**Ask a monkey a physics question thread**

[wufwugy]

Naw I'm pretty sure you're right. It was a confusion I had because in class it was presented as if the entire quantity had to be 100C for the phase change to begin. However, this doesn't pass the logic test, where you can say look at the ocean, parts boil while the rest doesn't. Maybe I misunderstood what was said in class, but in my defense my teacher was not good.

[OngBonga]

Well it's a chemistry question so probably he won't mind too much.

It's totally physics, not chemistry. No chemical reaction is taking place, just a transition from liquid to gas.

[Poopadoop]

a transition from liquid to gas.

I always thought that was chemistry, but if not then oops.

[OngBonga]

I stand to be corrected, by I thought chemsitry was the study of chemicals and how they react with one another, while physics is the study of matter and energy. We're talking about a change of state of matter, change of density, pressure, heat transfer... this is all matter and energy, there's no checmical bonding or breaking taking place. The gas is still H20, the molecule remains intact.

[Poopadoop]

Yup I get your point. I seem to remember being taught about phase transitions in chemistry class though. I guess that doesn't mean it's necessarily chemistry by definition.

[MadMojoMonkey]

Hopefully, in your chem lab, you were constantly stirring the ice/water to keep the water well mixed. Also, you were probably using filtered, de-ionized water.

Impurities in the water make a tiny difference in the boiling temperature. Atmospheric pressure changes the boiling temp., too.

In a pot of just normal kitchen water on the stove, the entire pot is ~100C at the time when the water is vigorously boiling, but prob. ~ 99C+ once the water is boiling at all.

If you really watch as the water first begins to boil, and if the pot is deep enough, you may see a bubble rise up from the bottom of the pan, and shrink to nothing before it reaches the top. This is, like poopadoop said, due to the fact that temperature near the heat source is usually greater than temperature far from the heat source. Water is a liquid, obv, so the heated water at the bottom will be displaced by cooler, less dense water at the top. This results in "pretty good" mixing due to convection w/o any added stirring on your part. Once the water is boiling, the rising bubbles make an increasingly capable stirring mechanism.

Nonetheless, unless you're talking about just water, no "large" solids, and constant stirring, then w/o the boiling rigorous enough to be effectively doing the stirring for you, then you can't really say with certainty that the whole pot is 100C.

[wufwugy]

Cool thanks.

I guess the next step is to see if I want to start brewing at the recommended 96C instead of the ~99C I've been doing.

[OngBonga]

Cool thanks.

I guess the next step is to see if I want to start brewing at the recommended 96C instead of the ~99C I've been doing.

Bear in mind that the act of pouring the water from the kettle causes a temporary but large increase in surface area, and thus an increased rate of heat transfer between the hot water and the cool atmosphere. Point being, there's very little chance that the water is ~99 by the time it's in your cup.

[wufwugy]

Bear in mind that the act of pouring the water from the kettle causes a temporary but large increase in surface area, and thus an increased rate of heat transfer between the hot water and the cool atmosphere. Point being, there's very little chance that the water is ~99 by the time it's in your cup.

See, I was totally gonna ask about this, but I thought that the answer would be too dependent on things.

Is there some rule of thumb for how quickly, say, a cup of boiling water cools down once pulled off burner or during the pour?

[OngBonga]

Quantifying this into "rule of thumb" numbers is well out of my depth, sorry! Maybe mojo can! It would just use intuition, and assume that it's gonna be cooled closer to ~95 degrees in the time that it took me to pick the kettle up, and then pour the water. If you really want numbers, well experiment. Get yourself a thermometer, and measure, taking note of how long the water was off the boil. Be consistent with height and rate of pour, and you'll be able to figure it all out in no time!

[OngBonga]

It'll be dependant mostly on the temperature of the atmopshere, which will effect the rate of heat trasfer as the water pours. That's where I'd expect the largest variables to be.

*edit - obviously the time it's sitting idle after boiling is the most important factor, I'm thinking here of secondry factors. I think atmosphere temperature will be more significant than height of pour, or pressure, or other factors that I'm overlooking.

[Savy]

See, I was totally gonna ask about this, but I thought that the answer would be too dependent on things.

Is there some rule of thumb for how quickly, say, a cup of boiling water cools down once pulled off burner or during the pour?

Boil your kettle, wait 30 seconds and it'll be ok for coffee.

Naw I'm pretty sure you're right. It was a confusion I had because in class it was presented as if the entire quantity had to be 100C for the phase change to begin. However, this doesn't pass the logic test, where you can say look at the ocean, parts boil while the rest doesn't. Maybe I misunderstood what was said in class, but in my defense my teacher was not good.

If you took 100ml of water and heat it up assuming some bits get hotter than others faster then if you look at the localised spots and say there's a spot of 10ml that is 100C then why would that 10ml not be boiling? It would in isolation.

There may fully well be a reason but makes sense to me.

[MadMojoMonkey]

Chart shows dropping water onto a heated surface, not bringing up the temp of water in a container.

At 100C water exists as either a liquid or a vapor, and the factors which determines this are the mass of water, the pressure in its container, and the amount of heat (energy) which has been added. If you're trying to apply a whole lot of heat really quickly, then the specific heat of water can come into play, or even the Leidenfrost Effect (called film boiling in the diagram).

Water at 100C doesn't boil unless you keep putting energy (heat) into it. It can't rise above 100C until it's a vapor, so it will exist in both phases until enough heat is added to vaporize the entire mass. Then the temp. can rise above 100C. Assuming 1 atm of pressure.

[OngBonga]

So I have a silver 2p in my coin collection. Turns out that it could be worth a lot of money.

Very rarely, a blank ten pence coin finds its way into the two pence press, resulting in a silver, instead of copper, 2p. These have sold to collectors for up to £1400.

More likely, it's been plated outside of the Royal Mint, probably a fake.

The coin is dated 1995. Back then, ten pence coins were made from 75% copper, 25% nickel. Have you any idea how it can be tested, from home, without damaging it to determine its composition?

It's worth noting that the coin appears to have small amounts of rust on the surface. Considering there was no iron in ten pence coins in 1995, this doesn't look good. Does nickel readily oxidise? Could it be copper and not rust? I should note that it was a Cu/Ni alloy, not nickel coated copper, so it's not likely the nickel has worn away to expose copper.

