# MITES 2010: Physics III Survey of Modern Physics Final Exam Solutions

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1 MITES 2010: Physics III Survey of Modern Physics Final Exam Solutions Exercises 1. Problem 1. Consider a particle with mass m that moves in one-dimension. Its position at time t is x(t. As a function of its position, Its potential energy is U(x = C, where C is a constant. (a Derive the equation of motion for the particle using Newtons second law. (b Derive the equation of motion for the particle, but this time, using the Lagrangian. (c Solve the equation of motion (i.e., what are x(t and dx dt?. Define all the free parameters in your solution (i.e. what are their physical meanings? Solution: (afrom Newton s Second Law we find mẍ = F (x = d dx U(x = d dx C = 0 = ẍ = 0 (b From the definition of the Lagrangian we find that the Lagrangian of this system has the form L = KE PE = 1 2 mẋ2 C From this Lagrangian we may derive equation of motion using the Euler-Lagrange (E-L equation. and so L x = x ( C = 0 d L dt ẋ = d dt ẋ = d dt (mẋ = mẍ ( 1 2 mẋ2 0 = L x d L = 0 mẍ = ẍ = 0 dt ẋ 1

2 (c Our equation of motion ẍ = 0 is a simple second order differential equation so we may solve it by separation of variables. ẍ = 0 dẋ dt = 0 dẋ ẋ(t = 0 dt ẋ 0 dẋ = 0 ẋ(t ẋ 0 (t = 0 = ẋ(t = ẋ 0 This equation for velocity provides us with another differential equation for the positon x(t. x(t ẋ = ẋ 0 dx dt = ẋ 0 dẋ = ẋ 0 dt x 0 dx = t 0 ẋ 0 dt x(t x 0 = ẋ 0 t = x(t = x 0 + ẋ 0 t In our solutions, the free parameter ẋ 0 represents the initial velocity of particle and x 0 represents the initial position. 2. Problem 2. Hanging mass The potential energy for a mass hanging from a spring is V (y = ky 2 /2 + mgy, where y = 0 corresponds to the position of the spring when nothing is hanging from it. (a What is the net force acting on the particle? (b Write down the Lagrangian for this system. (c Use the Euler-Lagrange equation to derive the equation of motion for this system. DO NOT use Newtons second law to derive it. What kind of motion does this correspond to? (d Solve the equation of motion, thereby obtaining y(t. What do all the free parameters in your solution physically mean? Solution: (a F (y = V (y = d dy (ky2 /2 + mgy = ky mg (b L = KE PE = 1 2 mẋ2 1 2 ky2 mgy 2

3 (c L y d L dt ẏ = y ( 1 2 ky2 mgy = ky mg = d ( 1 dt ẏ 2 mẏ2 = mẍ 0 = L y d L dt ẏ = ky mg mÿ = ÿ + k m y + g = 0 If we let δ = y + mg/k then we have δ + ω 2 δ = 0 where ω 2 = k/m. This is the simple harmonic oscillator equation which models all free undamped oscilatory motion. d The solution to the SHO EOM, δ + ω 2 δ = 0, is δ(t = A sin ωt + B cos ωt And from our definition δ = y + mg/k, we find that y(t is y(t = A sin ωt + B cos ωt mg k ω = k m. In order to find what A and B represent, we solve for the initial conditions of our mass. Setting t = 0 in our position equation, we find y 0 = y(t = 0 = A sin 0 + B cos 0 mg/k = B mg/k = B = y 0 + mg k B represents the initial displacement from the equilibrium position y eq = mg/k. Similarly, differentiating y(t and then plugging in t = 0 gives us v 0 = ẏ(t = 0 = Aω cos 0 Bω sin 0 = Aω = A = v 0 ω A represents the inital velocity of the bead divided by the angular frequency. All of this information gives us a general solution x(t of the form y(t = v ( 0 ω sin ωt + y 0 + mg cos ωt k 3. Problem 3. Zipper problem (Model for unzipping of DNA. A zipper has N links; each link has a state in which it is closed with energy 0 and a state in which it is open with energy ɛ. We require, however, that the zipper can only unzip from the left end, and that the link number s can only open if all links to the left (1, 2,..., s 1 are already open. (a. Show that the partition function can be summed in the form [ ] 1 exp Z = 1 exp 3 (N+1ɛ k B T [ ɛ k B T ]

