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/sci/ - Science & Math

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2733648 No.2733648 [Reply] [Original]

So there's this common understanding that, in nuclear fusion and fission, mass can actually be converted into energy. That's how we get the whole E = mc^2 thing, right?

But I've been reviewing some theoretical nuclear fusion reactions, and they seem to balance out. Protons and neutrons are conserved, so what exactly is lost? You can't lose only a bit of a proton. How the hell is it determined that one atom is less massive than another?

>> No.2733666

the mass of an atom (lets say deuterium) is larger than the mass of the individual protons and neutrons because of the binding energy in the atoms. so deuterium mas > proton plus neutron mass

>> No.2733665

>>2733648 Protons and neutrons are conserved

Look again.

>> No.2733674

The mass loss doesn't come from a particle disappearing. It comes from the particles loosing mass themselves.

The total mass loss is about a fifth of the mass of a proton.

>> No.2733698

>>2733665
Gif related looks pretty conserved to me, bro. Are you gonna argue with a gif?

>>2733666
So... huh. That was kind of a theory I had. That strong force somehow translates into mass. But that's crazy. So crazy it just might work.

>>2733674
This is the part that totally bewilders me. So what if you keep fusing the same proton? Do you get a super-tiny proton?

>> No.2733701
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2733701

>>2733698
Forgot my gif. Derp.

>> No.2733711

take potential energy inside the atom
use e=mc^2
now it is mass!
break atom apart
use e=mc^2
now the mass is energy again!

>> No.2733713
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2733713

>>2733648

>> No.2733722
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2733722

>>2733713
Oh yeah. Because someone who's trying to get their head around nuclear chemistry must be a real fuckin' dumbass.

>> No.2733734

>>2733698
Don't think of it like a piece of proton disappearing.

Intuitively, mass is a fundamental property of an object, and it indicates how much "stuff" the object has. That's no longer tenable in special relativity.

By saying that the mass of the nucleus decreases, I mean that it's easier to accelerate it now compared to before.

>> No.2733730

Protons, neutrons and electron have (microscopic) mass

Heavier elements have more protons, neutrons and electrons

Ergo certain atoms are less massive than others,

>> No.2733747

>>2733734
Huh. Alrighty, then. That clears things up a lot. Thanks!

>> No.2733755

>>2733713
This is why I left this board ages ago, because people who think they know a lot about science look down on a person who asks a question. Science is about asking questions and understanding, why do you need to scoff and belittle their attempt at that? I'm guessing it makes you feel bigger about yourself.

>> No.2733756

>>2733698
If you add up the individual weights of isolated protons and neutrons that make up an atom, you'll find it doesn't match the mass of the atom as a whole. Why? Binding energy. Certain forms of matter are more stable. This stablity translates into decreased energy, and therefore decreased mass. Other forms are unstable, very energetic, and ready to release that energy to get to a lower energy state.
The protons and neutrons ARE conserved, and individually, they would have the same mass. But they're not individual. Atomic masses are the component particles plus the energy needed to bind them. For fission of elements heavier than iron, you get energy out and the total mass of the produced atoms is lower than what you used because the products are more stable. For fusion of elements lighter than iron, the atom you create has less mass than the components because the product is more stable.

>> No.2733761

not op but i have a question. dint want to create another thread just for this. why cant U-238 be used for fission in nuclear power stations?

>> No.2733762

the extra mass that "lost" isn't part of a proton or other particle, the mass comes from the energy in the atom (because of the strong force). so the energy you get is just stored energy in the atom, E = mc^2 relates the extra mass of the atom the the extra energy.

>> No.2733764

>>2733747
No probs, if you have any other questions, ask away!

>> No.2733782

>>2733761
U-238 is stable and doesn't yield much energy when breaking into its components. It takes a lot of energy to start the reaction.

>> No.2733791

>>2733782

thanks brah