I can send it to the Royal Mint Museum to have it tested for free, but they tell me it'll take up to 8 weeks. They also warn me that the vast majority of coins the wrong colour (98%) were coated outside of the Royal Mint, meaning they are worthless. So I'm not optimistic. But it's definitely worth looking into.

[MadMojoMonkey]

Check the density of the coin
Compare to expected density of silver (10.49 g/cm^3)
Profit

The easiest way to do this is to have or fashion a scale which can measure tenths of a gram or better.
Yes, you can make a DIY balance scale with common household items which can measure those tiny amounts.

Next you need a plan. Your mission is to compare the mass of your coin to the mass of an equal volume of water. The most common way to do this for objects that sink is to
1) find 2 containers which hold water
2) fill one of the containers and position it such that if you add anything at all to the full cup, it will overflow into the other cup.
3) Measure the mass of the empty cup or put it on the scale and TARE the scale (zero it).
4) GENTLY place your coin into the full cup, so that it spills its volume's worth of water into your empty cup.
5) Measure mass of displaced water. If you didn't TARE the scale, then take this final measurement minus the initial measurement to find the change in mass due to the displaced water. If you did TARE the scale, then it's reading this amount directly.
6) Retrieve your coin, thoroughly dry it off and measure its mass. (Prob should have done this first, TBH, sigh)

Since the density of water is 1 g/cm^3, then simply dividing the mass of the coin by the mass of the displaced water is your goal.

If it's very close to 10.49, then prob silver or at least worth checking further.


***
However... Unless you have a beautiful, perfect, sharp relief coin which has NEVER been in the sun or in a pocket or had any thermal or mechanical stresses applied to it, then don't expect to get much more than the 2p of value from it. Coin collectors are the most tedious perfectionists in the world. If it's not just a mint coin, but an exceptionally perfect mint coin, they're not going to shell out big monies for it.

[OngBonga]

Sorry when I say silver, I mean in colour, not metal. It's definitely not Au (far too valuable to make 2p or 10p coins out of). It's either junk (probably steel), or it's a copper/nickel alloy (not junk). What would the expected density of Cupro-Nickel at 3:1 be?

6) Retrieve your coin, thoroughly dry it off and measure its mass. (Prob should have done this first, TBH, sigh)

Haha I didn't fully read your post before starting to reply... I considered how to measure density, thought about sinking it in water and measuring the displacement to give us its volume, then thought about weighing it on a float (again, displacement), and realised it need to be dry, so I should weigh it first. Then I read your instructions!

I think I can make water scales. I'm aware the accuracy is down to how accurately I can measure the displacement, and I'm aware it needs to be dropped carefully to minimise ripples overflowing.

I think the codition of the coin would mean it simply won't reach its maximum value, but I suspect it's rare enough in any condition (assuming it's genuine) to be valuable.

[OngBonga]

Sorry when I say silver, I mean in colour, not metal. It's definitely not Au. It's either junk (probably steel), or it's a copper/nickel alloy (not junk). What would the expected density of Cupro-Nickel at 3:1 be?

I suppose I can find this out by weighing a normal ten pence piece.

[OngBonga]

Well... I have some digital scales for checking my weed that weighs to 0.1g... they're not very accurate because it weighs a 2p at 7.1g (I know it's 7g because I'm a stoner and 2p coins are regularly used to weigh a quarter of an ounce).

A ten pence is coming up 6.6g.

My silver 2p is... drumroll please... 6.6g.

I think I'm just going to send it to the Mint for analysis, based on that alone. My scales are accurate enough to tell me that it is approximately the same weight as a 10p, not 2p. I can't be fucked to create a water scale when I have dodgy Chinese import digis telling me what I wanted to see.

[Poopadoop]

Well... I have some digital scales for checking my weed that weighs to 0.1g... they're not very accurate because it weighs a 2p at 7.1g (I know it's 7g because I'm a stoner and 2p coins are regularly used to weigh a quarter of an ounce).

A ten pence is coming up 6.6g.

My silver 2p is... drumroll please... 6.6g.

I think I'm just going to send it to the Mint for analysis, based on that alone. My scales are accurate enough to tell me that it is approximately the same weight as a 10p, not 2p. I can't be fucked to create a water scale when I have dodgy Chinese import digis telling me what I wanted to see.

The nice thing about a dodgy scale is it tends to be off by roughly the same amount every time. So if it says the 10p coin and your rare coin weigh the same, I'd be pretty confident.

Here's another question though: Mint coins fetch more than rusty and/or worn ones. You can get rid of the rust, but how does your coin look underneath it? Are all the ridges and other details still clear or are they worn down?

[OngBonga]

I can probably get rid of the "rust", but I need to first identify the "rust". If it's iron oxide, the coin has iron in it and it's worthless.

The "rust" needs to be nickel oxide or copper oxide. I would assume the methods for removing these rusts would be different.

[OngBonga]

I need to get it analysed by experts. I can do that by sending it to the Royal Mint Museum, and they'll analyse it for free. They'll also provide a certificate of authenticity it if's genuine, which will make it much easier to sell.

Once I get their results, I think then I'll then look into how to clean it.

[MadMojoMonkey]

I can probably get rid of the "rust", but I need to first identify the "rust". If it's iron oxide, the coin has iron in it and it's worthless.

The "rust" needs to be nickel oxide or copper oxide. I would assume the methods for removing these rusts would be different.

Iron oxide is red/orange - the familiar rust color
Copper oxide is green - the Statue of Liberty is made of copper
Nickel oxide is pale green or black - depending on the number of oxygen atoms bonding to the number of nickel atoms - It can appear a dull orange if some non-annotated pics on various web pages on the topic of nickel oxide are to be trusted.

Nickel does oxidize, but it's a slower process than iron or copper. Typical rare-earth magnets are a ceramic with a nickel coating. I don't have any rusty magnets, and I've had them for years. Granted, they've been in a much gentler environment than a coin in circulation.

WD-40 or Naptha (commonly sold as Zippo fluid or in the paint thinner aisle of a hardware store) should remove Nickel Oxide. I just read that, and I'm not sure if either of those reacts with copper oxide or iron oxide.


