## PHYS 342 Class Info

IMPORTANT: The tests will be in the classroom. If you miss a test, an excused absence needs to be verified through the Office of the Dean of Students: https://www.purdue.edu/advocacy/students/absences.html See the syllabus for the grading policy for excused absences. Unfortunately, an unexcused absence will count for 0 points.

TEST 1 will be Thurs. Feb 23 and will cover Chaps ?. Bring a nongraphing, dumb calculator (can only do logs, trig functions, exponentials, powers, roots; one or two line TI-30X and TI-36X models; if you are uncertain, ask me; similar to this rule) and a writing device.

TEST 2 will be Thurs. Apr 13 and will cover Chaps ?. Bring a nongraphing calculator and a writing device.

FINAL will be ???? May ?, ?-? pm. The final will be comprehensive. Bring a nongraphing calculator and a writing device.

The text book for the class is Modern Physics 4th Edition by Kenneth S. Krane.

We will cover most of Chaps 2-7 and a wide variety of contemporary results.

Syllabus

Office hours: Wednesday 2:00-3:00, Thursday 2:00-3:00 (Contact me by email if you want to arrange a special time.)

Class
Notes

Chap 2; Chap 2 slides.pdf; Chap 15; Chap 15 slides.pdf; Chap 3; Chap 3 slides.pdf; Chap 4; Chap 4 slides.pdf;

Homework

HWK 1 (Due Thu Jan 12, 5 problems total): Chap 2: Problems 2, 4 but for this problem change the speed of light, c, to 330 m/s (if you get their speed is larger than c then you're doing something wrong), 10, 14, 22

HWK 2 (Due Thu Jan 19, 5 problems total):

Prob 1: (a) An electron is accelerated in a uniform electric field of -10

^{6}V/m. It starts at the origin with 0 velocity. Using relativity, how far does the electron travel before it's speed is 0.09 c? 0.9 c? 0.99 c? 0.999 c? (Hint: What is the potential energy for a charge in a uniform electric field? Hint

^{2}: Can you use a conservation law to solve?) (b) Repeat (a) but using non-relativistic equations.

Prob 2: Chap 2 prob 44

Prob 3: Low mass stars have a stage where three

^{4}He atoms combine into one

^{12}C. Using the data in Appendix D "Table of Atomic Masses". Calculate how much energy is released for each

^{12}C formed.

Prob 4: Jun Ye's group have made some of the most accurate atomic clocks. For example, they used them to measure the effects of General Relativity. Here, they claim a relative clock accuracy of 3.1 X 10

^{-18}. Suppose they have two of these clocks that are at different heights in their lab. Can they measure the difference in the rate that the two clocks tick when the vertical separation between the clocks is 10 cm? 1 cm? Roughly what is the smallest vertical distance where they can measure the effects of General Relativity?

Prob 5: According to the interwebs, the most massive black holes are over 10 billion solar masses while the black hole at the center of the Milky Way is a piddly 4.3 million solar masses. Calculate the Schwarzschild radius of Holmberg 15A and of Sagittarius A* using the masses here. Compare these to the radius of the sun and the orbital radius of (say) Neptune.

HWK 3 (Due Thu Jan 26, 5 problems total):

Prob 1: Chap 15 prob 2

Prob 2: From Fig. 15.9 of the textbook (the right figure on pg 10 of Chap 15 slides.pdf): a) At 30 kpc, estimate the actual tangential velocity (dots) and the tangential velocity expected from visible matter (blue line). b) Use these numbers to estimate the fraction of matter that is visible and the fraction that is "dark". (For the universe, from NASA https://science.nasa.gov/astrophysics/focus-areas/what-is-dark-energy

there is approximately 5X more dark matter than visible matter).

Prob 3: You have a light source of 700 nm wave length that emits 60 W of light uniformly in all directions. Assume nothing absorbs the light. a) What is the intensity 1 meter from the source? 100 meters from the source? (Hint: What is the average power through the surface of the sphere and what is the surface area of the sphere?) b) What is the average electric field (square root[E

^{2}

_{0}]) 1 meter from the source? Repeat for 100 m from the source? c) The sun outputs 3.8X10

^{26}W. What is the intensity of the light at the orbital radius of the earth? at the distance to Alpha Centauri? d) What is the average electric field for both distances in part c)?

Prob 4: A light beam pulse is moving in the x-direction and has 10 J of energy. a) What is the momentum of the pulse? b) Suppose a stationary grain of dust in space (M = 2 X 10

^{-9}kg) completely absorbs the light pulse. What velocity will it have? c) Repeat b) but for a pulse with energy of 2.1 eV absorbed by an initially stationary atom with mass of 23 amu.

Prob 5: In a recent calculation, we simulated the effect of an X-ray pulse hitting an electron. The photon energy was 9.0 keV and the peak intensity of the X-ray pulse was 3X10

^{21}W/cm

^{2}(be careful of the intensity units which are not SI). a) What is the wave length of a photon in this pulse? b) At the peak intensity, what is the rate that these photons hit a circular area with a radius of 2X10

^{-10}m?

HWK 4 (Due Thu Feb 2, 5 problems total):

Prob 1: Use the book's table of work functions. Cobalt is illuminated with light. (a) What is the largest wavelength that will cause photoelectrons to be emitted? (b) What is the largest kinetic energy for ejected electrons when you shine light of 150 nm on Cobalt?

Prob 2: When you shine light of 241.257 nm on a gas composed of sodium atoms, electrons are ejected and you make positive sodium ions. When you shine light of 241.274 nm on a gas composed of sodium atoms, the light is not absorbed and no electrons are ejected. (a) What happens when you shine light of 700 nm on the sodium atoms? (b) What happens when you shine light of 200 nm on the sodium atoms? Make sure to give their kinetic energy if electrons are ejected in one of these cases.

Prob 3: (a) Chap 3 prob 23(a) (don't do 23(b)). (b) At what wavelength does the Sun emit its peak intensity?

Prob 4: Chap 3 prob 26

Prob 5: Hydrogen atoms absorb photons with a wavelength of 121.6 nm. Approximately 10 nanoseconds after absorbing a photon, the hydrogen atom will emit a photon of the same wavelength in a random direction; it will then be ready to absorb another photon. Suppose a hydrogen atom with a velocity of 50 m/s is flying at you. You are shooting 121.6 nm photons at it in pulses of light separated by 0.1 seconds. About how many photons would need to be absorbed before its speed is approximately 0? (For what it's worth, this is essentially the method we use to cool the antimatter version of hydrogen atoms. CERN press release, Purdue press release, various news reports)

Tests/Solutions

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