# Modern physics Definition and 54 Discussions

Modern physics is a branch of physics either developed in the early 20th century and onward or branches greatly influenced by early 20th century physics. Notable branches of modern physics include quantum physics, special relativity, and general relativity.
Classical physics is typically concerned with everyday conditions: speeds are much lower than the speed of light, sizes are much greater than that of atoms, and energies are relatively small. Modern physics, however, is concerned with more extreme conditions, such as high velocities that are comparable to the speed of light (special relativity), small distances comparable to the atomic radius (quantum mechanics), and very high energies (relativity). In general, quantum and relativistic effects are believed to exist across all scales, although these effects may be very small in everyday life. While quantum mechanics is compatible with special relativity (see relativistic quantum mechanics), one of the unsolved problems in physics is the unification of quantum mechanics and general relativity, which the Standard model (of particle physics) currently cannot account for.

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1. ### I Does charging my phone increase its gravitational force?

If the statement above is correct, I do not understand this concept. I guess by charging my phone I am not producing matter. Does it mean in this case, energy converts to mass (not matter)? Can someone please explain this?
2. ### I On the Rydberg Constant and the Emission Lines

With regard to Rutherford's atomic model, and Rydberg's discovery in general for the hydrogen distribution lines, what does Rydberg's constant physically mean? Its unit is m ^ -1, as if it were a rate, but it was not clear to me its physical meaning. And why does it grow with atomic mass...
3. ### Intro special relativity problem regarding time dilation

This example is worked out in the book, and at the beginning, they make the assumption that the muon is traveling at c, and then find the change in time from the Earth reference frame using delta t=100km/c. Then delta t is plugged into the time dilation equation on the left side and we solve for...
4. ### I The center of mass & relativistic collisions

In special relativity (especially relativistic collisions), is the center of mass frame as useful as Newtonian mechanics?
5. ### Photoelectric effect question

The full questions is in the picture. I already solved a) and found 5.6E14 electrons per second For b) i first found the power of the light but just multiplying the intensity with the area: (6.0 W/m2)(3.5E-4 m^2) = 0.0021 W Then I tried to use the voltage from the graph but i am not sure which...
6. ### Find the wavelengths Kα, Kβ , Lα and Lβ from elemental Cu

Homework Statement Problem: "What are the wavelengths of the characteristic Kα, Kβ , Lα and Lβ x-rays emitted from copper? Remember to use a Rydberg constant with the reduced mass correction made for elemental copper." Givens: Energy levels given: Kα (2,1) Kβ (3, 1) Lα (3, 2) Lβ (4, 2)...
7. ### I Radioactive decay constant

What is the significance of the radioactive decay constant?
8. ### Showing that the P(r) is maximum at r=a_0/Z

Homework Statement Homework Equations where The Attempt at a Solution I tried to integrate (7-32) over all values of r (i.e., from negative infinity to positive infinity) and set it equal to 1, but the result was too messy and was divergent. Am I making the right approach?
9. ### I Moving frames

I am trying to understand why maxwell equations are correct in any reference frames? While i started to understand of his laws of physics a bit i could not imagine why he uses hyperbolic functions such as coshw instead of spherical ones in position and time relation between moving frames...
10. ### Finding the 10 lowest energy levels

Homework Statement Homework Equations The Attempt at a Solution I understand the equation, and I understand the concept. My question is this: What is the best way to go about solving this problem? My line of reasoning concludes that the fourth lowest energy level is E211. However, the...
11. ### Quantum Most accurate modern physics book

I have been plagued by modern physics and quantum mechanics books before that employ the traditional approach where they start from Photoelectric effect, Blackbody radiation, etc. (This is really not an issue but in fact illuminating since it shows how old QM developed) until you read about...
12. ### Intensity of Light using Quantum theory

Homework Statement Approximately 4% of the intensity of light is reﬂected at a glass-air boundary. Classically one expects roughly 8% of light to be reﬂected from a thin glass plate (4% at the front and back boundary). Outline brieﬂy what quantum theory predicts for a single photon instead of...
13. ### Looking for a project for modern physics class

