A First Course in Physics · Ch. XXII, §§ 608–614 · Chapter 22: Invisible Radiations
72. Radio-activity and the energy in atoms
Goal: Your student can tell the story of Becquerel's and the Curies' discoveries, describe alpha, beta and gamma rays, explain that radio-active atoms change into other elements, and compare the book's predictions with what came after.
You'll need
- The book, open to these pages
- Paper and pencil for a timeline (1895–today)
- A notebook and pencil
- A calculator (optional, for the problems)
What's in the book
Becquerel's uranium rays (1896); Madame Curie's discovery of radio-active thorium and of radium in pitchblende; alpha, beta and gamma rays (Rutherford); the spinthariscope; Rutherford's theory that radio-active atoms are breaking down; helium produced from radium; the slow decay and birth of radium; the enormous energy stored in atoms, and the closing question whether man can ever use it.
The lesson, step by step
Hook question
SayThe book ends with a question: could people ever use the energy locked inside atoms? What do you think happened?
Read together
Read §§ 608–614, including the last paragraph. The book's final question:
Is it possible for man to gain control of this tremendous store of subatomic energy and to use it for his own ends?
Watch only: radio-active materials
Watch only: never handle radio-active materials. Radium, uranium ore (pitchblende) and old radium-painted watch dials are dangerous; radium cost the health of many people in the early 1900s, including workers who painted glowing dials. Look up a video of a cloud chamber, where you can safely see the tracks of alpha and beta particles.
Explain the idea
Some heavy atoms are unstable. Now and then one breaks apart and shoots out a particle: alpha (positive, stopped by thin foil), beta (electrons), or gamma rays (very penetrating, § 610). What is left is a different element (§ 612). Each atom releases enormous energy, far more than burning coal (§ 614).
What we know now
Several of the book's guesses have been corrected. An alpha particle is a helium nucleus, about four times as heavy as a hydrogen atom (not twice). Radium's half-life is about 1600 years (the book estimates 1280). Gamma rays come from the nucleus itself. The earth is about 4.5 billion years old, which radio-active dating helped show. And the book's question was answered: in 1942 the first nuclear reactor ran in Chicago, where Millikan and Gale taught; nuclear power plants and nuclear weapons followed. The sun is powered by nuclear fusion, as § 614 guessed.
Make a timeline
Together, put these on a timeline: Röntgen's X rays 1895, Becquerel 1896, Thomson's electron 1897, the Curies' radium 1898, Rutherford's alpha, beta and gamma rays (1899–1903), this book 1906, Rutherford's nucleus 1911, the first reactor 1942.
Say it back
Your student explains why radium must be continually forming if it decays (§ 613).
Wrap up the course
Look back at the table of contents together. Ask your student which chapter was most surprising, and which experiment they would like to repeat.
Afterwards
Read a short biography of Marie Curie, the first person to win two Nobel Prizes.