Simple Nature - Light and Matter

p/The strong nuclear force
cuts off very sharply at a range of
about 1 fm.

and protons, and protons and protons with roughly equal strength.

The strong nuclear force does not have any effect on electrons, which

is why it does not influence chemical reactions.

Unlike electric forces, whose strengths are given by the simple

Coulomb force law, there is no simple formula for how the strong

nuclear force depends on distance. Roughly speaking, it is effec-

tive over ranges of∼1 fm, but falls off extremely quickly at larger

distances (much faster than 1/r^2 ). Since the radius of a neutron or

proton is about 1 fm, that means that when a bunch of neutrons and

protons are packed together to form a nucleus, the strong nuclear

force is effective only between neighbors.

q/1. The forces cancel. 2. The forces don’t cancel. 3. In a

heavy nucleus, the large number of electrical repulsions can add up to

a force that is comparable to the strong nuclear attraction. 4. Alpha

emission. 5. Fission.

Figure q illustrates how the strong nuclear force acts to keep

ordinary nuclei together, but is not able to keep very heavy nuclei

from breaking apart. In q/1, a proton in the middle of a carbon

nucleus feels an attractive strong nuclear force (arrows) from each

of its nearest neighbors. The forces are all in different directions,

and tend to cancel out. The same is true for the repulsive electrical

forces (not shown). In figure q/2, a proton at the edge of the nucleus

has neighbors only on one side, and therefore all the strong nuclear

forces acting on it are tending to pull it back in. Although all the

510 Chapter 8 Atoms and Electromagnetism