For my modern physics course our group(3 people) need a project to build in 12 week,(so not short, 2-3hours work for each person per week) Our topics are: Relativity Waves and Particles I Waves and Particles II Bound States: Simple Cases Unbound States: Obstacles, Tunneling and Particle-Wave...
14. ### Ratio of energy densities of black body radiation

Homework Statement Homework Equations The Attempt at a Solution The energy density is given as ## u = \frac { 8 \pi {\nu }^2}{c^3}~ \frac { h \nu} {e^{ \frac { h\nu}{k_B T}} – 1}.## EDIT : I put the constant C. ## \frac { u( 2 \nu) } {u(\nu)} = C \frac { {e^{ \frac { h\nu}{k_B...
15. ### Compton scattering

Homework Statement Homework Equations The Attempt at a Solution ## \lambda' = 2 \lambda ## ## \lambda' - \lambda = \lambda = \frac { h} { mc} \cos{ \phi } ## Here, ## \phi = \frac { pi} 4 ## is the angle between photon's original line of motion and its line of motion after...
16. ### 300 Level Classes

I'm not if this is the right place for this, so mods can feel free to move this. I'm a part-time student going into my second semester of 300 level physics class. This past semester I oscillated between feeling just fine and questioning whether physics was right for me (usually around test...
17. ### Energy needed to remove both electrons from a He atom

Homework Statement Homework Equations The Attempt at a Solution [/B] Both electrons are in 1s orbit. For taking out the second electron, I will have to put slightly more energy than 24.6eV. So, the energy required to remove both electrons should be slightly more than 49.2 eV. So, I...
18. ### De – Broglie wavelength of O_2

Homework Statement Homework Equations The Attempt at a Solution [/B] De – Broglie wavelength of ## O_2 ## molecule is ## \lambda = \frac h p ## Assuming that the speed of molecule is small enough to take non – relativistic calculation, ## \frac...
19. ### Doppler effect: calculate speed of a moving star

Homework Statement Homework Equations The Attempt at a Solution When the source is not moving, ## \lambda = 6250 A ## ...(1) When the source is moving, ## \lambda ' = 6500 A ## ...(2) From (1) and (2), ## \lambda ' > \lambda ## ...(3) This means that the source is moving away...
20. ### Change in the energy of photon due to recoil of the nucleus

Homework Statement Homework Equations The Attempt at a Solution Since energy of photon is very low compared to the rest mass energy of the nucleus, I consider non – relativistic calculation. Conservation of linear momentum gives : momentum of nucleus = momentum of photon = p...(1)...
21. ### The amount of radioactive substance after 38 days

Homework Statement Homework Equations The Attempt at a Solution Is there anyway to choose the correct option without solving it? ## \lambda = \frac { 0.693 } { t_{\frac 1 2}} ## [/B] ## N = N_0 e^{- 0.693 {\frac { t } { t_{\frac 1 2 }}}} ## ## N = 10.24 e^{- \frac { 0.693 * 38 }...
22. ### Wavelength of em radiation of H - atom

Homework Statement Homework Equations The Attempt at a Solution E = (13.6 – 3.4 = 10.2)eV = ## \frac {hc } { \lambda } ## =1.22 * 10^{-5} cm So, the correct option is (a). Is this correct?
23. ### Special Relativity: Traveling to Regulus

Homework Statement A visit to Regulus is on my bucket list. However, it is 77 light years away. Assuming I will live only another 40 years, can I make it to Regulus? How fast would I have to travel (at constant speed) to get there in 40 years? Homework Equations t1 = t0...
24. C

### Is modern physics as a course necessary for all future physicists?

So I'm about to start my second year of college soon, I'm majoring in physics. I've noticed that some universities (places like Princeton, MIT, etc) don't have a course on modern physics, where a more superficial treatment is given to things like special relativity, quantum mechanics...
25. ### Special Relativity: Length Contraction Problem