It's worth noting that there is going to be some impurity metal (often copper) in silver. Pure silver is too soft for daily-use objects. A slight addition of copper helps to stabilize the crystalline structure, adding strength while preserving color. Jewelry grade (Sterling) silver is 7.5% copper by weight. I'm guessing that coins are Sterling Silver, too.


The value of rare coins goes up exponentially as the grade of the coin goes up. I used to know a coin collector and he could tell if a coin had been left in a car on a hot day by looking at it. He could tell if it'd been in a pocket without a protective plastic shell. He could tell if it'd bounced off the wall of the shell sometimes. The odds that you have a big money find from a publicly circulated coin are basically 0. It still may be worth a buck or so, which isn't bad considering it's a 2p piece, but I'd be shocked of you got as much as $10 for it, even if it's the real deal.

[OngBonga]

Thanks for that.

The rust is not very prominent at all, it's small and dark and probably needs better light and a magnifying glass to be certain what colour it is. In this light, it appears red or orange.

The coin would have been in circulation for a good few years, but it has been in my coin jar for at least a decade. That said, it's hardly a rust-proof environment. It's been in the shed for two years.

I'll give it a rub with some WD-40 and see if it comes up shiny. Cheers.

[OngBonga]

Yeah so I googled "how to clean coins" and the general advice is don't. Even circulated coins can lose value as a result of abrasive and corrosive substances. Scratching it would be bad. I'm not going to clean it, instead I'll send it for analysis first, then take advice from an expert if it's genuine.

It's magnetic. That confirms the presence of cobalt, iron or nickel. So that isn't really helpful, but I checked anyway.

Is it possible the "rust" is the consequence of reactions with other coins it's been in contact with? The jar is full of dirty coins of different metals, there's even a chunk of zinc in there.

[Poopadoop]

Wonder how many valuable coins go around in circulation or sit in a coin jar without anyone ever realising it. Must be quite a few.

[MadMojoMonkey]

Is it possible the "rust" is the consequence of reactions with other coins it's been in contact with? The jar is full of dirty coins of different metals, there's even a chunk of zinc in there.

Well, yeah. Put 2 different metals together and, in general, you've made a battery. Not all metals will corrode each other in this fashion, though. Aluminum and Steel are commonly used together, e.g., and are stable.

IDK what Nickel reacts with, though, aside from Cadmium (Ni-Cd batteries).

Cool to think that batteries produce power by rusting metal, though.
Yeah, your cell phone runs off of rust.

[OngBonga]

update - I just had the coin scanned with an xray thingy to determine its compisition.

99.9% iron.

It's worthless.

Sad face.

[MadMojoMonkey]

[MadMojoMonkey]

Just surfing some old vids and this one is great.

[chemist]

Thanks it really was a good video, very simple descriptions of Quarks, Leptons and Bosons.

[Eric]

http://www.dailymail.co.uk/sciencete...e-planets.html says there is a promising star system 39 light years away. It then says the following:

experts suggest that it would take humans hundreds of thousands of years to reach the new system using current technology

How close will we be able to get to light speed as our technology improves?

[CoccoBill]

What is the airspeed velocity of an unladen swallow?

[BananaStand]

How close will we be able to get to light speed as our technology improves?

I saw something on Netflix, can't remember what' it's called, but it evaluates the possibility of humans moving to another planet. The most viable solution seems to have been developed in the 60's. And I can't remember the name of the project, but essentially, you build a massive vessel, and every few minutes it shits out a nuclear bomb. The bomb explodes in space, and the explosion pushes the vessel forward. A few minutes later, another bomb, and the vessel moves even faster.

They said within some months the vessel would be going approximately 6% of the speed of light, which is fast enough to get us to another potential planet within 150 or so years.

Obviously a fuck-ton can go wrong. Most notably, the planet could be unlivable. But they did say that the engineering is probably the easy part. The real challenge is figuring out what happens to babies born in zero-gravity. The way our bones grow and how we learn to walk is all driven by gravity that pulls us downward.

It's possible that our species de-volves into gelatinous sacks of goo before we ever get to the new planet.

[MadMojoMonkey]

http://www.dailymail.co.uk/sciencete...e-planets.html says there is a promising star system 39 light years away. It then says the following:


How close will we be able to get to light speed as our technology improves?

There's no theoretical limit.

Practically, we use jets (of matter) to create thrust. That means we are limited by how much fuel we start with and the specific performance of the engine, which has to impart kinetic energy to the fuel. The current method is to bring a fuel and imparts this energy to itself with a bit of provocation... it burns, releasing energy from chemical bonds. The engine's job is mostly to keep pumping fuel into the right spot so that it can be accelerated such that the equal and opposite acceleration happens to the engine, and therefore the ship it's attached to.

Project Orion was a plan to explode nukes behind a ship to ride the shock waves to accelerate. That is not impractical in design, only in getting it off the ground w/o massively polluting the Earth. That would be capable of much higher speeds, due to the ludicrously more energy held in atomic bonds compared to molecular bonds.

[OngBonga]

Photons are stupid. I was talking to one today who thinks the earth is flat. Silly photon.

[MadMojoMonkey]

I saw something on Netflix, can't remember what' it's called, but it evaluates the possibility of humans moving to another planet. The most viable solution seems to have been developed in the 60's. And I can't remember the name of the project, but essentially, you build a massive vessel, and every few minutes it shits out a nuclear bomb. The bomb explodes in space, and the explosion pushes the vessel forward. A few minutes later, another bomb, and the vessel moves even faster.

They said within some months the vessel would be going approximately 6% of the speed of light, which is fast enough to get us to another potential planet within 150 or so years.

Yep. That's Project Orion. And the "spaceship" can be city-sized using 1960's tech.

It's still a perfectly viable option if we don't launch it from Earth. It's a monumentally heavier design, which is stopping us from building one in orbit in order to take joy rides to Jupiter.
I'd have to look at the data again, but I think the radiological fallout is A) not ALL pulled toward the humans by gravity, and B) has to get through the Earth's magnetic shield*. Once you're out far enough, blowing up nukes to get around is actually legit.

*Totally not OP for your home planet to start the game with a force field, IMO.

It's possible that our species de-volves into gelatinous sacks of goo before we ever get to the new planet.

Changes in physiology and not the genome don't count.

