PFE020: Nukes

Nuclear weapons are probably the most famous invention/discovery from the physics community. Never mind things like the atom leading to all modern chemistry. Or the laser. Not to mention that nukes were only used in wartime twice. Of course, those two uses killed a ton of innocent people which is kind of hard to forget.

Avoiding the political aspects both past and present still leaves enough interesting history that it's worth discussing.

We know the theory behind it, that we can get energy from mass by E=mc². The actual mechanics are interesting too.

The first thing to cover is that everything radiates energy. Every atom, in my body, is giving off energy. Photons are radiating away from us so fast!  This is from a notion known as "black-body-radiation."

 Physicists just got chills. Everyone else got bored.

Some particles aren't content to just throw away photons, some want to do more for those around them. So they toss out big particles (alpha particles, or He nuclei), with a big mass, changing the particle to something else altogether. When a radioactive atom ejects a particle like this, it decays into smaller atoms, but more importantly, the total mass also decreases creating a huge amount of energy.

The rates at which particles do this vary wildly. Some atoms will break apart in less than a second, while others will hang around for thousands of years. To make a bomb, you need something in between. For a given atom, there is a "critical" set of conditions (amount of the element and how close together they are) that will lead to a nuclear reaction.

All of these processes can be sped up. As atoms eject alpha particles, these alpha particles can hit other atoms and make them decay too. So if you pack a bunch of these highly radioactive particles together, they will create a huge explosion from the chain reaction.

There are a number of different bomb designs and I won't go into too much detail on them as they are still classified in some sense of the word and I don't want any budding terrorists pointing the finger at me.

The most basic design (and believe me, it is anything but easy to build) is a standard fission bomb. In this case, you take a sub-critical radioactive sphere (one that won't blow up in your hands, although carrying it around with you as you eat lunch is probably still a bad idea) and place explosives all around it, and set them to all go off at exactly the same time. This squeezes the ball in, it goes critical, and boom. This is the design of the fat man bomb dropped over Nagasaki on August 9, 1945.

The first bomb dropped on Japan, over Hiroshima on August 6,

was called the little boy, and was an example of the gun assembly model. In this case, there are two radioactive parts, one is a sphere with a hole bored half way through it, and the other the bullet. At the opportune moment, the bullet is shot down the length of the bomb into the core sending the bomb past the critical requirements.

Most nuclear bombs today are what is known as "hydrogen bombs", although this is a bit of a misnomer. They are typically two stage devices, the first of which is a standard fission bomb like the fat man. The second stage uses a fusion rod, along with a fission core, to greatly expand the effect of the blast. The name hydrogen bomb comes from a "booster" that is used to increase the effectiveness of the standard fission reaction. The problem is that if the fission material explodes outward too rapidly, it won't all undergo fission. The booster material is typically "heavy" hydrogen - deuterium or tritium.

For comparison, the fat man was between 50% and 100% more powerful than the little boy. A hydrogen bomb (depending on the model) is 450-600 times more powerful than the fat man.

That's nukes.

PFE019: Static Electricity

I was shocked at least 740 times in the last 24 hours. Seriously, what's up with this?

There are two things that have to happen to get shocked. This first is that you have to build up a charge on your body. Anyone who has ever a) lived anywhere with a real (or even moderate) winter or b) played with a balloon knows, you can build up a charge on your body by rubbing things together. Shuffling your feet on the ground is a great example of this. How much charge you can build up depends on each material being rubbed together. (This sounds like a perfect do-it-at-home: find out what works the best.) Either way, a charge builds up on your body and since water (remember we're mostly water) conducts electricity somewhat better than air, all of this extra charge builds up on the surface of your skin.

But here's the deal, all of these like charges next to each other want to repel each other so they'd love to jump off your skin onto something else, hopefully something metal, which will conduct them away in no time. Air is working very hard to stop it.

As we all know, of course, we're more likely to be shocked sometimes than others right? Not only is friction important to build up that charge, a low humidity is important to keep it. That is, water will suck away a charge since it is alright to be slightly charged or not. So on humid days it is much harder to hold a charge, but when it's really dry out [like today apparently] charges will build up no problem all the time.

