Modern Physics

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PHY251 Midterm I Exam PHY 251 Midterm I Exam March 7 2001 I. Short Answer Questions (110 points) 1. List the crucial experimental evidence that showed that classical Newtonian mechanics was a low- velocity approximation of relativistic mechanics. Solution: (5 points) Michelson-Morley, who showed that all electromagnetic waves (predicted by Maxwell's equations) propagate with the same speed c in vacuum, regardless of inertial frame of the source or receiver. 2. List three types of experimental evidence that testify to the quantum character of the theory of radiation. Solution: (9 points) Planck's blackbody radiation, photoelectric effect, Compton scattering 3. List two types of experimental evidence that can be understood by assuming that sub-atomic particles like electrons have a wave-character. Solution: (6 points) Electron diffraction and interference, Bragg scattering of electrons (Davisson-Germer), Tunneling, Bohr's explanation of the hydrogen spectrum. 4. Consider a moving clock and a moving meter stick (moving parallel to its long dimension) both speeding at constant v = 0.8 c past a stationary observer (you). a. Calculate the length you will measure the speeding meter stick to have. Solution: (5 points) g = 1/ Ö (1-0.64) = 1/0.6 = 10/6; L ' = L 0 / g = 0.6 m b. Calculate the time that passed on the speeding clock if a clock in your rest frame indicates that 3.0 ns have passed. Solution: (5 points) Moving clock will "tick" slower: t' = gt 0 t 0 = 3.0 ns/ g = 1.8 ns c. Indicate how you would set up the experiment for the measurement in part b) above. Solution: (5 points) Several ways: set up two clocks that are perfectly synchronized, and move them c ×3 ns = 0.30 m apart, along the direction of the speeding clock. Rig a trigger close to each of the observers\'s clocks, so that when the moving clock passes, a nearby camera takes a picture. Comparison of the two pictures will then show the time differences on the
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