It's still evolution, not de-volution. A process which changes over time is an evolving (or evolution) process.

You'd have to reverse causality to have de-volution, I think.

[MadMojoMonkey]

Photons are stupid. I was talking to one today who thinks the earth is flat. Silly photon.

[MadMojoMonkey]

What is the airspeed velocity of an unladen swallow?

What do you mean? An African swallow or your mom?

[BananaStand]

Changes in physiology and not the genome don't count.

It's still evolution, not de-volution. A process which changes over time is an evolving (or evolution) process.

You'd have to reverse causality to have de-volution, I think.

So at the end of all that scientific semantics, we're still a race of blob-people right? Or am I missing something in your post?

[OngBonga]

I posted this as my fb status to see how many of my friends are clever enough to get the joke. One like in one hour, and that's from my dyslexic brother-in-law.

[MadMojoMonkey]

So at the end of all that scientific semantics, we're still a race of blob-people right? Or am I missing something in your post?

Yeah, but blob people who could still mate and bear offspring capable of their own reproduction with non-blob people, so no speciation.

As for nitpicking on semantics:
This is the physics thread. I will shamelessly pick on semantics wherever I deem the potential for misunderstanding. We're not the only ones who read this thread. I want the scientific definition of evolution to the be one we favor in this thread. That is why I clarified. (I hope) I'm not so pedantic in other threads.

Semantics can make all the difference between a good explanation and a terrible explanation.

Or as Mark Twain is credited: The difference between the right word and a similar word is the difference between lightning and a lightning bug.

[a500lbgorilla]

Just surfing some old vids and this one is great.

I heard a particularly beautiful explanation which said that "things" which "exist" are all in interaction. If something doesn't interact with anything else, it doesn't exist at all. Existence exists in the collision.

[MadMojoMonkey]

Observation is interaction.
I mean... the only way for any thing to experience another thing is through some generalized form of interaction.

If that is the whole story with all it's lack of nuance, then I agree that there is no meaning in saying, "That thing exists."

However, the seemingly unbroken stream of existence-interactions makes me quite comfortable to suppose that the things exist in between interactions.

[Poopadoop]

Why does it take so long for a star to go through all it's fuel? A fusion bomb (afaik) would do it's thing almost instantaneously. If the sun is producing its energy through fusion then why doesn't the sun just explode like a bomb? Not saying I want it to obv. ...

[MadMojoMonkey]

Why does it take so long for a star to go through all it's fuel? A fusion bomb (afaik) would do it's thing almost instantaneously. If the sun is producing its energy through fusion then why doesn't the sun just explode like a bomb? Not saying I want it to obv. ...

It's about scale.
"Like a bomb" is a bit of an understatement.

The sun is violently exploding at a temp of millions of degrees in its core. The explosion is so violent that the sun spews out ~ 4 billion kilograms of photons per second. That's just counting the (lol "massless") photons. Calculation and some context shown in this post.
The sun will burn through it's Hydrogen in another ~9 billion years.

The sun is so large that you cannot actually understand how large and massive it is. I'm not condescending. I dwell on this topic and I feel pretty confident that "really big" is as technical as my understanding goes w/o invoking numbers with big powers of 10 behind them.

It takes so long for a star to go through its fuel because stars are enormous. Mind-bogglingly enormous.
The "middle-sized" sun is 2(10)^30 kg enormous.
10 to the stinkin' 30!

It takes less than 100 kg of uranium or plutonium to go fissile on it's own as pure metal. So just having a ball of the stuff has practical limitations which are quite small. The Fat Man bomb that dropped on Nagisaki used less than 7 kg of fissile material, of which only ~20% underwent fission.

Ahh, but that's fission and not fusion. What about fusion bombs. Spoiler... they're mostly fission with a little bit of fusion to boost them.
At any rate... despite their seeming large size, human made bombs are less than miniscule when compared to a star.

Here's one fact that I cannot forget.
Like, 99.9% of the solar system is the sun itself.
[fact-checking]
Of the total mass of the solar system, 99.866% of that mass is the sun.
Dayum, sun.

Nearly 3/4 (71.2%) of what's left is Jupiter.
Dayum.

Interestingly... given a mass is large enough, and "mostly Hydrogen" enough to fuse - given that, the smaller it is, the longer it will last. The fusion rate dominates lifespan over mass. Stars with more mass will have faster fusion rates, which are increasing fast enough that more mass always means shorter life for a star.
The "cool" red dwarfs last longest. They are barely on the edge of sustaining fusion, and therefore the smallest stars. The smallest of them have expected lifespans of ~100 trillion years.
The most massive stars, hypergiant stars, have "short" lifespans of only a few million years.

[CoccoBill]

Loved this:

http://joshworth.com/dev/pixelspace/...larsystem.html

[Poopadoop]

I know the Sun is really really really big. What I don't get is why this keeps it from using all its fuel instantaneously like in a nuclear explosion. What am I missing?

[OngBonga]

A nuclear bomb doesn't spend its fuel "instantaneously", it takes time.

[OngBonga]

Smaller bombs will detonate quicker than bigger bombs because of greater surface area relative to volume.

One also has to consider that the sun is so big that its gravity is going to play a significant role, in that it will "slow down" an explosion... it is collapsing gravitationally at the same rate as it is exploding... gravitational equilibrium. A bomb will explode quicker than it otherwise would if the surrounding medium is lower pressure.

I guess bigger stars burn quicker than smaller stars because they get hotter? Is this the same logic as a small fire burning for longer than a large fire?

[MadMojoMonkey]

I know the Sun is really really really big. What I don't get is why this keeps it from using all its fuel instantaneously like in a nuclear explosion. What am I missing?

The short answer is that none of these processes are instantaneous. Explosion, fusion and fission are chain reactions.

A better answer is that quantum mechanically, wave function collapse is an instantaneous event, and therefore the exploding / fission / fusion interaction is instantaneous at the moment of oxidation / split / join for those particles. The chemical or atomic makeup of the constituent parts suddenly changes from one setup to another. In exothermic events, this releases more energy than it took to cause the event to happen (this is really a definition of exothermic). These events are exothermic, which means at least enough energy to cause another identical group of particles to undergo the identical reaction gets released each time it happens.

