Deimos and I were discussing the use of "ask" topic tags this week, and we agreed it might be a good idea to get a consensus on these.

At the moment, Tilders are using four "ask" tags on topics:

• ask

• ask.survey

• ask.recommendations

• ask.help

(There may be more "ask" tags created in the future, but these four are what we're all using at the moment.)

Anything that's a question gets tagged with "ask". Some specific types of question will then get tagged with "ask.survey" or "ask.recommendations" or "ask.help", depending in the type of question being asked.

• "ask.survey" is for questions about preferences and favourites. "What's your favourite horror movie?" "What's the best place you ever visited?" "What's your favourite type of holiday?" The asker is collecting data about people's likes and dislikes (even if they're not going to publish the results in a report later!).

• "ask.recommendations" is for questions asking for recommendations. "What's a good browser to use?" "What book should I read next?" "Which brand of phone should I buy?" The asker is looking for people to recommend things to them.

However, Deimos and I wondered about "ask.help". One interpretation we came up with was that "ask.help" is for questions looking for a specific answer, where it should generally be possible for people to think "yes, this is the right answer to the question". This would include questions seeking help learning about an academic topic, such as happens in /r/AskScience and /r/AskHistorians over on Reddit. Another interpretation we came up with was that "ask.help" is for questions looking for guidance on doing something, like a "how to" type question. This would be more like the types of questions in /r/Help and like the Help menus in software and the F1 key - helping people get things done.

What do you think about the "ask" tags? In particular, what should the "ask.help" tag be used for? In general, are the existing "ask" tags okay? Do we need more "ask" tags? Do we need different "ask" tags?

"How could a low-budget, black and white movie from the 60s possibly scare be that scary." I thought to myself as I was purchasing my ticket for the movie. I was going to see the movie, because I had always heard that it was a good movie. I thought that this movie couldn't possibly be scary, so it had to have other merit to have it be considered a good movie.

I left the cinema that day quite spooked. I was amazed at with all of it. The true horror weren't the wondering ghouls, but the interaction between the people inside the house. It was a social experiment more than anything else. What would happen if six people were placed inside a house, with a wounded child, and impending doom closing in on them? This is the question answered by the movie. A power struggle between Ben and Harry, about the safest place in the house to hide, led to the death of Harry. Everybody sided with Ben, and in the end he only survived the longest because he hid in the cellar as Harry had suggested from the beginning.

TL;DR
I found the fighting between the survivors in the house to be very spooky.

Introduction

I want to give an introduction on several physics topics at a level understandable to laypeople (high school level physics background). Making physics accessible to laypeople is a much discussed topic at universities. It can be very hard to translate the professional terms into a language understandable by people outside the field. So I will take this opportunity to challenge myself to (hopefully) create an understandable introduction to interesting topics in modern physics. To this end, I will take liberties in explaining things, and not always go for full scientific accuracy, while hopefully still getting the core concepts across. If a more in-depth explanation is wanted, please ask in the comments and I will do my best to answer.

Previous topics

Bookmarkable meta post with links to all previous topics

Today's topic

Today's topic will be light emitting diodes, better known as LEDs. As the name suggests, we'll have to discuss light and diodes. We will find out why LEDs can only emit a single colour and why they don't get hot like other sources of light. Let's start by discussing diodes, in case you are already familiar with diodes note that I will limit the discussion to semiconductor (p-n with a direct bandgap) diodes as that's the type that's used in LEDs.

What's a diode?

A diode is an electronic component that, ideally, only lets electric current through in one direction. In other words it's a good resistor when the current flows in one direction and a really good conductor when the current flows in the other direction. Let's look a bit closer at how diodes function.

Semiconductors

Diodes are made out of two different semiconducting materials. In everyday life we tend to classify materials as either conducting (metals being the prime example) or non-conducting (wood, plastics, rubber). Conductance is the flow of electrons through a material, a conducting material has a lot of electrons that can move freely through a material while an insulator has none. Semiconducting materials fall in between these two categories. They do conduct but not a lot, so in other words they have a few electrons that can move freely.