5 Computing the average energy, we find E = log Z β = 1 Z Z β = 1 1 e β(n+1ɛ Z β 1 e [ βɛ ] = 1 ɛ(n + 1e (N+1βɛ (1 e βɛ ɛe βɛ (1 e (N+1βɛ Z (1 e βɛ 2 [ ] = 1 e βɛ ɛ(n + 1e (N+1βɛ (1 e βɛ ɛe βɛ (1 e (N+1βɛ 1 e β(n+1ɛ (1 e βɛ 2 ɛ(n + 1e (N+1βɛ = 1 e (N+1βɛ + ɛe βɛ 1 e βɛ Now, if we take βɛ >> 1, then we can take the following approximations 1 e βɛ 1 e βɛ >> e (N+1βɛ So that our average energy becomes E ɛe βɛ and the average number of open links is n = e βɛ. (c. Give a physical meaning of what ɛ >> k B T means. In particular, give a physical justification for what the approximation ɛ >> k B T means and why you can ignore some of the terms in your partition function in (b under this approximation. Recall that k B T is the typical thermal energy in a system. [Hint: How accessible are the states with energy much larger than the typical thermal energy?] Solution: ɛ is defined as the energy necessary to open one link in our zipper, so ɛ >> k B T means that it requires a lot of energy to open one link. Consequently, it will take a lot more energy to open multiple links. Therefore, if we consider this problem as a describing a state with a limited amount of energy, then the approximation ɛ >> k B T restricts the number of states the system can access. Since it requires such a large amount of energy to open mutliple states, the most probable states will be the ones with the lowest energy. This allows us to shorten our partition function so that it only contains the configurations with the lowest energy Z = 1 + e βɛ + e 2βɛ + + e (N 1βɛ + e Nβɛ 1 + e βɛ 4. Problem 4: Applying the rules of Quantum Mechanics. Consider a particle whose energy can only take on the following values: E 1 = 3 hω, E 2 = 6 hω, E 3 = 9 hω, E 4 = 0

6 These are the only allowed energies that the particle can have. They have corresponding eigenstates: E 1, E 2, E 3, E 4. Ĥ is the Hamiltonian operator - operator representing measurement of the particle s energy. Consider a particle in the following indeterminate state. ψ = 1 E 1 + eiθ 2 E eiϕ E 4 (i. If you measure the energy of 10,000 identical particles, all in the above indeterminate state just before your measurement, how many of them do you expect to yield energy value of E 2? Solution: The probability of obtaining E 2 is e iθ 2 = 1. So, out of 10,000 measurements, we expect 6 6 to yield energy E 2. 10, particles (ii. On average, what value of energy would your measurement yield? Solution: E avg = c 1 2 E 1 + c 2 2 E 2 + c 3 2 E 3 + c 4 2 E 4 = hω hω hω = 3 2 hω (iii. For which value of θ is the probability of a particle having energy E 2 zero? Solution: There is no value of θ for which the probability of obtaining E 2 is zero. First of all, this is because the coefficient of the state E 2 is proportional to e iθ and e iθ never equals zero. Second of all, the magnitude of e iθ is always 1 regardless of θ, so the probability of being in E 2 is independent of θ Prob(E 2 = e iθ / 6. (iv. Suppose you measure the energy of a particle. You find that it has energy E 1. Does this mean that the particle was in state E 1 just before your measurement? Suppose you measure the energy of the same particle for the second time. What is the probability that you get E 4 as the energy this second time? Solution: No, the particle was not necessarily in the state E 1 before the measurement. A postulate of quantum mechanics states that a quantum state is in a superposition of many states before a measurement and only collapses to one state after the measurement. Also, since the state is now in E 1 then there is zero probability that it is in any other state. 6

7 (v. Suppose you measure the energy of the particle and find that it has energy E 2. Write down the ket-vector representing the particle s state immediately after this measurement. Solution: Since we measured energy and obtained the value E 2 we know that the particle s state has now collapsed to the state corresponding to E 2. That is, the particle is in state ψ = E 2.. Problem. Sequential measurements An operator Â representing observable A, has two normalized eigenstates ψ 1 and ψ 2, with eigenvalues a 1 and a 2 respectively. Operator Â representing observable B, has two normalized eigenstates φ 1 and φ 2, with eigenvalues b 1 and b 2. The eigenstates are related by ψ 1 = 3φ 1 + 4φ 2 ψ 2 = 4φ 1 3φ 2 (a. How do we know that observables Â and ˆB do not commute (i.e. observables? (1 Â and ˆB are incompatible Solution: We know that the observables Â and ˆB do not commute 1 because they do not have the same eigenstates (i.e. φ 1 ψ 1 and φ 2 ψ 2. Compatible observables (observables with commuting operators must have the same eigenstate because the order of their operation does not matter. For example, let B 1 be an eigenstate of ˆB with eigenvalue b1. Then we can perform the following operation for commuting operators ˆB and Ĉ 0 = [Ĉ, ˆB] B 1 = Ĉ ˆB B 1 ˆBĈ B 1 ˆBĈ B 1 = Ĉ ˆB B 1 ˆB(Ĉ B 1 = b 1 (Ĉ B 1 So we see that Ĉ B 1 must be an eigenvector of ˆB with eigenvalue b 1. This means that we can write Ĉ B 1 as a multiple of B 1 such as Ĉ B 1 = k B 1 But, this is just the definition of an eigenvector B 1 of Ĉ with eigenvalue k. So we see that commuting operators share the same eigenvectors. Conversely, noncommuting operators will have different eigenvectors. (b. Observable A is measured, and the value a 1 is obtained. What is the state of the system (immediately after this measurement? Solution: By one of the postulates of quantum mechanics, the state has collapsed into the state corresponding to eigenvalue a 1. So, the state is in ψ 1. (c. If B is now measured, what are the possible results, and what are their probabilities? Solution: The state is currently in ψ 1. ψ 1 = 3 φ φ 2 1 Commuting observables satisfy Â ˆB = ˆBÂ or [Â, ˆB] = 0 7

