.mouser.com/ProductDetail/KYOCERA-AVX/SCMT32H755MRBB0?qs=l7cgNqFNU1iVPH0cf9oilA%3D%3D this one] is 7.5F (!) at 6v. They're not very large and only cost $9. Touching the terminals when it is charged will hurt a lot, but it will hardly kill you. [[User:Shamino|Shamino]] ([[User talk:Shamino|talk]]) 14:40, 25 June 2025 (UTC)
::It really doesn't (directly) depend upon the voltage, either. It's a function of the volts ''and amps'' (but ultimately, how much energy there is, and ''where'' it manages to go).
::Personal anecdote: Physics lesson (tertiary education level), one experiment used a High Tension Power Supply to provide a high (selectable) voltage to some equipment, already set up with a rope(-like) barrier around it that one had to stay beyond when it was powered up. HTPSU's selector-switch had a screw 'stop' to limit the selection to only 'very high' voltage (already way beyond 230+V mains, with generally up to 13A fuse). During a classroom break, I thought I'd see what results a 'ludicrously high' voltage would give, unscrewed the stop, turned the dial up beyond it, and from beyond the boundary-'barrier' turned the power on. *ZAP*, I actually got shocked! (Can't now remember if it blew a fuse/RCD, or if it was part of the experiment that a discharge naturally stopped it, it was decades ago and the finer details of the encounter are well and truly blurred, including what the activity was - but a Jacob's Ladder could well have needed thousands to tens of thousands of volts running through it, if it was that, or the HTPSU was ultimately capable of running one, rather than what 'low voltage' thing I was doing.)
::Hurridly turned it all off, possibly earthed the bit you had to remember to earrh when you were leaving it for the next person's turn, dialled it back down, reinserted the limiter-screw, went off on the break that the rest of the class (and supervising tutor) had gone off to. It would have been very low ampage (lucky for me), maybe also I only got a fraction of the discharge, sharing it with nearby lab-fixtures (sink/gas-taps?), etc, and it probably did not cross my body (the most dangerous effect) but I felt it (and remember not being sure from where the shock might have actually jumped).
::Nobody the wiser (''possibly'' the next experimenter found the fuse blown, when they tried to power up, but maybe even not that if was just temporarily RCDed at most), except maybe myself... Tended to respect 'screwstops' on dials from then on.
::And only other significant 'shock' I've ever had, apart from static ones resulting from man-made fibres in clothjng/carpets, was when I touched a plucked dandelion stalk (or similar) to an electric fence when ''much'' younger, curiosity getting the better of me in a slightly different scenario that turned out to be more shocking than I perhaps expected. Was I thinking it'd be a mere tingle, the current having to pass through a plant stem..? I think I already knew the old adage about "there are good electricians, bad electricians and dead electricians" - good ones have a current-detecting screwdriver-thingy (sufficiently high resistance), bad ones forget to bring one and have to test maybe-off wires with the back of their hand (if not off, muscles contract and contact quickly lost), dead ones ''grasp'' any mysterious wire and then ''can't let go'' until the power cuts for any other reason (high-rating household fuse, maybe, could be too late). OK, so there's some acinowledged inaccuracies (or historical assumptions) in the above, but the gist is pretty much there. [[Special:Contributions/82.132.244.34|82.132.244.34]] 15:47, 25 June 2025 (UTC)

We now have an exact answer to the question "how tall is Cueball?"--[[Special:Contributions/86.13.226.126|86.13.226.126]] 09:16, 25 June 2025 (UTC)
:Assuming that Cueball's holding the stick in a plane parallel to the comic frame. [[User:Legowerewolf|Legowerewolf]] ([[User talk:Legowerewolf|talk]]) 13:23, 25 June 2025 (UTC)
:If the stick really is parallel, Cueball's height is 1.78 m, or 5'10" for the Americans, or 9.02×106 ħc/eV for the Proxima b-ans. [[User:MinersHavenM43|MinersHavenM43]] ([[User talk:MinersHavenM43|talk]]) 15:19, 25 June 2025 (UTC)

In agreement with the first discussion point, this description remains low quality. It claims that after the unwarranted panic, Megan and White Hat "ask why he [Cueball] is carrying it [the 1-farad capacitor] around." This does not occur at any point in the comic or the Title Text, and should be removed. [[Special:Contributions/198.147.146.254|198.147.146.254]] 10:21, 25 June 2025 (UTC)

I'm sure the Farad is going to drive all sorts of commentary. My nitpick with the description: a 1F 30V+ capacitor can be held in hand (e.g. Cornell Dubilier DSM105Q030W075PB, Nichicon LNR1V105MSE). I don't recall the hazard criteria for stored energy right off, so I can only say that @30V there is no shock hazard in dry environment human handling, but the energy stored still present other hazards (e.g. fire or burns from conductors) [[Special:Contributions/12.171.61.178|12.171.61.178]] 14:39, 25 June 2025 (UTC) JourneymanWizard

Could the 1V battery be not a 1.5V alkaline battery, but a 1.1V lithium battery? Still somewhat discharged, but not nearly as much. [[Special:Contributions/2600:1009:B092:310F:4D22:1073:190A:E328|2600:1009:B092:310F:4D22:1073:190A:E328]] 17:04, 25 June 2025 (UTC)

"This litre of water is at 1 ''T''P" [[Special:Contributions/60.240.13.138|60.240.13.138]] 22:30, 25 June 2025 (UTC)

For different capacitors of a given physical size, what is the approximate relationship between capacitance and max voltage, all other factors also being equal? Simple inverse? Is it even a reasonable question to ask? Clearly, a gigantic capacitance and low voltage isn't dangerous; a tiny capacitance and extremely high voltage also isn't, if for no other reason than that it would discharge through the air. [[User:BunsenH|BunsenH]] ([[User talk:BunsenH|talk]]) 00:39, 26 June 2025 (UTC)