As a side note, as air increases in temperature the total amount of H₂O that the air can hold onto also increases. That's why the air is typically dryer in the winter than the summer.

But just having a charge doesn't really do anything. We feel a "shock" when all of this charge finally is released into a metal. When this happens, a lot of energy is released at once. So why aren't huge arcs flying out of me each time I drag my feet on a dry day (that never happens, dry or humid)?

The "electrical breakdown" in air happens at about 3,000,000 volts per meter. Three million volts? If you've ever licked a 9 volt battery (bad idea kids) you know that you can get a sizable tingle just from that. But three million is way more than nine. So how come we don't all fry ourselves every time we touch a door knob? What electrical breakdown really means is that to get a meter long arc we need 3,000,000 volts of separation between us and the door knob. God help us if we ever shuffle that much. Typically breakdown occurs at about one millimeter sending somewhere around three thousand volts.

Still, 3000 looks like a lot of volts. I mean, it's a lot of batteries lined up. And it is. That's why, on a large enough shock, if you're not paying attention, you'll jump at that instant. It's a lot of energy released all at once. But there's the key. It's all at once. There is no steady flowing of charge running through you at three thousand volts, so nothing really heats up from the spark.

Shocking yourself on a door knob is, of course, the same principle as lightning strikes.

 We all know lightning looks like this.

A charge separation is built up between the clouds and the ground, and once it's enough, lightning strikes and a huge amount of energy is released. Of course, this is enough to fry you so don't get hit by lightning.

I imagine you all shuffling around on the carpet touching door knobs. (Interestingly I can get a spark at all in my room while I couldn't touch anything metal without one today. I think it was my shoes.)

Thanks Josh for the topic.

That's static electricity.

PFE018: Water

I'm pretty fond of water. I drink quite a bit [of water] every day. Why this affinity? Perhaps it's because I am, in most senses, water.

Or perhaps it's because water is not only super common on earth, but also has some really unique properties.

This post is a little long, but it's been awhile and seriously guys, I love water.

Water, as we all know, freezes at 32^oF and evaporates at 212^oF (Fahrenheit because I only understand Celsius abstractly anyways). This means that water is found in solid, liquid, and gas forms on earth all the time which is super convenient (because each is helpful in different ways).

Also, because of its shape (the fact that two hydrogen atoms are attached to an oxygen atom, but aren't directly opposite from each other

as you might expect. While the effects of this are not immediately obvious, it turns out that it means that water is great at storing heat. This is both a blessing and a curse in the sense that heat can be transferred really easily by water which is great because there's so much of it and so it is great for cooling. On the other hand, when we try to heat up water for our pools/showers, it is very difficult to do so.

Those were some of the technical or precise things that make water so interesting. But let's look at some practical things that are a little bit more difficult to model by conventional means.

We all know that water evaporates at 212^oF, but if we think for a second we can realize that this isn't actually true. Of course, when we boil water, the water at the bottom of the pot hits 212^oF. But this isn't the only way water evaporates, because water "dries up" all the time even though it never hits 212^oF outside. So where's the mix-up? Any official explanation in a physics or chemistry text book will start talking about relative vapor pressures which always seemed unnecessarily confusing to me. The interesting phenomenon that is occurring here has to do with the aggregate behavior of water. When we say that water is at 73^oF, that is to say that the water has an average temperature of 73^oF, but some water molecules may have a lower temperature and others a higher. And sometimes, when a molecule has a really high temperature and is near the surface, it will fly off into the air and evaporate. The rates at which these happen depend on a bunch of things including the temperatures of the water and the air, the humidity of the air, what kind of gunk is mixed in with the water, and probably the day of the week. But what we do know is that it happens.

Perhaps more interesting that water evaporating below it's supposed to, is looking at the freezing point. Although president Leebron seems to think that water turns to ice at 32^oF people from the north (such as, you know, me) know that this isn't really the case. That is, in practice, it requires a colder temperature than 32^oF. See, when water freezes into ice it forms these wacky crystals.

