1mo ago

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

95 comments

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
- 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.
- Wait so this is like one mistake away from turning that stickman into a fried stickman?
  
  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.
- That is why I like supercapacitors.
- Worked with an industrial robot one that had 700V 0.5F electrolytic capacitors on its power supply. Those things were massive.
  
  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...
- Operation Sundial 2.0, electric boogaloo.
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.
- 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.
  
  The kilogram is just a thousand grams, so if they're tied together, they would still be tied together.
  
  But kilogram is the base unit
  
  You sent me down a freaking rabbit hole, thanks! :)
  From what I found is that there is the simple reason that the weird ones are distance, time and weight - the rest I looked into are based on formal non-normalized definitions (including lumen, which surprised me).
  My guess is that in depends on where the unit comes from: science or day to day use.
  I learned about the Siemens, the Weber and the Gray on the way.
  Thanks again!
"This magnet has one tesla"
- That stucks ;-)
- It is not that much though. You could easily make an electromagnet with magnetic flux density of multiple tesla in it's core.
  
  And you can get 1F capacitors in bulk from China for a few dozen dollars each.
  Those things are still dangerous and scary. The 1T magnet way more so than the capacitor.
- Most neodymium magnets have like 7 Tesla at their surface.
This capacitor will cost around one Bitcoin
- We use it to ignite the tokamak.
- Actually no, depending on the voltage it costs from 5$ to 100$ Also here's 5.5v 1f capacitor
Only criticism is the use of non-metric weight units when everything else is SI-based.
- Wikipedia currently says:
  the international avoirdupois pound, [...] is legally defined as exactly 0.45359237 kilograms
  So, technically, a pound is a metric weight, only a niche one whose use may or may not be permitted by local regulations.
  Similar is true of the inch, which is defined as precisely 25.4 millimetres.
  The US, UK and a handful of others collectively signed this into their respective laws in 1959. You might think we don't use the pound in the UK any more but it still shows up often in informal situations. Ditto inches and feet.
  
  That's similar to saying "Auf Wiedersehen" translated to English is "until I see you again", therefore "Auf Wiedersehen" is technically English. Just because there's a recognised translation to a thing, that doesn't make it that thing.
  
  I would argue that a legally defined conversion to a meter or another metric value does not make a unit metric in and of itself. Those units have to adhere to the system, which is clearly based on decimal values, not just some arbitrary conversion with an absurd precision that was only signed into law to minimize the inconvenience caused by non-standard units.
- The joke wouldn't have worked as well.
  A gram is actually a pretty small unit of weight, and the joke relies on the base units. It's actually a weird little abberation in the metric system that the "base" unit is considered the kilo gram. so a 1 gram rock would be a little pebble, strangely small.
Guys you're not gonna believe this:
- pfff at 2.7V that aint much of a danger, now show me 50F @ 2kV
- Yes, I do not believe it. It must be a misprint, right?
  
  No it's real! I can't verify the exact rating since it OL's my meter, but with some circuitry it can power my Pi for a few minutes. I got them from element14, so it's unlikely to be a fake product.
However, 1 farad is really goddamn big.
Lol, explainXKCD
Ah, Randall is alive! I kept thinking my bot had broken as it's so rare for him to miss an upload.
Their names are Cueball, Megan, and White Hat?
- It is my understanding that XKCD's "characters" are somewhere between an actual character and an archetype. It isn't clear...and kind of doesn't matter, if Black Hat is the same guy in every comic or if he's a different devious schemer in each. Randall hasn't bothered to name any of them so the community has given them unofficial nicknames.
  
  Randall hasn’t bothered to name any of them so the community has given them unofficial nicknames.
  Megan is actually named in multiple comics though, ~~so is Danish (Black Hat's girlfrenemy)~~ (actually that was picked semi-arbitrarily by the community). Cueballs also have different names occasionally but they're all drawn the same.
  And actually I do believe that certain non-named characters are the same comic-to-comic. Black hat and Beret guy almost certainly are.
- I remember reading about their names in explainxkcd. I think the only one never named in the comics is Cueball.
  For a while, there was a blog, but I don't think it named any character.
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?
- A rock that weighs one stone (14 lbs).
- 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?
  
  A chain is 66 feet long.
- hogshead is my favourite
  
  Imagine what it’s like to calibrate an instrument like that.
- 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.
  
  https://en.wikipedia.org/wiki/List_of_human-based_units_of_measurement
I’d be the clueless guy in the room. “I’m not familiar with that unit of measurement.”
- 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.
  
  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
  
  So what you're saying is that if I touched that in the wrong manner I'd get blown across the room?
🚲
- this guy gets it.
Thats literally exactly what it is. They aren't derived from the metric system like all units in the system, theyre a specialized edge case where a conversion was specifically written in because america sucks. There are no other conversions in the metric system, units are derived from constants based on a set of specific rules that the pound and yard do not adhere to.
- So, you don't understand the joke, but you detected an excuse to say America Bad. Your employer must be pleased.
  The joke of this comic is based around most units in both the metric and imperial system being fairly mundane quantities. A one meter long stick is about half the height of a man, a one pound rock is about the size of his fist, a one volt battery has the potential of a dead AA.
  The Ferad is the SI unit of capacitance, equal to 1 coulomb per volt. A one ferad capacitor can hold enough energy to do a number of exotically dangerous things.
  
  Lmao what makes you think i didnt understand the joke? We're talking about units of measurement. And yeah i dont need an excuse to say America Bad. It is bro, have you been paying attention?
  Edit* my comment was meant to be a reply to another comment about the metric system, not top level. Not sure how that happened aince i clicked reply right from my inbox. Stick by what i said about america
- And I have to say, the chemistry math that resulted from the metric system is elegant. I've forgotten most of it by now, but at some point, I was able to envision the machinery behind it all, and it was beautiful.

95 comments