Hello! I have a small problem with a task professor gave us. I tried many options (you will see below) but I cannot seem to get the right solution. Any advice or guideline how to solve this would be really helpful. In advance I thank you for helping me. Homework Statement Our professor of...
26. ### Lorentz Transformations vs Galilean Transformation

Homework Statement 2. The attempt at a solution 3. Relevant equations In the first problems of that book i was using the Galilean transformations where V1 = V2 + V But if i use that then V1 = 0.945 - 0.6 V1 = 0.345 Is not the same result, so I am confused. In this new problems we are...
27. ### Neutron interference

Homework Statement the question details are given in the attachment Homework Equations momentum = √2mE where m is mass of neutron and E is its kinetic energy The Attempt at a Solution initially i accounted for the energy change during the path followed by the neutron but later i got...
28. ### A Why is the Matter wave analogous to photon and derived from it?

The electromagnetic wave derivation uses the fact that charge enclosed is zero and it goes to obey plane wave equations. Lets say we were deriving a wave equation from maxwell's equations for electron wave motion, but we assume that charge enclosed is not zero, and come up with some...
29. ### Area of the football field

Homework Statement A football field is given in the following shape, where, ABCD is a square of side-length and AEB, CFD are semi-circular arcs. If an observer is moving with uniform velocity .along AB, what is the area of the football-field measured by the observer? ( is the velocity of light...
30. ### B Einstein's Photoelectric Equation and its graph

According to the equation, the graph of kinetic energy of emitted photoelectrons from a metal vs the frequency of incident radiation gives a straight line. My doubt is, what factors does the slope of this line depend on? I think it depends on the nature of metal used. Correct me if I am wrong.
31. ### Crystallography, diffraction, identifying the pure element

Homework Statement The only things you know about the sample are: (i) it has some kind of cubic lattice, and (ii) it is a pure element. Identify the element in the crystalline sample. SC: R = 0.5a FCC: R = 0.25a√2 BCC: R = 0.25a√3 Distances from Bragg peaks: d1 = 0.2037 nm d2 = 0.1746 nm d3...
32. ### Given wavenumber find spring constant harmonic oscillator

Homework Statement The separation between energies of an oxygen molecule is 2061 cm-1 (wavenumber). Treating the molecule as a simple harmonic oscillator whose fundamental frequency is related to its spring constant and reduced mass, calculate the spring constant for an O2 molecule. meff =...
33. ### Finding wavelength of visible light in a well

Homework Statement Determine what colors of visible light would be absorbed by electrons in an infinite well, N = 3.1 nm. The effective mass for an electron is one-fifteenth of the standard electron mass. Homework Equations En = n2h2/(8mL2) E = hf f = c/λ The Attempt at a Solution E1 =...
34. ### Infinite potential well

Homework Statement Determine what colors of visible light would be absorbed by electrons in an infinite well, N = 3.1 nm. The effective mass for an electron in GaAs is one-fifteenth of the standard electron mass. Homework Equations En = πh2/[2*N2*me/15]*n2 L = nλ/2 Ψ = √(2/L)sin(nπx/L) The...
35. ### Ground state energy for exciton in Si

Homework Statement Estimate the ground state energy (eV) for an exciton in Si. εSi = 12 ε = 1.0359×10−10 Effective masses me* = 0.26me mh* = 0.36me effective mass = 0.15me Values of h 6.626×10−34 J⋅s 4.136×10−15 eV⋅s Values of ħ Units 1.055×10−34 J⋅s 6.582×10−16 eV⋅s Homework Equations E1 =...
36. ### Bohr Model applied to Excitons

An exciton is a bound electron-hole pair (in a semiconductor). For this problem, think of an exciton as a hydrogen-like atom, with a negatively charged electron and positively charged hole orbiting each other. The permittivity of free space (ε0) is replaced with permittivity of the...
37. ### Testing Difficulty with Modern Physics Midterm

I am currently in Honors Physics 3 which is the third introductory course of my physics degree program and covers modern physics beginning with special relativity. So far we have covered Lorentz transformations and velocity additions, relativistic energy and momentum, blackbody radiation, photon...
38. ### Modern Physics question -- an atom ejecting a relativistic electron

Homework Statement . An atom at rest can undergo radioactive decay, ejecting an electron at a maximum speed of 0.5c. If the atom in a particle accelerator is observed to produce an electron traveling at 0.75c, at least how fast must the atom itself have been moving? Homework Equations u0 x...
39. ### I What is quantum decoherence?