This stuff is crazy.

All on its own! But when it comes to freezing water, this doesn't happen easily. So any movement in the water and it basically loses all progress and has to start over. This structure is what gives rise to snowflakes and I don't even need to tell you beautiful they are.

Ok, maybe I do if you didn't grow up in the North.

If you're interested in some do-it-yourself science involving explosions, you can easily separate water into hydrogen gas and oxygen gas. While this experiment is very easy to do, it is also very easy to get carried away so I won't sort out the details here, but you can find them easily on the World Wide Web but please don't blow yourself up.

That's water.

PFE017: Centrifugal Motion

The notion of a centrifugal force is often rather poorly understood. In high school, I was told, explicitly, that there is no such thing as a centrifugal force. Unfortunately, I passed this information on to others before I was corrected.

A centrifugal force is only felt from the point of view of someone moving in a circle. A car going around a turn. One of those carny rides.

You feel... pushed outwards. It feels like, if there were no wall, or side to your car, you might just go flying straight out. So there must be some force going out.

At this point, some people might tell you that there is no such force. None of the four fundamental forces (gravity, electromagnetism, strong and weak nuclear forces) can be tied to the present phenomena.

In general, physics is usually conducted in what is known as an "inertial reference frame" or a non-accelerating reference frame. An accelerating reference frame would be from when you step on the gas until your car maxes out its speed. Or, for example, on a spiny carny ride.

The reason why these situations tend to be avoided is because they add unnecessary complications. The study of forces is the study of accelerations, and adding additional accelerations adds a sort of "fictitious force", although I find that term is rather misleading simply because we are on the earth. And the earth rotates on its axis. The the earth orbits the sun. And the sun orbits the galaxy. So clearly we are in a non-inertial reference frame.

Effects from the earth spinning are measurable, in theory, but small. The main practical difference is that if you hang a plumb bob (weight on a string) it will deflect from the center of the earth. That is, the direction that we think of as "down" is not exactly toward the center of the earth as we would predict from gravity. This can give rise to deflections of nearly a tenth of a degree depending on latitude (or about 2 inches in 100 feet) (more extra credit! (pdf)). So then why don't buildings fall over all the time? Simply put, the forces felt on the plumb bob, "fictitious" or not, are the same felt throughout the whole building.

PFE016: Fermions (Halloween edition)

My Halloween costume this year focused more on execution than actual appearance. Jeff and I were identical fermions, specifically electrons.

This may be a little bit outside the scope of this blog, but is fun anyways and shows some of the truly bizarre properties of physics.

As you may or may not be aware, everything that you see around you is made up of particles. Teeny little things that each have different properties. Most of your regular everyday stuff is protons, neutrons, and electrons. Light is also a particle, (sort of). There are many more, enough that in the 60s the term "particle zoo" was coined to describe all of them.

All of these particles can be classified into either fermions or bosons based on their spin. Spin should not be thought of as anything like a baseball spinning, but rather as simply a property of the particle. For example, protons, neutrons, and electrons are all fermions while photons (light particles) are bosons.

One of the main properties of fermions is that two identical fermions cannot exist in the same state at the same time. So two electrons cannot be at the same energy and the same spin direction. Electrons can be spin up or spin down so, for Halloween, Jeff and I wore the same thing and were never in the same room unless one was standing and the other sitting (spin up vs. spin down).

$$|\downarrow\rangle_{PD}\otimes|\uparrow\rangle_{JM}\quad\quad\quad\quad=\quad\quad\quad\quad|\uparrow\rangle_{PD}\otimes|\downarrow\rangle_{JM}$$

I included the relevant braket notation for the physicists present. It is interesting to note that each state is identical. That is, in the eyes of physics, there is no way to differentiate between electrons. So one of them spin up and the other spin down is entirely indistinguishable from when the spins are flipped.

Those are some awfully scary fermions.