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Einstein Jr.
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If an electron moves at light speed, how do we know that it has a definite mass(9.1 x 10^-31kg)? According to relativity, shouldn't its mass be infinite?
It is its rest mass. And because electron has mass, it doesn't move at light speed. I would say near light speed. But compared to what?Einstein Jr. said:If an electron moves at light speed, how do we know that it has a definite mass(9.1 x 10^-31kg)? According to relativity, shouldn't its mass be infinite?
Einstein Jr. said:What is the difference between rest mass and normal mass? Even if an electron moves at near light speed, wouldn't we get a huge value for its mass, rather than a really small value like 9.1x10^-31?
Einstein Jr. said:Now, according to you guys, an electron orbiting an atom will have a huge mass. (this is what happens in an atom, if I'm not wrong, since it travels really fast) Therefore, shouldn't the atoms themselves also have huge masses, along with everything made of atoms?
A common misconception that can be attributed to the concept of relativistic mass is that an object changes its internal structure by gaining mass when it travels at relativistic speeds. The object’s internal structure is independent of its velocity and it will always appear to be the same in its rest frame. The source of this confusion is that relativistic mass depends on the frame in which the object is observed and the concept of mass is typically regarded as a property of an object. See also our FAQ on the mass energy equivalence.
Please banish the idea of relativistic mass from your mind. A moving electron has the same mass as an electron at rest. Mass is invariant. The FAQ linked to above is a good starting point. When physicists are talking to each other they practically never mean "relativistic mass" when they say just "mass".Einstein Jr. said:Now, according to you guys, an electron orbiting an atom will have a huge mass. (this is what happens in an atom, if I'm not wrong, since it travels really fast)
DaleSpam said:This KE will contribute to the mass of the atom as a whole, but it is not specifically mass of the electron.
anorlunda said:We can illustrate that by shifting the electron in the atom to a higher energy state, The energy of the atom increases but not its mass.
Nugatory said:Consider two deuterium nuclei. Do they have more, less, or the same rest mass as a single helium-4 nucleus? These two configurations contain the same number and type of particles, and differ only in their internal energy.
anorlunda said:Good point, even two fully ionized duterium atoms and one helium-4 nucleus have binding energy mass discrepancies. But still adding KE to the electrons by moving them to a higher energy state does not change their rest mass does it? Dalespam's comment sounded like a nucleus capturing an electron with higher KE makes the atom have more rest mass. The excess KE of the captured electron goes to KE of the atom, not its rest mass.
Nugatory said:If you consider the entire atom to be a single system, than the excitation energy of the electrons counts as internal energy of the atom, and the rest mass of the atom will change with the absorption and emission of photons.
My wording glossed over some stuff in the middle. The KE of the electron contributes to the internal energy of the atom as a whole. The internal energy of the atom as a whole contributes to its mass. If you shift an electron to a higher energy state you do, in fact, increase the mass of the atom.anorlunda said:I believe that what DaleSpam should have said was "This KE will contribute to the energy of the atom as a whole." The (rest) mass of an atom does not increase when we add KE to it.
We can illustrate that by shifting the electron in the atom to a higher energy state, The energy of the atom increases but not its mass.
DaleSpam's statement was correct, but it can be misleading in this context.
anorlunda said:Wikipedia quotes the "isotope mass" of Helium-4 as 4.002602 u. Is that number valid only for the ground state of the nucleus and [minimum energy] of all the electrons?
ChrisVer said:Also for the whole discussion I find it a bit awkward because it mixes the kinetic energy with the potential energy somehow?
anorlunda said:Wikipedia quotes the "isotope mass" of Helium-4 as 4.002602 u. Is that number valid only for the ground state of the nucleus and [minimum energy] of all the electrons?
The mass of an electron is approximately 9.11 x 10^-31 kilograms.
The mass of an electron is determined through various experiments and measurements, such as the Millikan oil drop experiment and the cyclotron frequency measurement.
According to Einstein's theory of relativity, the mass of an electron does increase as its speed approaches the speed of light. However, at everyday speeds, this increase is negligible and the mass can be considered constant.
The mass of an electron and its charge are two fundamental properties of the particle, and they are independent of each other. The charge of an electron is -1 and its mass is approximately 9.11 x 10^-31 kilograms.
Yes, according to Einstein's famous equation E=mc^2, mass and energy are interchangeable. This means that the mass of an electron can be converted into energy, and vice versa.