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Thermo & Stat Mech - Spring 2006 Class 16 1 Thermodynamics and Statistical Mechanics Probabilities

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Thermo & Stat Mech - Spring 2006 Class 162 Pair of Dice For one die, the probability of any face coming up is the same, 1/6. Therefore, it is equally probable that any number from one to six will come up. For two dice, what is the probability that the total will come up 2, 3, 4, etc up to 12?

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Thermo & Stat Mech - Spring 2006 Class 163 Probability To calculate the probability of a particular outcome, count the number of all possible results. Then count the number that give the desired outcome. The probability of the desired outcome is equal to the number that gives the desired outcome divided by the total number of outcomes. Hence, 1/6 for one die.

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Thermo & Stat Mech - Spring 2006 Class 164 Pair of Dice List all possible outcomes (36) for a pair of dice. Total CombinationsHow Many , , 3+1, , 4+1, 2+3, , 5+1, 2+4, 4+2, 3+35

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Thermo & Stat Mech - Spring 2006 Class 165 Pair of Dice Total CombinationsHow Many 71+6, 6+1, 2+5, 5+2, 3+4, , 6+2, 3+5, 5+3, , 6+3, 4+5, , 6+4, , Sum = 36

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Thermo & Stat Mech - Spring 2006 Class 166 Probabilities for Two Dice

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Thermo & Stat Mech - Spring 2006 Class 167 Probabilities for Two Dice

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Thermo & Stat Mech - Spring 2006 Class 168 Microstates and Macrostates Each possible outcome is called a “microstate”. The combination of all microstates that give the same number of spots is called a “macrostate”. The macrostate that contains the most microstates is the most probable to occur.

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Thermo & Stat Mech - Spring 2006 Class 169 Combining Probabilities If a given outcome can be reached in two (or more) mutually exclusive ways whose probabilities are p A and p B, then the probability of that outcome is: p A + p B. This is the probability of having either A or B.

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Thermo & Stat Mech - Spring 2006 Class 1610 Combining Probabilities If a given outcome represents the combination of two independent events, whose individual probabilities are p A and p B, then the probability of that outcome is: p A × p B. This is the probability of having both A and B.

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Thermo & Stat Mech - Spring 2006 Class 1611 Example Paint two faces of a die red. When the die is thrown, what is the probability of a red face coming up?

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Thermo & Stat Mech - Spring 2006 Class 1612 Another Example Throw two normal dice. What is the probability of two sixes coming up?

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Thermo & Stat Mech - Spring 2006 Class 1613 Complications p is the probability of success. (1/6 for one die) q is the probability of failure. (5/6 for one die) p + q = 1, or q = 1 – p When two dice are thrown, what is the probability of getting only one six?

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Thermo & Stat Mech - Spring 2006 Class 1614 Complications Probability of the six on the first die and not the second is: Probability of the six on the second die and not the first is the same, so:

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Thermo & Stat Mech - Spring 2006 Class 1615 Simplification Probability of no sixes coming up is: The sum of all three probabilities is: p(2) + p(1) + p(0) = 1

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Thermo & Stat Mech - Spring 2006 Class 1616 Simplification p(2) + p(1) + p(0) = 1 p² + 2pq + q² =1 (p + q)² = 1 The exponent is the number of dice (or tries). Is this general?

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Thermo & Stat Mech - Spring 2006 Class 1617 Three Dice (p + q)³ = 1 p³ + 3p²q + 3pq² + q³ = 1 p(3) + p(2) + p(1) + p(0) = 1 It works! It must be general! (p + q) N = 1

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Thermo & Stat Mech - Spring 2006 Class 1618 Binomial Distribution Probability of n successes in N attempts (p + q) N = 1 where, q = 1 – p.

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Thermo & Stat Mech - Spring 2006 Class 1619 Thermodynamic Probability The term with all the factorials in the previous equation is the number of microstates that will lead to the particular macrostate. It is called the “thermodynamic probability”, w n.

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Thermo & Stat Mech - Spring 2006 Class 1620 Microstates The total number of microstates is: For a very large number of particles

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Thermo & Stat Mech - Spring 2006 Class 1621 Mean of Binomial Distribution

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Thermo & Stat Mech - Spring 2006 Class 1622 Mean of Binomial Distribution

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Thermo & Stat Mech - Spring 2006 Class 1623 Standard Deviation ( )

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Thermo & Stat Mech - Spring 2006 Class 1624 Standard Deviation

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Thermo & Stat Mech - Spring 2006 Class 1625 Standard Deviation

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Thermo & Stat Mech - Spring 2006 Class 1626 For a Binomial Distribution

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Thermo & Stat Mech - Spring 2006 Class 1627 Coins Toss 6 coins. Probability of n heads:

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Thermo & Stat Mech - Spring 2006 Class 1628 For Six Coins

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Thermo & Stat Mech - Spring 2006 Class 1629 For 100 Coins

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Thermo & Stat Mech - Spring 2006 Class 1630 For 1000 Coins

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Thermo & Stat Mech - Spring 2006 Class 1631 Multiple Outcomes

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Thermo & Stat Mech - Spring 2006 Class 1632 Stirling’s Approximation

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Thermo & Stat Mech - Spring 2006 Class 1633 Number Expected Toss 6 coins N times. Probability of n heads: Number of times n heads is expected is: n = N P(n)

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