H133: 1094 Session 6

Write your name and answers on this sheet and hand it in at the end.
After the indicated time, move on to the next activity, even if you are not finished!

1. Group Warm-Up Problems [10 minutes]

1. Do Q12T.7 and Q13T.1. [Give brief explanations.]
   
   
   
   
   
   
2. In your body, are there more neutrons or protons? More protons or electrons? Explain.
   
   
   
   
   
3. Do Q13T.5 and Q13T.6. [Give brief explanations.]
   
   
   
   
   
   
   

2. Q14.5: Decay Rates [10 minutes]

1. Consider problem Q14T.7. First, what is the "half-life" (see page 256) in this case? What is the answer to the problem? [BONUS: Using (Q14.31) and (Q14.29), how many undecayed nuclei are there at t = 30 minutes?]
   
   
   
   
   
2. Do problem Q14T.9. (Hint: After how much time would you expect the relative abundance to be one-half of that in living tissue?)
   
   
   
   
   

3. Beta Decay Physics [15 minutes]

1. Both ²²Mg and ²²O are unstable. Predict how they will decay until they reach a stable nucleus (using the "box" model of the nucleus). Note what particles (e.g., electron) are emitted.
   
   
   
   
   
   
   
   
   
2. Check your predictions: go to http://www.physics.ohio-state.edu/~ntg/H133/h133_applets.php and follow the "Beta Decay" link. Try to account for incorrect predictions.
   
   
   
   
3. Do problem Q13S.5.
   
   
   
   
   
   
   
4. A radioactive nucleus can emit a position, e⁺. This corresponds to a proton in the nucleus being converted to a neutron. The mass of a neutron, however, is greater than that of a proton. How then can positron emission occur?
   
   
   
   
5. Z changes in beta decay processes but A is constant. Explain how to find, for a given A, an expression for the most stable Z. (Note: How do you account for the neutron weighing more than the proton?)
   
   
   
   
   
   

4. Rutherford Scattering [10 minutes]

Start up the PhET applet "Rutherford Scattering". The simulation is of the famous experiment of Geiger and Marsden, in which they scattered alpha particles from nuclei. At the time, physicists didn't know for sure where the protons in the atom were located (neutrons weren't discovered until 1932). Two models were the "Rutherford Atom" and the "Plum Pudding Atom". In the latter, protons were spread evenly through the volume of the atom, rather than being concentrated in a small volume (the nucleus).

1. What is the force that deflects the alpha particles?
   
   
   
2. Look at both the Plum Pudding and Rutherford atoms models. Select "show traces" for each. What should observers in the laboratory measure to distinguish the models? (Remember that they don't see into the atoms!)
   
   
   
   
3. Why are there no large deflections in the plum pudding model (the simulation won't show any deflections)?
   
   
   
   
4. List two adjustments you can make in the simulation to make the alphas in the Rutherford Atom approach closer to the nucleus. [BONUS (do this if you have time at the end): Determine quantitatively the maximum energy for the alpha particle in the simulation.]