While each individual particle experiences the "burning" (if I may cudgel this in as a blanket term for these different processes) in an instantaneous way, the energy released by this is predominantly in the form of photons. Those photons, as you know, travel at the speed of light. This finite speed limits the amount of time necessary for the "nearby" particles to absorb that released energy. That speed of information transfer is at the universal max, still, it is finite.

Another reason the sun doesn't burn faster is that it is only dense enough and hot enough in its core for the thermal energy of the Hydrogen ions (protons) to overcome their Coulomb repulsion (like electric charges repel), which is proportional to 1/r^2. That r gets very tiny when you're examining interactions for proton-proton bonding. The entire sun is not undergoing fusion, only the center of it is. The center of the sun is millions of degrees, whereas the "surface" is only ~6,000 Celsius.

The transport of energy from the core to the surface of a star, and the light energy and neutrino mass flooding out of it, all serve to limit the buildup of energy within the core of the star, which also limits the rate of fusion.

[MadMojoMonkey]

One also has to consider that the sun is so big that its gravity is going to play a significant role, in that it will "slow down" an explosion... it is collapsing gravitationally at the same rate as it is exploding... gravitational equilibrium. A bomb will explode quicker than it otherwise would if the surrounding medium is lower pressure.

The gravity is the origin of the pressure and confined volume, which serve to increase the temperature in that volume.

The gravity is why it's fusing, not the inhibitor to the fusion.

You mean hydrodynamic equilibrium. Gravitational equilibrium is not a term I'm familiar with. It could mean flight, or buoyancy, or orbit... it's potentially confusing. Have you heard this phrase in use or did you just use it 'cause it's close enough for a layman's description?

[BananaStand]

Why doesn't the moon crash into the earth? For that matter, why doesn't the earth crash into the sun?

Isn't gravity supposed to pull things together?

[OngBonga]

Have you heard this phrase in use or did you just use it 'cause it's close enough for a layman's description?

It was the best term I could pull out of my arse! Hydrodynamic equilibrium, I like that more.

Why doesn't the moon crash into the earth? For that matter, why doesn't the earth crash into the sun?

Isn't gravity supposed to pull things together?

No, that's just your observation because by far the most familiar aspect of gravity is the earth pulling us down. But that's not the whole story. Gravity is spacetime curvature, and orbits are geodesics, straight lines through curved spacetime.

The simplest way to imagine it is that the moon's vertical motion relative to earth is cancelled out by its horizontal motion. If you throw a ball off a mountain fast enough parallel to the ground, then it's curvature towards the ground as it falls is matched by the curvature of the ground itself, thus it is constantly falling and never landing. If not for the friction of the air, the ball would go round the world and hit you on the back of the head. This is orbit.

The head bending way of looking at it is that motion is relative, and the moon is constantly moving in a straight line through spacetime that is curved by the presence of earth. This curvature manifests itself if the form of a geodesic... an elliptical orbit around the source of the curvature.

When we fall off a building and hit the floor, what has happened there is that the floor has intercepted our geodesic with disasterous consequences. If not for the electromagnetic properties of the ground (and let's disregard friction while we're at it), then we would fall towards the centre, through it, then out as far as we came from, then start falling again. We would be following a geodesic that passes directly through the centre of the earth.

The moon simply has enough lateral movement to avoid a geodesic that passes through the centre of the earth... it misses by the distance it is away from us at it perigee.

(caveat - I think)

[OngBonga]

Understanding curvature is important. Are we moving around the sun? Not quite. Earth is travelling in a straight line. It's just our path is curved when observed from the source of the curvature... the sun. But it is still a straight line. This can be demonstrated by looking at the moon, and Newton's first law.

The law of inertia states that a body in motion will continue along its path in a straight line at constant velocity unless acted upon by another force. In the case of the earth/moon system, the "force" in question is earth's gravity. If gravity is a force acting upon the moon, causing it to change direction, it would be accelerating (undegoing a change in motion), and this would be reflected in a loss of relative velocity. It would slow down, and start falling towards earth. That is has not done so is evidence that the moon is not accelerating... it is not undergoing a change in motion, its inertia is maintained. The moon is moving in a straight line at a constant velocity. If it were literally moving in an ellipse, then it would be slowing down, or would require a source of energy to stop it from doing so.

Gravity is not a force that pulls things together. It's better to say that the presence of mass causes curvature of spacetime, and we see this curvature in the form of gravity. Things being "pulled togther" is just spacetime curved to the point where collision is inevitable.

[OngBonga]

The gravity is why it's fusing, not the inhibitor to the fusion.

It won't inhibit the fusion, but it surely will inhibit an explosion? An explosion will cause matter to move away from the source of the reaction, which means there is less reactant to fuel further reactions. Gravity binds a reaction together, causing it to continue in a conrolled manner.

To be fair, I have no idea when it comes to nuclear processes. I deleted a block about surface area, because I wasn't sure how it applied in this regard. Where oxygen is necessary, surface area is critical, and a big sphere has much less surface area relative to volume than a small sphere. For you bog standard explosion, I figure that's an important thing to be thinking about, but fuck know when it comes to nuclear.

[Poopadoop]

Another reason the sun doesn't burn faster is that it is only dense enough and hot enough in its core for the thermal energy of the Hydrogen ions (protons) to overcome their Coulomb repulsion (like electric charges repel), which is proportional to 1/r^2. That r gets very tiny when you're examining interactions for proton-proton bonding. The entire sun is not undergoing fusion, only the center of it is. The center of the sun is millions of degrees, whereas the "surface" is only ~6,000 Celsius.

This part makes sense. To follow up then, in essence the centre of the sun is 'exploding', the bits are being expelled, and these radiate as heat, light, energy. The reason it doesn't all go up at once is because it's only hot enough at the centre.

[BananaStand]

Can someone explain what a magnet is? In layman's terms?

[Poopadoop]

It won't inhibit the fusion, but it surely will inhibit an explosion? An explosion will cause matter to move away from the source of the reaction, which means there is less reactant to fuel further reactions. Gravity binds a reaction together, causing it to continue in a conrolled manner.

To be fair, I have no idea when it comes to nuclear processes. I deleted a block about surface area, because I wasn't sure how it applied in this regard. Where oxygen is necessary, surface area is critical, and a big sphere has much less surface area relative to volume than a small sphere. For you bog standard explosion, I figure that's an important thing to be thinking about, but fuck know when it comes to nuclear.

