3106: Farads

Farads

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Title text: 'This HAZMAT container contains radioactive material with activity of one becquerel.' 'So, like, a single banana slice?'

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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 (V, 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 like an AA, C, or D cell (1.5 V) might read as 1 V if significantly depleted.

In contrast, 1 farad is an unusually large amount of capacitance. Capacitance is the ability to store charge, a large amount of which could be dangerous. 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 it could be very dangerous.

For the same charging voltage and load/resistance, a larger capacitor won't deliver greater current or instantaneous power than a smaller one, but the total amount of energy and duration of discharge would be proportionally longer. If the capacitor's wires accidentally touch each other or a third piece of metal, an accidental "short circuit" is created, and all of the capacitor's stored energy discharges very quickly. For example, a 1 farad capacitor charged to 10 volts stores 50 joules of energy, and discharging all of that into a copper wire could cause the wire to heat up by a few dozen degrees Celsius faster than the blink of an eye and cause serious burns (see the calculation).

Large capacitors are often associated with larger voltages and heavy machinery, which can contribute to the feeling of caution around large capacitances.

The title text explores the inverse situation, where “1” of a unit is a very small amount. A becquerel (Bq) amounts to one radioactive atomic decay per second, 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 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). For comparison, a 70 kg human body itself has an activity of 8000 Bq, and the 1986 meltdown of the Chernobyl nuclear power plant released 2×10¹⁸ (that's 2,000,000,000,000,000,000) Bq! The earlier common unit for radioactivity is the 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. Radioactive material emitting 1 curie that is small enough to fit into a container for hazardous materials is dangerous enough that it probably should be in one. Bananas as a unit of measurement for radiation exposure are also discussed in the Radiation chart.

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!!

Trivia

Of the three 'normal' unit quantities, the battery gets a comment that it might not necessarily be. Common household versions of 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 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 primary cell.

Also, of all four measures, the pound is the only non-SI unit given. A mass of 1 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 metric prefix (the only one of the SI base units, even including the 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 normal three-a-week comic schedule having it come out on Monday, June 23, and being listed on xkcd's archive list for that day, it was actually released well into the next day. This is one of very few times other than April Fools' comics that Randall was so late. The next comic, 3107, came out well within its nominal day (Wednesday, and not just by US time zones), such that possibly this comic, of all comics in the usual three-a-week cycle, spent the least time as the "latest" comic before being replaced by the next in its cycle.

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Discussion

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." [1] 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. 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. 82.132.246.160 13:07, 25 June 2025 (UTC)

Kilogram is a unit of mass, while pound is a unit of force (weight is a measure of the force of gravity on a particular mass). So no, the kg is not quite the corresponding unit to the lb; that would be the Newton (N), equal to about 0.223 lb. 170.85.70.249 18:12, 10 July 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. --PRR (talk) 03:27, 25 June 2025 (UTC)

[I used to work with a guy who'd fall for all of the gags, going off to fetch striped paint, a glass hammer (there are such things in fact), a spirit level bubble, etc. Turns out he'd just goof off, completely aware he was on a wild goose chase.] 81.109.188.229 (talk) 19:47, 27 June 2025 (please sign your comments with ~~~~)

This explanation would benefit from some elaboration on how and why supercapacitors are dangerous. 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. 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? 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). 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. 2001:16B8:C731:2E00:9AC:BBD8:8775:315D 15:34, 25 June 2025 (UTC)

I agree with Troy0 that having a non-SI unit in there (1 pound) is incongruous, and it should instead be a sugar crystal weighing 1 gram. 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 dielectric absorption... Interesting, but completely different. -- 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 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. 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. 82.132.244.34 15:47, 25 June 2025 (UTC)

We now have an exact answer to the question "how tall is Cueball?"--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. 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. MinersHavenM43 (talk) 15:19, 25 June 2025 (UTC)

in my neck of the woods we usually use centimetres for people's heights. It's more convenient to not need to use a decimal separator. Bicorn (talk)

I've done the measuring, and I found a beautiful easter egg: The stick is held at a 23° angle, and when rotated to vertical is 100 px! So I get Cueball at 180 cm, or 1,8 m, as it's written in my neck of the woods. --80.71.143.166 09:38, 10 July 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. 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) 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. 2600:1009:B092:310F:4D22:1073:190A:E328 17:04, 25 June 2025 (UTC)

"This litre of water is at 1 T_P" 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. 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. 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.2602:FF4D:128:D56:8114:9FE5:5A4D:499F 16:19, 26 June 2025 (UTC)

Yes, and "low-energy" needs definition. It's easy to find caps in the ballpark of 1F, >30V rating, hundreds of milliohms ESR, under $50, and small enough to easily hold in your hand. This could have an energy around 1000 J and could supply tens of amps for a significant fraction of a second. It's not as dangerous as, say, a grenade, but enough that I would be careful to avoid accidental self-discharge, especially across the body. And this assumes that the previous user didn't charge it *above* the rated voltage! 174.126.217.139 17:46, 26 June 2025 (UTC)

All that panic about Farad ... I wonder what would they say if he was holding something creating a magnetic field of one Tesla. -- Hkmaly (talk) 18:32, 3 July 2025 (UTC)

I tried to something similar with an item one Kelvin, thinking that people would be interested, but just ended up being given the cold shoulder. 92.23.2.228 19:27, 3 July 2025 (UTC)

See also: A joke about measurement. He constructs a consistent system of measurement based around the speed of light, the calorie, and Middle C. Once you have that you can start deriving other units, such as power. It's measured in "middle-C calories" and works out to 1094.64 Watts, the "first derived unit that's actually a convenient size." 69.14.74.137 03:38, 13 July 2025 (UTC)
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