Although he threw in one pound, I think the point is that some base metric units are impracticaly large or small. The MKS system has a kilogram as a "base" unit rather the gram. The CGS system used grams but centimeters instead of meters. Nobody uses ares or bels. [[Special:Contributions/2600:8800:4880:66B:809:D867:2F4C:D77A|2600:8800:4880:66B:809:D867:2F4C:D77A]] 03:41, 26 June 2025 (UTC)

I'd just like to point out that the confidence with which the explanation declares the capacitor to be low energy and therefore safe is exactly the sort of confidence that gets you electrocuted.[[Special:Contributions/2602:FF4D:128:D56:8114:9FE5:5A4D:499F|2602:FF4D:128:D56:8114:9FE5:5A4D:499F]] 16:19, 26 June 2025 (UTC)

3106: Farads

2025-06-26T16:17:25Z

2602:FF4D:128:D56:8114:9FE5:5A4D:499F: /* Explanation */

{{comic
| number = 3106
| date = June 23, 2025
| title = Farads
| image = farads_2x.png
| imagesize = 677x253px
| noexpand = true
| titletext = 'This HAZMAT container contains radioactive material with activity of one becquerel.' 'So, like, a single banana slice?'
}}

==Explanation==
This comic shows [[Cueball]] showing off several items that (he claims) comprise approximately one of a given unit, with [[Megan]] and [[White Hat]] reacting appropriately.

The first three — meters, pounds and volts — are all units of which “1” is a not extraordinary amount for an item that can be easily held in the hand. As such, they elicit minimal reaction from Megan and White Hat. A meter (a unit of length) is visually verifiable; a pound (a unit of weight) is easy to hold in the hand; and a volt (a unit of electric potential) would cause minimal harm even if discharged. White Hat's remark that the battery "might need a recharge" refers to the fact that 1 V batteries are somewhat uncommon, but a battery with a larger voltage might read as 1 V if it is significantly depleted.

In contrast, 1 {{w|farad}} is an unusually large amount of capacitance. In common use, most consumer electronics use capacitors in the picofarad to millifarad range, and 1 millifarad is already considered a "large" capacitor. A 1-farad capacitor is considered a supercapacitor. Cueball claiming to have a 1-farad capacitor elicits panic from Megan and White Hat, who fear that {{w|Capacitor#Hazards and safety|it could be very dangerous}}. In reality, it's not the capacitance that kills you, it's the amperage, and any hand-held 1 farad capacitor ''should'' have a very low energy limit and therefore not be in any way dangerous. They are probably extrapolating from the effects of capacitors of similar size but lower capacitance, which can therefore store higher voltages. Such a capacitor can give a nasty shock.

Capacitance (C) is the ratio between charge (coulombs or amp-seconds) and electromotive force (V, volts or joules per coulomb).

The title text explores the inverse situation, where “1” of a unit is a very small amount. A becquerel (Bq) amounts to one radioactive decay per second on the atomic level, which is a really low level of radioactivity. As observed, the material in question could be a single slice of a banana (primarily due to the decay of trace {{w|potassium-40}} in the total potassium it contains, a natural proportion of 117 parts per million). Hence, it is both impractical and unnecessary to contain it inside a container for hazardous materials unless the material is dangerous for other reasons (such as corrosiveness, flammability, or overripeness). In comparison, the human body itself [https://web.archive.org/web/20200220103556/https://radioactivity.eu.com/site/pages/Activity_Doses.htm has an activity of 8000 Bq]. The earlier common unit for radioactivity is the {{w|Curie (unit)|curie}}, originally defined as the decay rate of 1 gram of radium. It has since been redefined to be 3.7 × 1010 decays/second, i.e. 37 GBq. A curie of radioactive material that is small enough to fit into a container for hazardous materials is dangerous enough that it probably ''should'' be in one.

==Transcript==
{{incomplete transcript|Don't remove this notice too soon.}}
: [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!!

==Trivia==
Of the three 'normal' unit quantities, the battery gets a comment that it might not necessarily be. Common household versions of {{w|electrochemical cell}} will normally be designed to produce around 1.5 volts (though some fairly common other versions go as low as 1.2 V or as high as 2.1 V), with single-/multi-cell {{w|List of battery sizes|batteries in general}} often being rated at a simple multiple of that (e.g. 1.5 V, 4.5 V, 9 V, ...). A cell producing nearer just a single volt, as is pointed out, might be significantly discharged and need recharging. If not replacing entirely, having aged due to too many recharges, as it also definitely would if it is a {{w|primary cell}}.

Also, of all four measures, the {{w|Pound (mass)|pound}} is the only non-SI unit given. A mass of 1 {{w|kilogram}} would also not be too odd a weight to have quoted here, being about 2.2 lb, but may not have been chosen due to its relative unfamiliarity to everyday US readers (even compared to the meter), or else because of its {{w|metric prefix}} (the only one of the {{w|SI base unit}}s, even including the {{w|SI derived unit|derived ones}}, to not be a bare unit) and the gram itself perhaps being even less relatable.

This comic was uploaded very late. Despite the next comic being scheduled for Monday, June 23, it was really released well into the next day. This is one of very few times other than [[:Category:April Fools' Day comics|April Fools' comics]] that Randall was so late. Comic [[3107]] came out well within its nominal day (Wednesday, and not just by US timezones) such that possibly this comic spent the least time as the "latest" comic of any that (together with its successor) belonged strictly to the default three-a-week comic cycle.

{{comic discussion}}<noinclude>
[[Category: Comics featuring Cueball]]
[[Category: Comics featuring White Hat]]
[[Category: Comics featuring Megan]]
[[Category: Physics]]