Working with Probabilities Physics 115a (Slideshow 1) A. Albrecht These slides related to Griffiths section 1.3
Consider the following group of people in a room: AgeNumber
Histogram Form
Consider the following group of people in a room: AgeNumber Total people = 14
Consider the following group of people in a room: Total people = 14 AgeNumberProbability ?
Consider the following group of people in a room: Total people = 14 AgeNumberProbability /
Consider the following group of people in a room: Total people = 14 AgeNumberProbability / ?
Consider the following group of people in a room: Total people = 14 AgeNumberProbability / /14
Consider the following group of people in a room: Total people = 14 AgeNumberProbability 141? 1511/14 163? 222? 242? 2555/14
Consider the following group of people in a room: Total people = 14 AgeNumberProbability 1411/ / / / / /14
Probability Histogram
Number Histogram
NB: The probabilities for ages not listed are all zero Total people = 14 AgeNumberProbability 1411/ / / / / /14
Assuming Age<20, what is the probability of finding each age? Total people = 14 AgeNumberProbability 141? 151? 163? 222? 242? 255?
Assuming Age<20, what is the probability of finding each age? Total people = 14 AgeNumberProbability 141? 151? 163?
Assuming Age<20, what is the probability of finding each age? Total people = 14 AgeNumberProbability 1411/5 1511/5 1633/
Total people = 14 AgeNumberProbability 1411/ / / / / /14 Assuming no age constraint, what is the probability of finding each age? Related to collapse of the waveunction (“changing the question”)
Assuming Age<20, what is the probability of finding each age? Total people = 14 AgeNumberProbability 1411/5 1511/5 1633/ Related to collapse of the waveunction (“changing the question”)
Consider a different room with different people: AgeNumber Total people = 15
Consider a different room with different people: AgeNumberProbability 1933/ / / / / /15 Total people = 15
Red Room Numbers
Red Room Probabilities
Combine Red and Blue rooms Total people = 29 AgeNumberProbability 1411/ / / / / / / / /29
Lessons so far A simple application of probabilities Normalization “Re-Normalization” to answer a different question Adding two “systems”. All of the above are straightforward applications of intuition.
Expectation Values
Most probable answer = 25 Median = 23 Average = 21
Most probable answer = 25 Median = 23 Average = 21 Lesson: Lots of different types of questions (some quite similar) with different answers. Details depend on the full probability distribution.
Average (mean): Standard QM notation Called “expectation value” NB in general (including the above) the “expectation value” need not even be possible outcome.
Average (number squared) AgeNumber(Number) 2 Probability 14111/ / / / / /14
In general, the average (or expectation value) of some function f(j) is Careful: In general
The “width” of a probability distribution
Discuss eqns 1.10 through 1.13 at board
Continuous Variables
Why not measure age in weeks?
Blue room in weeks
Conclusion: Blue room in weeks not very useful/intuitive
Another case where a measure of age in weeks might by useful: The ages of students taking health in the 8 th grade in a large school district (3000 students).