Well he said the photons get emitted at the speed of light, so I think the answer is that gravity doesn't inhibit the explosion. Gravity in the first place causes the fusion through the build up of pressure (i.e. heat) if I understand it correctly. Gravity at the edge of the sun is not enough to cause fusion to occur (again if I understand it correctly).

[OngBonga]

Can someone explain what a magnet is? In layman's terms?

Now gravity I can get my head around. Electromagnetism, not so much.

Well he said the photons get emitted at the speed of light, so I think the answer is that gravity doesn't inhibit the explosion.

Gravity affects light, hence black holes.

An explosion can only happen as fast as waves can propogate through a medium. That will be the local speed of sound. If gravity affects light, I imagine it affects sound, too.

I'm sure I read somewhere that it takes millions of years for a radiated particle emitted from the centre of the sun to emerge at the surface.

[Poopadoop]

Now gravity I can get my head around. Electromagnetism, not so much.

My admittedly rudimentary understanding of magnetism is that ions have a charge and if enough of them are lined up in the same direction the charge gets multiplied and you have a magnet.

[MadMojoMonkey]

Why doesn't the moon crash into the earth? For that matter, why doesn't the earth crash into the sun?

Isn't gravity supposed to pull things together?

OK. Everything (?) Ong said looks actually legit for Einstein's description of gravity
Einstein's General Relativity is the most accurate description of gravitation known so far.

I'll describe it in a Newtonian way. Newtonian gravity is less accurate, but a very good approximation for non-relativistic speeds.

I don't know your level of physics understanding, so I'll try to start with basic principles. Definitely ask questions if I get too technical. I love to break things down to a non-physicist's level of understanding. (I like to think Ong's excellent description of Einstein's gravity is at least a little bit due to this thread.) As I become better acquainted with your level of understanding, my answers will be more directly to the point you're asking.

I think, but I'm guessing, that the misunderstanding is between force, velocity and acceleration.

A re-phrasing of Newton's 2nd Law of motion:
F = ma
Force equals mass times acceleration. Bold letters indicate vectors. A vector has a magnitude and a direction.
Acceleration is defined as change in velocity over time, a = dv/dt.
Velocity is a vector. It has a magnitude (size, or length, if you like), called speed, and a direction.
Speed is not velocity. Velocity is speed in a direction. Speed is the magnitude of velocity.
Yes, outside of a physics conversation, these are inter-changeable, but in physics they're different.

OK. Forces cause accelerations. An acceleration is a change in velocity. Velocity is speed and direction.
On the face of it, any acceleration should be making the object speed up or slow down. It is true when the acceleration is parallel to the direction of motion that the object will speed up. It is true that when the acceleration is anti-parallel (pointing exactly opposite the direction of travel) the object will slow down. It's the direction bit that's messing with us.

Side track:
The intermediate value theorem
If f is a continuous function and f(x1) = y1, and f(x2) = y2,
then for value, y3, between y1 and y2,
there is at least one value, x3, which is between x1and x2
for which f(x3) = y3.

It basically says it's like this: If you're walking along some path, and at one point your elevation is X and at another point your elevation is Y, then you must have passed through all the in-between elevations at least once along your path, because you can't suddenly go from one height to another height w/o passing through the in-between heights.

Back on track:
So if there is some direction of acceleration which makes you slow down, and another direction of acceleration which makes you speed up, then there MUST be at least one direction of acceleration in between those 2 directions which will not change your speed, because directions are continuous.

This class of acceleration directions are those which are perpendicular to the object's direction of motion. These perpendicular accelerations cause the object to change its direction of motion (which is a change in velocity). So it is possible to accelerate w/o changing speed.

We're primed to talk about Newtonian orbits.

For any object moving in a circular path at a constant speed, the direction of acceleration is toward the center of the circular path. For an orbiting body, this acceleration is due to the gravitational interaction between the 2 bodies. Technically, the acceleration is toward the center of mass of the system, but with only 2 object, that center of mass is always between them, so the direction of acceleration is always pointed toward that in-between location. Since perspective is a matter of choice, it is always true that both bodies are orbiting around each other. Neither is stationary, because Newton's 3rd says that equal and opposite forces characterize all force interactions. Both bodies are pulled toward their center of mass, but neither falls into it, because they have some velocity which is perpendicular to the acceleration and keeps making them miss the exact center. Unless the size of the objects is such that they collide, they will perpetually orbit each other, 'cause no friction in space.
(Note that the "perpetually" part is subtly not true in Einstein's model.)


Technically, a satellite is any orbiting body. The moon is a satellite of Earth. Earth is a satellite of the sun, etc.

This still holds for elliptical orbits.

The orbiting body has some velocity which is perpendicular to the direction of acceleration. The direction of acceleration is still toward the center of mass of the system, but that center of mass is NOT always perpendicular to the object's path. Some of the acceleration is along or against it's path, either speeding it up or slowing it down, but some of the acceleration is making it turn toward the center of mass. Again, the object has some velocity which is toward or away from the center of mass of the system, but some which is perpendicular to it. If it doesn't collide with anything in its path, and some other details, it's in orbit.


I discussed orbits in detail in this post.

[MadMojoMonkey]

The simplest way to imagine it is that the moon's vertical motion relative to earth is cancelled out by its horizontal motion.

The moon is, in fact, spiraling away from the Earth at a rate of ~3 inches per year, if I remember correctly.

(caveat - I think)

Well said, man.
You even covered Newtonian orbital model and Einstein orbital model.
You know that the reason things are "hard" is due to electromagnetism.
You mentioned conservation of energy principles, too.


My one correction is trivial and pedantic.

[MadMojoMonkey]

The law of inertia (Newton's First) states that, in an inertial reference frame, a body in motion will continue along its path in a straight line at constant velocity unless acted upon by another force. In the case of the earth/moon system, the "force" in question is earth's and moon's gravity. If gravity is a force acting upon the moon, causing it to change direction, it would be accelerating (undegoing a change in motion), and this would be reflected in a loss of change in relative velocity. It would slow down not move in a straight line, and start falling but accelerate towards earth. That is has not done it is doing so is evidence that the moon is not accelerating Newton's Law is an apt description... it is not undergoing a change in motion, its inertia is maintained in an inertial reference frame.

