Why is the total energy of an orbiting system negative?

Assume it’s an circular orbit. Object A orbits around object B. Take object B as frame of reference. .E=KEa+GPE .E=12mav2a+(−GMbmAr) .E=12ma(GMbr)+(−GMbmar) .E=−GMm2r<0 What does negative total energy at any instant of time mean? Answer Negative energies are totally fine, because you had to pick a zero-point for energy. In your calculation you picked it to … Read more

Why doesn’t hydrogen have a neutron?

Why doesn’t hydrogen have a neutron? Answer Your question boils down to why didn’t protons in the very early universe combine with neutrons to form deuterons (a proton plus a neutron). The answer is that they do – there is a brief window of time where deuterons can form and the universe is cool enough … Read more

Why do almost all nuclear reactions release energy?

I’m just wondering why almost all nuclear reactions release energy, in a typical way, the mass defect is transformed into energy ? Is there a nuclear reaction that gains mass (resp. energy)? or most nuclear reactions always produces energy ? Your answer would be more helpful if you provide examples. Thanks. Answer If they didn’t … Read more

How to explain E=mc2E=mc^2 mass defect in fission/fusion?

What is the nature of nuclear energy? This is closely related to the correct explanation of mass defect. I did some research of that topic and cannot come to a single comprehensive and consistent description. Below are related statements I gathered or can think of, describing the problem area. To me, each of them seems … Read more

If the energy of an ant is smaller than an elephant, does that mean the ant is more stable than the elephant?

I know that when a system is in its lowest level of energy, it is most stable. However, what if system 1 has lower energy than system 2, does it keep meaning so? Or do we need to examine their binding energies of them? If the systems have only two bodies, easy: the one which … Read more

Why bound states in QFT have higher mass than single particle states?

In standard textbooks in QFT while discussing e.g. the Kallen-Lehmann formula (see e.g. Section 7.1 in the Peskin-Schroeder book) it is always assumed that bound states of two or more particles have higher mass than the one particle states. Why this should be true? Let us compare this for example with the classical two particles … Read more

What exactly is binding energy?

I’ve been reading on radioactivity but along the way I got confused, if binding energy is the amount of energy used in holding the nucleus together then why is binding energy also the amount of energy used in breaking the nucleus apart, shouldn’t the energy required to break the nucleus apart be greater than the … Read more

Why is the binding energy per nucleon not zero for hydrogen atom?

The lone proton has not to be worked on against any electrostatic force. So where does the energy come from? What is mass defect for a hydrogen nucleus? Answer The hydrogen nucleus has exactly zero nuclear binding energy, for the reason you gave in your question. The nuclear binding energy is the energy it takes … Read more

Why is it energetically favourable for molecular bonds to form from a QM point of view?

For example, if you have two hydrogen atoms and an oxygen atom, they are all electrically neutral and don’t attract each other. But then if they manage to get “close enough” somehow they snap together releasing energy, and to get them apart again requires energy be input into the system. This says that the bound … Read more

What percentage of a Proton’s mass is potential/kinetic energy?

So in an hydrogen atom, the total mass of the atom is equal to the masses of the proton, the electron, minus their net binding energy of around 13 eV. Making the total less massive than the sum of its parts by about 1 part in 100 million. As it also turns out, the electron … Read more