Simple Nature - Light and Matter

o / A nuclear power plant at Cattenom,France. Unlike the coaland oil plants that supply mostof the U.S.’s electrical power, anuclear power plant like this onereleases no pollution or greenhousegases into the Earth’s atmosphere,and therefore doesn’tcontribute to global warming. Thewhite stuff puffing out of thisplant is non-radioactive water vapor.Although nuclear powerplants generate long-lived nuclearwaste, this waste arguably posesmuch less of a threat to the biospherethan greenhouse gaseswould.p / The strong nuclear forcecuts off very sharply at a range ofabout 1 fm.8.2.5 The strong nuclear force, alpha decay and fissionOnce physicists realized that nuclei consisted of positively chargedprotons and uncharged neutrons, they had a problem on their hands.The electrical forces among the protons are all repulsive, so the nucleusshould simply fly apart! The reason all the nuclei in yourbody are not spontaneously exploding at this moment is that thereis another force acting. This force, called the strong nuclear force, isalways attractive, and acts between neutrons and neutrons, neutronsand protons, and protons and protons with roughly equal strength.The strong nuclear force does not have any effect on electrons, whichis why it does not influence chemical reactions.Unlike electric forces, whose strengths are given by the simpleCoulomb force law, there is no simple formula for how the strongnuclear force depends on distance. Roughly speaking, it is effectiveover ranges of ∼ 1 fm, but falls off extremely quickly at largerdistances (much faster than 1/r 2 ). Since the radius of a neutron orproton is about 1 fm, that means that when a bunch of neutrons andprotons are packed together to form a nucleus, the strong nuclearforce is effective only between neighbors.Figure q illustrates how the strong nuclear force acts to keepordinary nuclei together, but is not able to keep very heavy nucleifrom breaking apart. In q/1, a proton in the middle of a carbonnucleus feels an attractive strong nuclear force (arrows) from eachof its nearest neighbors. The forces are all in different directions,and tend to cancel out. The same is true for the repulsive electricalforces (not shown). In figure q/2, a proton at the edge of the nucleushas neighbors only on one side, and therefore all the strong nuclear490 Chapter 8 Atoms and Electromagnetism