N-type semiconductors

We are able to change a semiconductor's conductivity by adding tiny amounts of other materials, this is called doping. As an example, we can take silicon (the stuff that the device you're reading this on is made out of) which is the most well-known semiconductor. Pure silicon will form a crystal structure where each silicon atom has 4 neighbours, and each atom will share 1 electron with each neighbour. Now we add a little bit of a material that can share 5 electrons with its neighbours (how generous!). What will happen? Four of its shareable electrons are busy being shared with neighbours and won't leave the vicinity of the atom, but the fifth can't be shared and is now free to move around the material! So this means we added more freely flowing electron and that the conductivity of the semiconductor increases. An illustration of this process is provided here, Si is chemistry-talk for silicon and P is chemistry-talk for phosphorus, a material with 5 shareable electrons. This kind of doping is called n-type doping because we added more electrons, which have a negative charge, that can freely move.

P-type semiconductors

We can do the same thing by adding a material that's a bit stingy and is only willing to share 3 electrons, for example boron. Think for a moment what will happen in this case. One of the silicon atoms neighbouring a boron atom will want to share an electron, but the boron atom is already sharing all of its atoms. This attracts other electrons that are nearby, one of them will move in to allow the boron atom to share a fourth electron. However, this will create the same problem elsewhere in our material. Which will also get compensated, but this just creates the same problem once more in yet another location. So what we now have is a hole, a place where an electron should be but isn't, that is moving around the crystal. So in effect we created a freely moving positive charged hole. We call this type of doping p-type. Here's an illustration with B the boron atoms.

Creating a diode

So what would happen if we took a n-type semiconductor and a p-type semiconductor and pushed them against one another? Suddenly the extra free-flowing electrons of the n-type semiconductor have a purpose; to fill the holes in the p-type. So these electrons rush over and fill the holes nearest to the junction between the two semiconductors. However, as they do this a charge imbalance is created. Suddenly the region of p-type semiconductor that is near the junction has an abundance of electrons relative to the positive charges of the atom cores. A net negative charge is created in the p-type semiconductor. Similarly, the swift exit of the electrons from the n-type semiconductor means the charge of the cores there isn't compensated, so the region of the n-type semiconductor near the junction is now positively charged. This creates a barrier, the remaining free electrons of the n-type cannot reach the far-away holes of the p-type because they have to get through the big net negative charge of the p-type near the junction. Illustration here. We have now created a diode!

How diodes work

Think for a moment what will happen if we send current* (which is just a bunch of electrons moving) from the p-type towards the n-type. The incoming electrons will face the negative charge barrier of the p-type and be unable to continue. This means there is no current. In other words the diode has a high resistance. Now let's flip things around and send electrons through the other way. Now they will come across the positive charge barrier of the n-type semiconductor and be attracted to the barrier instead. The electrons' negative charge compensates the net positive charge of the barrier on the n-type and it will vanish. This destroys the equilibrium situation of the barrier. The p-type holes are no longer repelled by the positive barrier of the n-type (as it no longer exists) and move closer to the junction, this means the entire barrier will fade and current can move through. We now have a conductor.

OK, but I don't see what this has to do with light

Now let's find out how we can create light using this method. When current is applied to a diode what happens is that one side of the diode is at a higher energy than the other side. This is what motivates the electrons to move, they want to go from high energy to low energy. If the p-type semiconductor is at a higher energy than the n-type the electron will, upon crossing the junction between the two types, go from a high energy level to a lower one. This difference in energy must be compensated because (as @ducks mentioned in his thermodynamics post) energy cannot be destroyed. So where does the energy go? It gets turned into light!

The energy difference between the p-type and n-type is fixed, meaning a fixed amount of energy is released each time an electron crosses the junction. This means the light is of a single colour (colour is how we perceive the wavelength of light, which is determined by the energy of the light wave). Furthermore, none of the energy is lost so there is no energy being turned into heat, in other words the LED does not get warm.

Conclusion

So now we know why the LED is so power-efficient; it does not turn any energy into heat, it all goes into light. We now also know why they only emit a single colour, because the energy released when an electron crosses the junction is fixed.

Next time

I think next time I will try to tackle the concept of wave functions in quantum mechanics.

Feedback

As usual, please let me know where I missed the mark. Also let me know if things are not clear to you, I will try to explain further in the comments!

