jtbell said:
Binding energy is negative.
Binding energy is always positive (or zero in the special case of
1H), but that's just a quibble over how "binding energy" is defined. Nuclear binding energy is the amount of energy that must be added to a nucleus to separate the constituent particles to infinite distance. The amount of energy bound up in the nucleus is negative.
Another way to look at it: The mass of the nucleus is always less than the sum of the masses of the neutrons and protons that form the nucleus.
mosad655 said:
How can we profit from this energy? How do they do it at nuclear power plants? Is binding energy the same as the kinetic energy of the atoms?
Start with fission. A
235U that captures a neutron will usually (about 82% of the time) undergo fission. It splits into, for example,
89Kr,
144Ba, 3 neutrons, and photons (gammas). Most of the energy that results from this fission is in the form of kinetic energy in the product nuclides. Those product nuclides will quickly collide with other nuclides, dispersing this kinetic energy. In other words, the kinetic energy quickly becomes thermal energy. Those product nuclides are highly unstable. They decay into something else, and this too becomes thermal energy. The same goes for the gammas. Most of them are also absorbed, their energy becoming thermal energy. Now it's just a matter of converting that thermal energy into useful energy. This is now just a problem of good old thermodynamics. Typically that thermal energy is used to boil water. The steam drives a steam turbine, and the turbine creates electricity.
Conceptually, fusion works in a similar fashion. A
4He nucleus has less mass than do (for example) a tritium nucleus and a proton. Conservation of energy (mass
is energy) dictates that that excess mass of
3H+
1H versus
4He be converted to some other form of energy, kinetic energy + photons + neutrinos. Most of this quickly becomes thermal energy. In practice, it is a lot harder to harness fusion in a sustained, controlled manner than it is to harness fission.