SMH

bold for emphasis
Newton's First Law is a definition of what is an inertial reference frame. It defines the context of the other 2 laws.

The moon is moving in a straight line at a constant velocity. If it were literally moving in an ellipse, then it would be slowing down, or would require a source of energy to stop it from doing so.

It's a straight line through curved spacetime in Einstein's model, but a curved line through flat spacetime in Newton's model.
The underlying mechanics behind the motion are different, but the descriptions which emerge are (to so many decimal places) identical in the context of Earth-Moon or Earth-Sun.

It is literally moving in an ellipse in an inertial reference frame in the Newtonian model. It is speeding up when it moves closer to the body it orbits and slowing down when it moves further away. Bodies in elliptical orbits do not move at constant speed. Conservation of energy states that when their separation increases, their gravitational potential energy increases, and their kinetic energy decreases. The opposite is also true. That's why when you drop something it accelerates (keeps getting faster) toward the ground. It is moving from a state of higher gravitational potential energy to lower, and it increases in kinetic energy as it does so.

Gravity is not a force that pulls things together. It's better to say that the presence of mass causes curvature of spacetime, and we see this curvature in the form of gravity. Things being "pulled togther" is just spacetime curved to the point where collision is inevitable.

You're somewhat confusing in writing off the Newtonian model altogether. While it is technically inferior, it is at least intuitive. In its inferiority, it still describes all of the solar system aside from some tiny perturbations in Mercury's orbit, so it is a quite useful approximation, too, while being MUCH less complicated in the math.

[MadMojoMonkey]

It won't inhibit the fusion, but it surely will inhibit an explosion? An explosion will cause matter to move away from the source of the reaction, which means there is less reactant to fuel further reactions. Gravity binds a reaction together, causing it to continue in a conrolled manner.

You're assuming that only the spent fuel is carried away in the explosion. In general, the explosion will disperse the reactant(s), too, leaving some of it unsploded.

The gravity pulls back everything, the spent fuel (Helium nuclei) and unspent. The 4x as dense Helium nuclei migrate to the center, and block fusion, which migrates to a shell around the "heavy" core. The denser core has a more pronounced effect on the curvature of spacetime, which increases the pressure and fusion rate in the "burning" shell. This continued process of mass migrating to the center and increasing the fusion rate around the core is what turns a nice mid-sized star like the sun into a red giant as it ages.

For bigger stars, the helium can start its own fusion processes, which drops it's Carbon-Nitrogen-Oxygen "ash" to the core, displacing the helium fusion to a shell. The bigger the star, the more shells it will eventually sustain. Each new shell accelerates the rate of fuel consumption and heat and pressure in the core, which all compound to making each new shell's lifespan a fraction of its predecessor's.

If a star is big enough to work its way up the periodic table to iron, then it's supernova time. Iron is the lowest number element on the periodic table for which fusion is endothermic, it absorbs energy. If the fusion process in the center doesn't push back out to maintain the hydrodynamic equilibrium, then gravitational collapse is inevitable. All that in-rushing mass tries to get to the same gravitational center, further increasing the fusion rate of iron, and now the rest of the periodic table, all of it endothermic. Eventually, the pressure is so great and density so great that quantum mechanical forces dominate. The in-rushing mass has accelerated to relativistic speeds and a few crazy and fascinating things can happen at this point. The exact mechanism is not well understood as QM and GR are doing something totally sneaky.

The supernova can leave a white dwarf star formed of electron degenerate matter. An object the mass of the Sun, but the size of the Earth.

It can leave a Neutron Star, a star whose gravity is so great that (nearly) all of the protons and electrons have been forced to bond into neutrons (releasing anti-electron neutrinos in the process.) They're about twice the mass of the Sun, but 10 km in diameter.

It can leave a black hole. You know about those.

To be fair, I have no idea when it comes to nuclear processes. I deleted a block about surface area, because I wasn't sure how it applied in this regard. Where oxygen is necessary, surface area is critical, and a big sphere has much less surface area relative to volume than a small sphere. For you bog standard explosion, I figure that's an important thing to be thinking about, but fuck know when it comes to nuclear.

Get the fully ionized nuclei moving really fast at each other. If they're moving fast enough, they will tunnel through each other's Coulomb repulsion and the strong nuclear force will bond the small nuclei to make a big nucleus. This increases the total potential energy for small nuclei (up to Iron). The fusion process radiates away this discrepant energy in the form of gamma ray photons (mostly) and neutrinos in the production of neutrons.

Stars accomplish this by being enormously massive and mostly Hydrogen, so that the heat and pressure in the confined volume conspire to make loads of potential fusing nuclei moving the right speed in all directions, so some will be heading right for each other and will tunnel into a fusion bond.

[MadMojoMonkey]

This part makes sense. To follow up then, in essence the centre of the sun is 'exploding', the bits are being expelled, and these radiate as heat, light, energy. The reason it doesn't all go up at once is because it's only hot enough at the centre.

Yes. That's as much as I've told you and you got it down.

Another reason is that the fusion is a "rare" occurrence. Most times a + charge proton gets close to another + charge proton, their Coulomb repulsion will cause them to scatter, not fuse. They have to "tunnel through" a potential barrier which is greater than their total energy.

This can only be understood with QM. It defies intuition. The principle is that if I roll a ball halfway up a hill with the exact same force a bazillion times, one of those times, it's get over the hill. That's certainly not a reasonable expectation in the macroscopic world you and I understand, but in Quantum Mechanics, the uncertainty principle allows for mandates this kind of nonsense.

Another limiting factor is that the protons can actually be moving too fast to fuse. This, again, defies intuition. Basically, if the protons are moving past each other too quickly, then their wave functions wont interact. There's a sweet spot of speed where they're slow enough to "see" each other, and barely fast enough to tunnel through each other's mountain of Coulomb repulsion.

This sets the energy of the core, basically. A star will collapse until the energy produced in this process is enough to prevent total collapse, which happens to be where the fusion rate is "rare" from a statistical stand point. Even in large stars, the fact that they're mostly Hydrogen and Hydrogen gives of loads of energy in its fusion, it will reach an equilibrium with Hydrogen shell burning and a high pressure non-fusing core of Hydrogen until that fills up with Helium.

