2. Announcements

3. Radioactivity The plot below shows activity as a function of time for three samples. Which sample has the longest half-life? Which sample has the shortest half-life?

4. Radioactive Dating A 10.00 g charcoal sample has a 14 C activity of 129.0 dis/min. A living tree has a 14 C activity of 15.3 dis/min per 1.00g. The half-live of 14 C is 5730. How old is the sample?

5. Alpha-Decay Very heavy nuclei are “crowded” – nucleons want to leave Although it is possible for them to emit single nucleons, this is very rare Although it is possible for them to emit large particles, it is easier for them to emit small well-bound particles Such a particle is a 4 He nucleus Because the 4 He nucleus has four nucleons, two of which are protons, Z decreases by 2 and A decreases by 4 The 4 He nucleus is also called an  - particle This process is called alpha decay

6. Beta Decay A neutron inside a nucleus is spontaneously decays into a proton, an electron, and an antineutrino. The number of protons changes, so the element changes. Though energy, momentum, angular momentum, nucleon number, and charge is conserved. A dominate mechanism for light nuclei.

7. Beta: Electron Capture A proton inside a nucleus absorbs an electron, and becomes a neutron The number of protons changes, so the element changes. Though energy, momentum, angular momentum, nucleon number, and charge is conserved. Another mechanism for light nuclei decay.

8. Gamma Decay The nucleus can lose the energy by emitting a gamma ray (high energy photon) After a nucleus undergoes a radioactive decay, the nucleus is often in an excited state.

9. MAGNETISM Normally denoted by B Magnetic forces affect moving charged particles* Forces are perpendicular to both magnetic field and direction of motion *Or objects made of charged particles charge q Magnetic Field B Force F velocity v

10. Called a Tesla (T) A Tesla is a big magnetic field Magnetic field units: N  s/C  m Magnetic units MRI magnet -> 1-4 Tesla

11. Electric Field Lines Graphical Illustration of Electrical Fields Lines start on positive charges and end on negative Number of lines from/to a charge is proportional to that charge Density of lines tells strength of field. + - - +

12. Magnetic Field Lines Graphical Illustration of Magnetic Fields Lines start on north pole and end on south pole Opposite poles attract, like poles reply Density of lines tells strength of field. Key differences between magnetic fields and electric fields: All magnets have a north and south pole! No such thing as an isolated north or south pole. (have magnetic dipoles, not monopoles) Different force laws!

13. Vector Product Vector product -> two vectors make a vector Geometric Algebraic C has magnitude ABsin  Direction perpendicular to the plane containing A and B.

14. The right hand rule velocity v Magnetic Field B Force F

15. Magnetic Force Can only affect moving particles! Force depends on charge just like electric fields Force is maximum when the velocity and field are perpendicular, and zero when they are parallel When the velocity and field are neither perpendicular nor parallel, the force still exists!

16. 3-d directions of vectors out of the plane into the plane The two vectors shown represent the velocity and the magnetic field. For a negatively charged particle, which way will the particle accelerate? A)C) B)D) velocity v Magnetic Field B