So I whilst understanding basics of some quantum phenomena like superposition, tunnelling, fluctuations etc I happened to watch the movie "Coherence" where there's a scientifically unsatisfactory reference to quantum decoherence. What exactly is this concept?

My college started physics course with light and blackbody radiation. I am facing difficulties in understanding relativistic energies, Compton effect and specific heats of multi atomic gases. Can you suggest a method for studying modern physics and suggest some good books for getting my concepts...
41. ### Other Junior-level Modern Physics textbook suggestions

Hello All, I am looking for suggestions for a junior-level Modern Physics textbook to read over the summer for the purpose of preparing me to take senior-level Quantum Mechanics in the fall. I have a solid Math background so a text that would possesses some level of mathematical rigorousness...
42. ### I Quantum field and mediating particles

I'm not sure if I posted this in the right category, it's something that came up just after the quantum mechanics section so I just chose this one. I've come across something that I simply can not find an answer for on my own. I'm taking Modern Physics course and the last chapter is some...
43. ### Expressing A Quantity In Polar Coordinates?

Homework Statement Express the quantity ∂2/∂x2+∂2/∂y2 in polar coordinates. Homework Equations x=ρcosφ y=ρsinφ ρ=sqrt(x2+y2) The Attempt at a Solution This is my first post, so I apologize for any weird looking equations, etc. I know that this is not a difficult problem, but I just cannot...
44. ### Kaon Decay -- Special relativity

Homework Statement A K0 particle is unstable and has a mass of 8.87x10-28 kg. It can decay into π+ and π- particles, each of mass 2.49x10-28 kg. Suppose that a K0 is moving in the +x direction and decays by this process, with the π+ moving off at 0.9c and the π- moving off at 0.8c. a) What was...
45. ### Deriving the Compton Effect

Homework Statement I need to derive the equation related to the Compton Effect from the equations for momentum and energy conservation. Homework Equations (1) Compton Effect: λ' -λ = h/me(1-cosθ) (2) Conservation of Momentum (x-direction): h/λ= (h/λ')cosθ+γumeucosΦ (3) Conservation of Momentum...
46. ### Modern physics: Atomic Structure

Homework Statement If we take the average radius of the electron wave function in the ground state of hydrogen to be the Bohr radius, calculate the average potential energy of the electron due to the Coulomb interaction with the nucleus. State answer in eV. Homework Equations I think the...
47. ### Atomic Best book on Modern Physics? 5 choices, suggestions?

Hi everyone, I'm looking for an introductury book (or several books) on modern physics that explains the history and experiments the led to the understanding of the atom, the electron, avogadros number, Plancks constant, rutherford, quantum mechanics etc. I want an idea of how all the...
48. ### Other Good pre-honours 'revise all physics' type book(s)

Hi - I'm doing an honors degree in physics by distance learning, having received my B.Sc around 20 years ago. I find there is a lot that is presented quite differently, and also stuff that I don't remember well enough - or not at all. So I'm looking for the fewest books that I can use to...
49. ### Readings for a general understanding of Quantum Field Theor?

Hi all, I have an undergraduate degree in Physics, but I've since specialized in more environmental applications of Physics. I've taken a few quantum Physics courses as part of my degree, but, if I'm being honest, what I took from those courses was more of a mathematical view of the field, such...
50. ### Sketch the wave, showing both x < 0 and x > 0.

Homework Statement [/B]A wave is represented by the wave function: y = A cos(2πx/λ + π/3) for x < 0. For x > 0, the wavelength is λ/2. By applying continuity conditions at x = 0, find the amplitude (in terms of A) and phase of the wave in the region x > 0. Sketch the wave, showing both x < 0...