# Naked Science Forum

## Non Life Sciences => Physics, Astronomy & Cosmology => Topic started by: marcelo on 11/07/2011 23:07:02

Title: What about the chemical nature of "m" in E=mc2?
Post by: marcelo on 11/07/2011 23:07:02
Hi,
I am no physicist, but this question came to mind the other day.
The famous Einstein's equation E=mc2 states the equivalence between mass and energy. However nothing seems to inform the equation about the composition of this matter...so it looks like as if in principle you would get the same energy out of accelerating a gram of feathers or a gram of uranium up to light speed. Is this the case?!

-puzzled

Title: What about the chemical nature of "m" in E=mc2?
Post by: Kryptid on 12/07/2011 01:13:08
You don't get the energy by accelerating the mass to the speed of light (which is impossible anyway). The equation is mass multiplied by the speed of light squared, not just the speed of light. It's a statement of the relationship of their magnitude, not velocity per se.

And yes, one gram of feathers contains just as much energy as one gram of uranium, but one gram of feathers is much larger in volume than one gram of uranium.
Title: What about the chemical nature of "m" in E=mc2?
Post by: imatfaal on 12/07/2011 10:11:52
SuperC is completely right - but just one more thing.  Marcelo you seem to be conflating E=mc2 which is the energy equivalence with matter and KE=1/2mv2.  It is kinetic energy that you gain through
Quote
accelerating a gram of feathers or a gram of uranium
Kinetic Energy = 1/2 mass times velocity squared = 1/2mv2
From this you can see the KE of a gram of feathers will be the same as the KE of a gram of uranium provided they are travelling the same speed
Title: What about the chemical nature of "m" in E=mc2?
Post by: Soul Surfer on 12/07/2011 18:03:58
It is also important to remember that when exothermic chemical reactions occur there is a tiny mass loss between the total mass of the reactants and the total mass of the product associated with the liberation of energy during the reaction this is too tiny to be measurable.  The mass difference is only measurable in much higher energy nuclear reactions.
Title: What about the chemical nature of "m" in E=mc2?
Post by: marcelo on 13/07/2011 01:23:39
Hey guys, thanks for the answers,a few replies since I got more puzzled now...

A basic aspect to clarify is the real-world situation that is described by the eq. E=mc2? Supecryptid, yes it is a statement about the relationship of E,m and c, but this certainly must summarize a statement about a physic phenomenon...which one? what's then the underlying phenomenon where the relationship E=mc2 can be verified?

Imatfaal I agree the phenomenon/situation is not just motion since the eq for KE describes the energy developed/required for that situation.

I just thought that E=mc2 described a special phenomenon (not only the motion) where the  transformation of mass into energy can actually occur. This is why I thought that the nature of the matter could have something to do with the final energy obtained. If all comes down to motion and (kinetic) energy then feathers and uranium are all the same. However both grams and feather definitely DO NOT contain the same energy when it comes to CHEMICAL energy (otherwise we may fill the tank of the car with the same mass of oil, feathers , water,etc)

Title: What about the chemical nature of "m" in E=mc2?
Post by: Soul Surfer on 13/07/2011 08:33:28
You must remember that ALL chemical energy is purely a superficial process with matter in which a few electrons on the outer surface of atoms are rearranged to create and destroy structured groups of atoms.  These energy levels are extremely tiny when compared with the mass energy of the whole atoms or the energy levels involved in nuclear changes.
Title: What about the chemical nature of "m" in E=mc2?
Post by: JP on 13/07/2011 15:38:15
E=mc2 is always a bit confusing.  What it's telling you is that if you have a block of mass m at rest (on your lab bench, for example) and you were to somehow turn it all into energy--say photons of light, for example, you would get mc2 units of energy out.  c is a huge number, so a tiny bit of mass can create a huge amount of energy.

It works the other way, too.  If you concentrate enough energy into a small enough space, you can create particles that have mass.  This is what's going on in particle accelerators: they give particles a lot of energy and collide them together.  All this energy allows the creation of many kinds of new massive particles.

Finally, it can also describe how "trapped" energy can something have mass.  The actual masses of all the tiny particles making up an atom are very small--not nearly enough to give you the total mass of the atom.  But when they're bound up together, the energy of those bonds gives the atom extra mass from m=E/c2.  Like Soul Surfer said, the chemical energy is just a tiny tiny fraction of this--most of this energy comes from bonds holding the nucleus together.
Title: Re: What about the chemical nature of "m" in E=mc2?
Post by: syhprum on 09/03/2019 20:51:37
I understand that the nuclear bomb that  exploded over Hiroshima actually converted about 2 grams of the U235 out of hundred Kg or so it contained into energy which must have been a disappointment to its designers if not to the inhabitants of Hiroshima.

Title: Re: What about the chemical nature of "m" in E=mc2?
Post by: PmbPhy on 09/03/2019 22:56:40
I understand that the nuclear bomb that  exploded over Hiroshima actually converted about 2 grams of the U235 out of hundred Kg or so it contained into energy which must have been a disappointment to its designers if not to the inhabitants of Hiroshima.

No. They got what they predicted.
Title: Re: What about the chemical nature of "m" in E=mc2?
Post by: Kryptid on 09/03/2019 23:53:40
I understand that the nuclear bomb that  exploded over Hiroshima actually converted about 2 grams of the U235 out of hundred Kg or so it contained into energy which must have been a disappointment to its designers if not to the inhabitants of Hiroshima.

It wasn't an antimatter bomb so I don't think anywhere near 100% conversion of mass to energy was expected.
Title: Re: What about the chemical nature of "m" in E=mc2?
Post by: Janus on 10/03/2019 02:15:36
I understand that the nuclear bomb that  exploded over Hiroshima actually converted about 2 grams of the U235 out of hundred Kg or so it contained into energy which must have been a disappointment to its designers if not to the inhabitants of Hiroshima.

Fission, the nuclear reaction that this bomb relied on, only converts a small percentage of the U-235 into energy. This energy comes from the difference in rest mass between the U-235 isotope and the daughter products of the fission.  Fusion, the basis of the H-bomb,  converts a greater percentage of mass to energy, but still only a small fraction.