Chapter 7, Problem 73PP

Nuclear fusion is the process that powers the Sun. Fusion occurs when two low-mass atomic nuclei fuse together to make a larger nucleus, in the process releasing substantial energy. This is hard to achieve because atomic nuclei carry positive electric charge, and their electrical repulsion makes it difficult to get them close enough for the short-range nuclear force to bind them into a single nucleus. Figure 7.25 shows the potential-energy curve for fusion of two deuterons (heavy hydrogen nuclei). The energy is measured in million electron volts (MeV), a unit commonly used in nuclear physics, and the separation is in femtometers (1 fm = 10⁻¹⁵ m).

FIGURE 7.25 Potential energy for two deuterons (Passage Problems 68-71)

When two deuterons are 4 fm apart, the force acting on them

1. a. is repulsive.
2. b. is attractive.
3. c. is zero.
4. d. can’t be determined from the graph.