8 From this state, there are two possible results upon measurement of B. associated probabilities are These results and their Results Probability 2 = 9 ( 3 b 1 2 ( 4 2 b 2 = 16 2 which fits our normalization requirement because 9/2 + 16/2 = 2/2 = 1 (d. Right after the measurement of B, A is measured again. What is the probability of getting a 1? (Note that the answer would be quite different if I had told you the outcome of the B measurement. Solution: There are two possible ways to get to a 1 from our initial a 1 measurement. We can obtain b 1 from the B measurement and then obtain a 1 from the A measurement. Or, we can obtain b 2 from the B measurement and then obtain a 1 from the A measurement. In order to calculate the total probability of obtaining a 1, we must calculate the probability of these independent paths and then add them. But first, we must write φ 1 and φ 2 in terms of ψ 1 and ψ 2 in order to obtain the probabilities for the second measurement of A after our measurement of B. Solving Eq (1by elimination, we find for φ 1 ψ 1 = 3φ 1 + 4φ 2 ψ 1 4φ 2 = φ 1 3 Substituting this result into our other equation, we find Now, solving for φ 1 ψ 2 = 4φ 1 3φ 2 ψ 2 = 4 3 (ψ 1 4φ 2 3φ 2 = 20 3 ψ φ 2 3φ 2 = 20 3 ψ φ 1 2φ 2 = 20 3 ψ 1 ψ 2 φ 2 = 4ψ 1 3ψ 2 ψ 1 = 3φ 1 + 4( 4 ψ 1 3 ψ 2 = 3φ ψ 1 12 ψ 2 9 ψ 1 = 3φ 1 12 ψ 2 φ 1 = 3ψ 1 + 4ψ 2 8

9 So, in summary, we have φ 1 = 3ψ 1 + 4ψ 2 φ 2 = 4ψ 1 3ψ 2 From this result, we may now construct our probabilities. There are two paths to get to a 1 from a measurement of B. These paths and their associated probabilities are Path Probability 1 From b 1 to a = From b 2 to a = For each path, we multiplied the probability of obtaining our first measured value of B by the probability of obtaining our second measured value of A. The total probability of obtaining a 1 is the sum of these probabilities and is therefore = 337 We can check that this procedure 62 for calculating probability is consistent with normalization by calculating the probability other two possible paths. Instead of obtaining a 1, we could have obtained a 2 in our second measurement and the paths and probabilities associated with this value are Path Probability 3 From b 1 to a = From b 2 to a = We have listed all possible outcomes of our measurements. The sum of the probabilities for theses outcomes must be one. Prob(Path 1 + Prob(Path 2 + Prob(Path 3 + Prob(Path 4 = = = 1 Our outcomes are normalized, so we know that our procedure is correct. 6. Problem 6. Eigenstates and Eigenvalues of a two level system. The Hamiltonian Ĥ of a particle with two possible energy states is represented by the 2 2 matrix ( 1 0 Ĥ = hω 0 2 (a. What are the possible values of energy for this particle (i.e. eigenvalues of Ĥ? Solution: We employ the standard procedure for the calculation of eigenvalues. 0 = det(ĥ ( EÎ hω E 0 = det 0 2 hω E = ( hω E(2 hω E = E 1 = hω, E 2 = 2 hω (b. For each eigenvalue of Ĥ you found in (a, find the corresponding eigenstate of Ĥ. 9

10 Solution: E 1 = hω 0 = (Ĥ E1Î E 1 ( hω hω 0 = 0 2 hω hω ( 0 0 = E 0 hω 1 E 1 The last line suggests that E 1 cannot have a second component. So we have E 1 = ( α1 0 = E 1 = Where we set α 1 = 1 in order to normalize our eigenstate. For the other eigenvalue, E 2 = 2 hω, we have ( = (Ĥ E2Î E 2 ( hω 2 hω 0 = 0 2 hω 2 hω ( hω 0 = E E 2 The last line suggests that E 2 cannot have a first component. So we have ( 0 E 1 = α 2 = E 2 = Where we set α 2 = 1 in order to normalize our eigenstate. ( 0 1 (c. The two eigenstates you found in (b, represent two orthogonal vectors. What is the physical significance of this fact? Explain your answer in terms of the measurement of energy of a particle that is in an arbitrary state before your measurement. Solution: The fact that the two eigenstates are orthogonal (i.e. E 1 E 2 = E 2 E 1 = 0 signifies the fact that when we are in one eigenstate of an observable then we can only be in that eigenstate. There is no overlap between the states so that if we measure the energy and obtain eigenvalue E 1 then when we perform another measurement we will obtain E 1 again because the probability of the particle being in state E 2 is zero. Probability of measuring E 2 after obtaining E 1 = E 2 E 1 2 = 0 10

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