Week 36 (Apr. 29 - May 3)

Reading: Radioactivity

Key Topics:
x-rays, cathode rays, JJ Thompson's experiments, Rutherford's experiments, alpha rays, beta rays, gamma rays, radioactive decay,

Homework Problems: Due Tuesday May 7.

  1. Write the alpha-decay process for Uranium-235 and Pa-231, including the chemical symbols and values for Z and A. (Brief answers: Uranium-235 spontaneously decays into Thorium-231 and an alpha-particle; Protactinium-231 spontaneously decays into Actinium-227 and an alpha-particle.)
  2. Write the beta-decay process for Cobalt-60, including the chemical symbols and values for Z and A. (Brief answer: Cobalt-60 spontaneously decays into Nickel-60 and a beta-particle.)
  3. Suppose a that a completely ionized alpha particle that is emitted from a radioactive nucleus is traveling in a straight line (along the z-axis, let's say) at a speed of 10,000 km/second. It passes through a region between two charged capacitor plates where the electric field is directed perpendicular to the z-axis (let's say the electric field is pointed directly along the x-axis.). The electric field strength is 1000 volts/meter. Naturally, the alpha particle will feel a force due to the electric field, so its trajectory will change. But suppose a magnetic field is also applied to the moving alpha particle in an attempt to make it go in a straight line. (a) In which direction must one aim the magnetic field (along which axis?) in order to make the alpha particle go in a straight line instead of curving? (b) How strong must the magnetic field be in order to make the alpha particle go in a straight line? (Solution: The helium nucleus has a positive charge of q = 2 * 1.6e-19 Coulombs, since it is fully ionized. The electric field will thus exert an electrical force, F = qE, on the helium nucleus that will try to push it in the +x direction. The strength will be F = 3.2e-16 Newtons. In order to make it go in a straight line, the electric force must be balanced by the Lorentz force exerted by the magnetic field. The magnetic field must be directed in the +y direction so that the Lorentz force F = qvB is directed in the -x direction. The magnetic field must be B = 1e-4 Tesla.)
General College Physics