A First Course in Physics · Ch. XXII, §§ 599–607 · Chapter 22: Invisible Radiations
71. Cathode rays, the electron, and X rays
Goal: Your student can describe cathode rays and the evidence that they are tiny negative particles (electrons), describe X rays and their discovery, and compare the 1906 picture of the atom with what we know now.
You'll need
- The book, open to these pages
- A strong magnet (to show, by a video or a description, how it bends cathode rays)
- A dental or medical X-ray picture if your family has one
- A notebook and pencil
What's in the book
Electric discharges in partial vacua (Geissler tubes); cathode rays at very low pressure, their heating effect and sharp shadows; their deflection by a magnet and by charges shows they are negative particles; their mass about 1/2000 of a hydrogen atom and speeds up to a third of light's; J. J. Thomson's electron theory of the atom; Röntgen's discovery of X rays (1895); X rays make gases conduct; X-ray pictures. Portraits of Röntgen and J. J. Thomson.
The lesson, step by step
Hook question
SayWhat is an atom made of? In 1906 that was one of the newest questions in physics.
Read together
Read §§ 599–607. The book's careful verdict:
This theory undoubtedly contains many germs of truth. As yet, however, it is in the formative stage
Watch only: vacuum tubes and X rays
Watch only: don't try this at home. Cathode-ray tubes and X-ray tubes need very high voltages, and X rays are harmful (§§ 599–607). Look up a video of a teacher's "Crookes tube" or "Maltese cross tube" demonstration and of a cathode-ray beam bent by a magnet.
Explain the idea
At very low pressure, invisible rays stream from the negative terminal. A magnet bends them the way it would bend a stream of negative charges (§ 601). Thomson showed (1897) that they are particles about 2000 times lighter than a hydrogen atom: electrons. Röntgen found (1895) that where cathode rays strike glass or metal they produce X rays, which pass through flesh but not bone (§ 607).
What we know now
The electron theory has been completely confirmed. But the details have changed. A hydrogen atom has just one electron (not about 2000 as § 603 suggests); almost all of an atom's mass is in a tiny nucleus, found by Rutherford in 1911, made of protons and neutrons. X rays are electro-magnetic waves like light, only with far shorter wave lengths. Doctors now use X rays at very low, carefully limited doses.
Say it back
Your student explains the evidence that cathode rays are made of negatively charged particles and not of light.
Look at an X-ray picture
If you have a dental or medical X-ray, look at what shows white (bones, teeth, metal fillings) and dark (soft tissue). Discuss why bones stop more X rays than flesh.
Afterwards
Read about J. J. Thomson and Ernest Rutherford in an encyclopedia, and draw the 1906 "solar system" atom next to a modern picture of the atom.