I say "rare" in quotes because it's still going on like mad, due to the extreme size of a star and number of protons zipping about.

[MadMojoMonkey]

Can someone explain what a magnet is? In layman's terms?

This one's a tough one. In layman's terms, you'll feel a bit cheated. In full description, you'll fall asleep.

The short answer is that there are multiple mechanisms which give rise to magnetism. There are multiple kinds of magnetism, too.

I have to premise this with the notion that electric charges create electric fields, and moving charges create changing electric fields. Changing electric fields create changing magnetic fields. And just to create a feedback loop, changing magnetic fields create changing electric fields. As if that wasn't complicated enough... whether or not a thing is moving is a matter of perspective. Ugh.

There is an intrinsic magnetic property to protons and electrons. By which I mean, we have no idea how it got there, so we call it intrinsic.

So each proton and each electron is a tiny magnet. Their movement is understood statistically and quantum mechanically, and hard to describe w/o loads of math. When you put a bunch of magnets close together, they exert a torque (spinning force) on each other and attempt to align all of their magnetic field lines, which is the lowest energy state. However, the material properties, crystalline structure and temperature can disrupt this aligning process.
Within a magnet, there will be "domains" or regions where all the magnetic field lines are aligned within that section, but that section's field lines are not aligned with the neighboring sections'. This also serves to weaken the total external field.
Increasing temperature is to increase random particle motions, which randomizes the alignments, weakening the magnetic field.

So ... why magnets do magnet thing?
Everything is made up of tiny magnets. In some materials, a statistical balance of different magnetic regions pointing in different directions is never reached. Those have a net external magnetic field and are magnets.

[MadMojoMonkey]

Well he said the photons get emitted at the speed of light, so I think the answer is that gravity doesn't inhibit the explosion. Gravity in the first place causes the fusion through the build up of pressure (i.e. heat) if I understand it correctly. Gravity at the edge of the sun is not enough to cause fusion to occur (again if I understand it correctly).

@bold
Pressure isn't heat, but perhaps misuse of i.e.
It's the buildup of both and density building helps, too.
Lots of particles zipping about real fast near each other.

Interestingly, the energy transport is pretty slow. The photons are always moving at the speed of light, that's a given. They don't travel in a straight line, without interacting on the way out, though. They are absorbed and re-emitted over and over again, often splitting into multiple, lower energy photons.

The tiniest peek of the energy from any given fusion reaction will make it to the surface and out within minutes, but the expected time for half of the total energy from a single fusion reaction to have escaped is on the order of thousands of years. ALL of the energy from any single fusion interaction is probably never going to escape. The long statistical tail and extreme numbers involved make it supremely likely that some of the energy from each fusion process with remain to the end.

[MadMojoMonkey]

Gravity affects light, hence black holes.

An explosion can only happen as fast as waves can propogate through a medium. That will be the local speed of sound. If gravity affects light, I imagine it affects sound, too.

I'm sure I read somewhere that it takes millions of years for a radiated particle emitted from the centre of the sun to emerge at the surface.

Gravity affects light but not by changing its speed. The black hole's gravity well changes the photon's frequency. As a photon moves up out of a gravity well, it increases its gravitational potential energy, and loses energy in the form of reducing its frequency, conserving energy. The energy of a photon is defined as its frequency time plank's constant. Plank's constant is the proportionality of the energy of a photon to its frequency.

In a black hole, the frequency is reduced to 0, as there is no amount of energy great enough to avoid this.
With 0 energy, there is nothing to detect. I.e. no photon.

***
The medium thing is cool. Like space explosions in movies. I always thought they were so fake 'cause no sound in space, right. Turns out, hearing explosions in space is real. The explosion moves at the speed of detonation, which is uninhibited by any medium, so travels quite fast. The debris from the explosion is moving outward in all directions and carrying the sonic information with it. When it contacts your ship's hull, that information will be transferred.

You will hear a satisfying roar as you slay your alien pirate foes.

***
Millions of year for "the" photon to escape the sun?
That's a naive application of water molecule based motion to photons in a plasma. My explanation above is (more) physically motivated.

[MadMojoMonkey]

My admittedly rudimentary understanding of magnetism is that ions have a charge and if enough of them are lined up in the same direction the charge gets multiplied and you have a magnet.

Not ions. Ions are atoms with an imbalance of protons to electrons.

Magnetic dipoles. Dipole means 2 poles. N and S.
When they are aligned, it increases the field.

If you can demonstrate a magnetic monopole, you will surely be awarded a Nobel Prize in Physics if you live long enough to have your experiments repeated and verified by the broader community.

[OngBonga]

Just about to go out, so will get round to replying later. Just wanna say that yes, certainly this thread has helped me better understand Einstein gravity. I must say I find it somewhat amusing, and fascinating, that I find Einstein's theories much easier to understand than Newton's. Intertia is still something I get confused about. It's interesting that GR seems to have come about due to Einstein's reservations about Newton's theory of inertia. I defintely want to understand Einstein's view on inertia.

[BananaStand]

Anti Matter. Explain please?

[Savy]

Anti Matter. Explain please?

Like the opposite of matter but not in every way and then exactly the same.

[Poopadoop]

I'd also like to know the deal with anti-matter. Specifically, is it a proven entity or a theoretical construct like parallel universes or the cat in the box? If the latter, obviously then what is the rationale for proposing its existence? If the former, where is it?

[Poopadoop]

Like the opposite of matter but not in every way and then exactly the same.

I sense an Ongdentity paradox arising here.

[Savy]

I'd also like to know the deal with anti-matter. Specifically, is it a proven entity or a theoretical construct like parallel universes or the cat in the box? If the latter, obviously then what is the rationale for proposing its existence? If the former, where is it?

Dunno why I'm answering these questions as I'm not 100% but yeah it's been detected, they can even play about with the stuff. Happens quite often/all the time in places like CERN & any other high energy particle physics.

There just isn't very much of the stuff (otherwise we wouldn't exist) I don't think they know why there is so much matter and so little anti-matter.

I sense an Ongdentity paradox arising here.

But it's like literally what it is. Some things the same like mass (I think) other things opposite like charge (& spin?).