How Things Work - Chapter 16 Demonstrations
Section 16.1 Nuclear Weapons
Demonstration 16.1.1: Hopping Toys
Description: You press the top and base of a spring-loaded toy together. After a few seconds, the suction cup that holds them together releases and the toy leaps into the air.
Purpose: To show that there are systems in which a strong, short-ranged attractive force can temporarily keep that system together, despite the presence of a weaker but much longer ranged repulsive force.
1 hopping toy (sold in various games and given away as premiums by some stores and restaurants—they have a spring between their top and base and a leaky suction cup that will temporarily keep the spring compressed when you squeeze the top and base together)
Procedure: Discuss the two forces in the toy: a relatively long-ranged repulsion (the spring) and a relatively short-ranged attraction. Squeeze the toy together and point out that you are doing work as you push the parts together—they are storing energy in the long-ranged repulsive force (potential energy). When the suction cup catches, point out that the short-ranged attractive force has now taken over and will keep the toy together.
However, there is a leak in the suction cup, so that it eventually lets go. When that occurs, the toy hops. The energy needed for the hop comes from energy you stored in it when you first squeezed its parts together.
Explanation: The spring is analogous to the long-ranged electrostatic repulsion between protons in a nucleus. The suction cup is analogous to the short-ranged attractive nuclear force that holds the protons together once they touch. The leakiness of the suction cup is analogous to the quantum tunneling that permits the nucleus to fall apart in spontaneous fission.
Follow-up: Squeeze together several of these toys at once and arrange them so that, when one jumps, it hits the others and causes them to "fission"—a chain reaction.
Demonstration 16.1.2: Radioactive Decay
Description: You hold a Geiger counter near various radioactive sources and listen to the random nature of their decays.
Purpose: To show that radioactive decay is a random, spontaneous event.
1 or more radioactive sources (appropriate licensing, training, and safety precautions must be followed)
1 Geiger counter
Procedure: Use the Geiger counter to monitor the decays of the various radioactive sources. Point out that the nuclei in these sources were given excess energy long ago, either in a star explosion (a supernova) billions of years ago or at a nuclear reactor facility in more recent years. This excess energy has been stored in the nuclei ever since but they haven't been able to release it because of various competitions between forces. Occasionally quantum tunneling allows one of the forces to win out over another and one of these nuclei spontaneous decays. It then releases some or all of its excess energy and the Geiger counter detects its energetic fragments. Point out that there is absolutely no way to predict when a particular nucleus will decay. The best one can do is make a statistical prediction that a certain fraction of the nuclei will decay during a certain time. If you look at enough nuclei, such statistical predictions will be quite accurate.
Explanation: In the case of spontaneous fission, the Coulomb repulsion is defeating the nuclear force because of tunneling effects. In the decays of other common radioactive sources, relatively isolated neutrons may be converting to protons, electrons, and antineutrinos (so-called "beta decay"). In still other sources, helium nuclei may be emitted (so-called "alpha decay").
Demonstration 16.1.3: A Mousetrap Nuclear Explosion
Description: You drop a small rubber ball into a field of set mousetraps, each loaded with two rubber balls. After bouncing around briefly, the rubber ball trips a mousetrap and the whole collection suddenly "explodes" in a shower of bouncing balls.
Purpose: To show how a chain reaction occurs.
65 or more small rubber balls
1 clear plastic rectangular top with a small hole on the top center (it should accommodate all 32 mousetraps, closely spaced, and have a height of about 30 cm so that the balls have some room to move)
Procedure: Set each of the mousetraps and place them close together on the board. Very carefully put one rubber ball on each side of the flip wire so that when the mousetrap trips, both balls will be thrown into the air. When all 32 mousetraps are set and loaded with balls, carefully place the plastic cover on top of them. When everything is ready, drop one of the remaining rubber balls through the hole in the plastic top. The ball may bounce once or twice, but it will probably trip a mousetrap sooner or later. When it does, the whole collection of mousetraps will explode into action. Discuss how this result resembles an explosive chain reaction in fissionable nuclei. When the Pandemonium is over, you might note that one or two mousetraps remain untripped. That would be consistent with a nuclear explosion, where some of the nuclei survive despite the violent activity around them.
Explanation: The average mousetrap "induced fission" yields three rubber balls (the original ball is still available). With such a rapid increase in the number of rubber balls bouncing in the box, the explosive chain reaction proceeds quickly.
Section 16.2 Medical Imaging and Radiation
Demonstration 16.2.1: Shadow CT Scan
Description: An opaque object is rotated about on an overhead projector while the students try to guess what it is.
Purpose: To show that many 2-D shadow views can be assembled to give you a 3-D image of an object.
1 overhead projector
1 or more interesting opaque objects
Procedure: Turn on the overhead projector and focus the white light from writing surface onto a screen. Now place an opaque object onto the projector’s writing surface and hold it still. Have the students try to guess what it is. Now begin to rotate the object slowly about so that they can see its shadow from many different directions. Have them guess again.
Explanation: Although a CT scan only makes a series of shadow views of a complicated 3-D object, these individual views can be assembled together into a very detailed 3-D image of the original object.