Frequently Asked Questions

The following are questions submitted to us. Helen Quinn, content provider for this web site, offers answers to the questions.

1. Why is so much energy produced when an atom is split or fused?
2. More about time dilation
3. How does a cyclotron work?
4. Have quarks been observed and isolated in the laboratory?
5. Is mass always conserved?
6. How can the exchange of a photon attract a proton and an electron, yet repel two electrons?
7. More about the speed of light.
8. I was looking for a SU3 chart of the quark model.
9. Why is the visual spectrum continuous if it is produced by electrons going from one quantum state to another within the atom?
10. Time, microphysical processes, and probability.

FAQ 1: Why is so much energy produced when an atom is split or fused?

What is meant here by "so much energy" -- much relative to what? The answer is relative to the mass of the fuel used. (If we burn enough fuel we can make as much electricity in a coal-fired power plant as in a nuclear one.) In any process the energy produced is determined by how much mass is converted to energy, following the rule E=mc². So what we are really interested in here is the fraction of the mass that is released in the process.

The short answer to your question is that the energy release in nuclear fission or fusion is a larger fraction of the mass of the fuel than in chemical processes because the binding forces (and hence the binding energies) between the protons and neutrons in nuclei are much larger than those for atoms in a molecule or solid.

To understand this first let's look at a coal-burning power plant (that is for combustion or for any other energy-releasing chemical reaction). The usual rule taught in chemistry is that mass is conserved. The precise version of this statement is that the sum of the masses of the atoms is the same before and after any chemical process, since atoms are not created or destroyed in chemical processes. But every stable molecule has a mass that is a tiny bit less than the sum of the masses of the atoms it contains. It is less by an amount (binding energy)/c-squared, where c is the speed of light (m=E/c² is just another way of writing E=mc²).

The energy released in any chemical combustion process is just the difference in binding energy between the molecules present before burning and those present after the burning. The typical binding energy of a molecule is a few parts in a billion of the mass-energy (mass times c-squared) of the molecule. (That's why we never see a measurable mass change in chemical reactions, our chemistry lab experiments never have a balance that's accurate to that level.)

Now lets consider nuclear processes. Here the role of atoms is replaced by the protons and neutrons and the role of molecules is replaced by the nuclei. Each nucleus has a mass which is a little less than the sum of the masses of the protons and neutrons that it contains. The energy release in fission or fusion is the difference in the sum of the masses (times c-squared) of the nuclei before and after the process. Since the total number of protons plus neutrons does not change in these processes, this energy release is again just the differences in the binding energies of the nuclei before and after. But now we are talking about nuclear binding energies, which are typically about 1-10 parts in 100,000 of the mass of the nuclei. The fraction is much bigger than for molecules because the binding forces between protons and neutrons in a nucleus are much stronger than the binding forces between atoms in a molecule.

Now for some additional info:

The full answer is that we never produce energy but merely transform it from one form to another. Indeed, mass (times c²) is just another name for the energy contained within a system, the sum of all forms.

So whether we are "converting mass to energy" or not actually depends on where we are measuring from. If we could measure the total mass of the system where all these conversions are occurring, from outside that system, the mass of the system does not change, as it is just a measure of the total energy in the system. But if we are inside the system, looking at the pieces, we call some of the energy mass, other parts heat or kinetic energy, other parts energy stored in electromagnetic fields or radiation. Then any increase in one kind must come at the price of some matching decrease in another kind.

The reason its not taught this way in chemistry is that the fractional mass changes in chemical processes are so small as to be unobservable in any analytic balance. Thus keeping track of mass is a good way to keep track of atoms --and the real conservation law of chemistry is conservation of the number of atoms of each type. Chemists always talk about binding energy as a separate concept from mass, and when they talk about mass they always mean the sum of the masses of the atoms. This is a very sensible way to proceed. It is inconvenient to mix up two quantities with scales several orders of magnitude different. It is much simpler to take the atomic masses out of the picture and just keep track of the differences in binding energy in chemical processes. But if you want to know what the precise mass of a molecule is, you must subtract the binding energy (divided by c²) from the sum of the masses of the atoms.

Indeed there is is a logical contradiction between conservation of mass (the full mass, including binding effects) and conservation of energy. The heat from a burning log must come from a decrease in some other form of energy. You can call that a change in binding energy if you prefer not to call it mass, but from the outside of the system you cannot in any way tell what fraction of the mass is in what form.

The extreme case here is a proton or neutron, less than a tenth of its mass can be attributed to the mass of its constituents (quarks and gluons), the rest is their kinetic energy and their interaction energy!