Addendum

*) Yes, current flow is defined to be opposite to the flow of the electrons, but I don't want to confuse readers with annoying definitions.

I was wondering where one can find (and submit) how long it takes to finish a game.

After a quick online search I did find one or two pages where petiole post this, but none looked really good. So I'm wondering if more regular gamers here have a recommendation.

Intro

Hello everyone,

@wanda-seldon has been giving us an introduction to quantum physics. For now, she will be given a short break to prepare new stuff. In the meantime I will be covering some classical mechanics, more specifically thermodynamics. In part 1, we need to work our way through some of the more dry concepts, so we can understand and appreciate the horrifying implications of the fun parts. So I promise, this will be the most verbose one.

Some of you may have briefly seen a version of this posted, that was due to me misunderstanding the schedule with @wanda-seldon. If you saw that one, I will mention I rewrote nearly all of it to be more readable.

Now, on today's agenda: The basics of heat, work and energy and how it's all related.

Previous posts can be found here: https://tildes.net/~science/8al/meta_post_for_a_laypersons_introduction_to_series

Important note

If @wanda-seldon in her posts mention "energy", it's most likely in the context of energy operators, which is a concept in quantum physics. I'm not going to pretend I understand them, so I will not be explaining the difference. We will cover what energy is in classical mechanics. So keep that in mind if you read something from either of us.

Subject

Summarized

What is heat? Using a lot of fancy words we can describe it as follows. Heat is an energy that is transferred between systems by thermal interaction. And what is work? Work is an energy that is applied in a way that performs... work. The combined energy in a system is called internal energy. This type of energy can be transformed or applied to other systems.

These are a lot of new words, so lets break that down a bit.

Systems

A system is just a catch-all term for something that can be defined with a boundary of sorts. Be it mass, volume, shape, container, position, etc. A canister, your tea mug, the steam inside a boiler, your body, a cloud, a room, earth, etc. They are all systems because you can in some way define what is within the boundary, and what is beyond the boundary.

In theory, you could define every single nucleid in the universe as an unique system. But that would be counter-intuitive. In thermodynamics we tend to lump things into a system, and treat it as one thing. As opposed to Quantum stuff that looks at the smallest quantity. Calculating every single water molecule in my coffee would be pure insanity. So we just treat my mug as the boundary, and the tea inside the mug as the system. And just so it's mentioned, systems can contain systems, for instance a tea mug inside a room.

Energy

Energy is some quantifiable property that comes in either the form of heat, work. It can be transferred to other systems, or change between the different energy types. An example of transfer is my coffee cooling down because it's in a cold room. That means heat has been transferred from one system (my mug) to another system (the room). Alternatively you could say my hot coffee mug is warming up the room, or that the room is cooling down my coffee. Thermodynamics is a LOT about perspective. An example of transforming energy types is when we rub our hands together. That way we convert work (rubbing) into heat. It's really not more complicated than that. An interaction in this case is just a system having an effect on a different system. So a thermal interaction means it's an interaction due to heat (like in the mug example).

This brings us to an extremely important point. So important, it's considered "law". The first law of thermodynamics even. Energy cannot be destroyed, it can only change forms.

Your battery charge is never really lost. Neither is the heat of your mug of coffee. It just changed form or went somewhere else. The combined energy of all types that is residing inside a system is called internal energy.

Heat and work

Let's say we have a system, like a room. And all windows and doors are closed, so no energy can leave. In this system, you have a running table fan connected to a power line, getting energy from outside the system. The table fan is making you feel cool. Is the fan cooling down the room, heating up the room, or doing nothing? Think about it for a moment.

http://imgbox.com/CKtQLLOQ

The first thought of many would be to think that this fan would cool the room down, it sure makes you feel cooler! But it's actually heating up the room. As we remember, internal energy is the energy inside a system (room, in this case). The fan is getting energy from outside, and uses this energy to perform work. The fan accelerates the air inside the room, and this accelerated air will evaporate some of your sweat, so you feel cool. But as we remember, energy cannot be destroyed. So we are importing energy into the system, increasing the internal energy. Some of the work from the fan is also directly converted to heat, since the motor of the fan will get hot.

So if we are not getting rid of any of this excess energy, we are increasing the internal energy. And therefore actively increasing the temperature of the room.

http://imgbox.com/SAtqk7YG

To use a more tangible example: Simplified, this phenomena is why green house gases are bad. Lets define earth as a system. Earth gets a lot of energy from the sun. And a lot of this energy will be reflected and sent back to space. Green house gases will reflect back some of this energy trying to leave earth. So instead of having a roughly equal amount of energy enter the system (from the sun, from us doing stuff, etc) that leaves out in space, we have an increasing amount of energy on earth. This, as a consequence, increases temperature.

Implications

Now, what are the maybe not so obvious implications of this?

Waste heat, from supplied energy or inefficient work is a constant headache in engineering. If we cannot remove enough heat, we will actively heat up objects until they are destroyed. Thats why good cooling systems are important in cars, computers, etc.

Whats next?

Now this was not so bad. In the future we will cover phase changes, equilibriums, entropy, the heat death of the universe and briefly touch upon engines. So thats most likely two more parts after this. After that @wanda-seldon will take over again.

I plan on doing one main part per week, but if something is asked that warrants a small topic I might do smaller ones inbetween.

Feedback

Something unclear? Got questions? Got feedback? Or requests of topics to cover? Leave a comment.

I find myself on a bit of an unending quest to organize my own thoughts, especially since my work evolved into multiple streams on different projects.

I have been looking for a tool to help me organize myself and focus on the things I want to do. More specifically, I keep wanting to improve my ability to remember things: Be able to remember faster, longer, recall more reliably, categorize, filter and export those things, etc.
Links, reading material, "watch later" material, todo lists, contacts, phone numbers/emails, identities, what I know about people, reminders, highlights, emails to respond to, work logging, etc. The more I think about it, the more I have this need for a tool that essentially acts as a permanent second brain.

I feel like I've tried everything. Note-taking apps like Keep, orgmode, wikis, journals, disorganized text files, issue trackers, Pocket, gmail itself, calendar reminders, even Magic. Nothing quite works. The issues I most consistently hit are:

• The method is not good enough at ingesting abstract data. Examples: Anything calendar-bound is not good at storing anything that isn't related to a point in time. Pocket cannot store things that aren't links to web pages.
• The method is far too cumbersome to be able to braindump into it or too impractical to retrieve data from. Examples: Wikis, Keep and other object-based note-taking systems are unfilterable unless you take a ton of time to attach a lot of metadata to each note. Magic is too asynchronous as you sometimes wait several minutes for responses (and it also gets far too expensive to use at the level I'd like).

Despite trying everything, I don't know if I want to build that tool myself, because I think it probably already exists somewhere (and it might be down to me not knowing how to use the things that are already out there). Although if someone does feel inspired to build that, hit me up. :)

My current flow looks like a frankenstein mix of Keep/Gmail/Calendar, which at least integrate with one another, and a ton of proprietary or dissociated methods (including Pocket, Discord, Spreadsheets/Drive, Magic, Kayak, 1Password and a ton of duplicate files and documents). Then it just becomes a matter of remembering what type of information is where, and how to best find it.

So Tildes, what do you use?

I'm rather sleepy, generally very reserved when it comes to sharing my work, and not a native user of English, but I have a couple poems in English, and I though I'd share one here and see what the folks think of it. I love the challenge of writing stuff in languages other than my native tongue.

a bird with no wings
a song no one sings
a sorrow when time brings
         nil.
ex nihilo nihil fit
et words have no wit
mouth knows only to spit
         nil.
time is scarse and gods wobble
in vain hurry naive men hobble
ignoring they will only nobble
         nil.

From the environmental standpoint shrinking of human population is often quoted to have desirable effects, and that's reasonable. But from the point of view of our daily lives and functioning of the human society, what negatives could we then expect? (I mean a soft decline due to lower birth rates, not some abrupt events.)

For example, with smaller population fewer music albums could be made every year than some time before, and people would maybe feel less inspired and satisfied. Less scientific research, less choices for relationships... and maybe other things? Would being more technically advanced compensate for the issues? Won't we feel ourselves in oblivion and romanticize the "numerous" past?