Week 10 of a 10 week survey course in physics

1. Module 6
Modern Physics
Dr. Paul H. Comitz
pcomitz@live.com

2. Agenda
 Light as an Electromagnetic Wave
 Maxwell's Equations
 Light as a Particle: The Photoelectric Effect:
 Quantum Theory
 The Atom and Atomic Structure
 The Most Famous Equation in the World
 Epilogue
 Schrodinger’s Cat

3. Course Modules
# Module Weeks Reading Quiz
1 Newton's laws 1 Ch 4,5 *
2 Conservation of Energy and
Momentum
2,3 Ch 6,7,8 Quiz 1
3 Thermodynamics 4,5 Ch 12,13,14
4 Electromagnetism 6,7 Ch 17,18 Quiz 2
5 Waves, Sound, and Light 8,9 Ch 16, 20, 21 Quiz 3
6 Modern Physics 10,11 Ch 23 Final Exam
* it is strongly recommended you read chapters 0 - 3

4. Module 6
 Reading: Chapters 23
 Chapter 23 Survey of Modern Physics
 Discussion (5%)
 See next slide
 Final Exam (20%)
 In class 8/23
 See class website for practice questions

5. Discussion – Due in class 8/23
 Submit one page – be prepared to discuss on class
 Although quantum information has been around for a
long time, we are starting to see more about it in the
media – especially in reference to quantum computers.
Discuss the following using the ITT Tech Virtual Library
other resources you find useful. Please list any
references you use in your post
 What is quantum computing?
 What can a quantum computer do that a classical computer
can't?
 What is required to build a quantum computer?
 When will there be a real quantum computer?
 Why could quantum computers be important?

6. Grading
 3 quizzes @ 5% = 15%
 Final @ 20%
 8 labs @ 3.75% = 30%
 HW Exercises 5@4% = 20%
 Discussion 3@5% = 15%

7. The Nature of Light
 Wave Theory
 Each point on a wave front
can be regarded as new
sources of small wavelets
that spread out uniformly in
the forward direction at the
same speed
 Particle Theory
 Light is made up of streams
of particles called photons
 Einstein
 Light behaves as both a
particle and a wave

8. Light as an Electromagnetic Wave
 Electromagnetic waves are produced by
moving electric charges
 Electric charges have an electric field
 Moving electric charges create a magnetic
field

9. Vector Fields
 A wave is formed by
vector fields
 Vector fields can be
characterized by
 Divergence
 gives the scalar magnitude
of a vector field's at each
point
 Curl
 Rotation of a 3D vector
field

10. Maxwell’s Equations
 Describe electromagnetic waves in terms
of divergence and curl
 Divergence of E
 Divergence of B
 Curl of E
 Curl of B

11. Maxwell's Equations
 Divergence of E
 E diverges outward from
positive charges and inward
from negative charges
 Curl of E
 E curls around changing B
fields
 Divergence of B
 B never diverges, always
loops around
 Curl of B
 B curls around currents and
changing E fields

12. Light as a Particle: The Photoelectric Effect
 Einstein’s view on light
 as a stream of particles, bundles of energy (photons)
 photons interact with matter one at a time
 high-energy photons dislodge electrons from certain
metals
 Einstein was awarded the Nobel Prize in Physics in
1921 for his paper on the Photoelectric effect
 Photon
 A particle representing a quantum of light or other
electromagnetic radiation. A photon carries energy
proportional to the radiation frequency but has zero
mass

13. The Photoelectric Effect
high-energy photons dislodge
electrons from certain metals

14. Quantum Theory
Quantization
 the idea that the natural world is granular rather
than smoothly continuous
Quantum
 any elemental particle that makes up matter or
carries energy

15. Quantum Theory
 Max Planck
 Particles can have energies in discrete levels
 Whole number multiples of Planck’s constant
 6.62607004 × 10-34 m2 kg / s
 Einstein
 Light is a stream of particles called photons
 No mass
 Travel at speed of light
 Bohr
 Light is emitted when electrons change orbits
 De Broglie
 Electrons behave as waves

16. Wave-Particle Duality
Wave-particle duality
 A photon behaves as a particle when emitted by
an atom or absorbed by photographic film or
other detectors.
 But it behaves as a wave traveling from a source
to the place where it is detected.
 In this sense, light can be both a wave and a
particle!
 This idea is extended to all matter in quantum
physics
 The Central Mystery of Quantum Mechanics

17. The Atom and Atomic Structure
 Bohr Model of the Atom
 Energy is quantized
 An electron can only emit or
absorb specific amounts of
energy
 Schrodinger Wave Model
 Developed wave model of the
atom
 Difficult to visualized
 Based on the Schrodinger Wave
Equations

18. Wave Model of the Atom
 The fundamental equation of quantum
mechanics is Schrödinger’s wave equation,
which is:
(Details of this equation are beyond the
scope of this course.)

19. Atomic Spectra
 Bohr model useful for some atoms
 Bohr model refers to fixed energy levels
 E = - kZ2/n2
 Z is atomic number
 number of positive charges in nucleus
 n is the energy level
 Also know as the quantum number
 k is a constant = 13.595 eV (electron volts)

20. Atomic Spectra
 What is the energy for :
 n = 2
 Z = 1
 What element is this ?
 E = - (13.595eV)(12)/22 = - 13.595/4 = -3.39 eV
 Element is hydrogen

21. Quantum Mechanics and Atomic Properties
 Quantum Mechanics:
 All matter has wave and particle properties
 Wave model of the atom
 When electron behaves as a wave, exact
location cannot be determined !
 Using Schrödinger wave equation, calculate
probability that electron exists in a region
 Orbitals referred to as s,p,d,f

22. Quantum Mechanics
 Progression from the Bohr model of the atom to
the modified model with de Broglie waves to the
Schrödinger model.

23. Wave Model of Atom
 Electron position referred to by principal and
secondary quantum number
 Principal n=1,2,3,4
 Distance from nucleus
 Secondary s,p,d,f
 Shape of orbital

24. Electron shells
 Each atom attempt to fill its outer shell
 Borrow, lend, share electrons
 Metals
 Lend electrons
 conductors
 Non-metals
 Borrow electrons
 Inert
 Chemically inactive

25. Nuclear Mass and Binding Energy
 Neutrons bind the nucleus together
 Otherwise nucleus would fly apart from Coulomb
repulsion
 Often referred to as the “strong nuclear force”
 Atomic Number Z : number of protons on the
nucleus
 Atomic Mass A: number of protons and
neutrons
 Example
 Carbon
 Z = 6 protons, A = 14 = 8 neutrons + 6 protons

26. The Most Famous Equation in the
World
 1905 Einstein wrote:
 If a body gives off the energy L in the form of
radiation, its mass diminishes by L/c².
 This is the original form of E = mc2, where L
refers to energy
 Implications
 If the neutrons and the protons in a nucleus are
separated, the mass of the nucleus is less than
the mass of the individual neutrons and protons !!!

27. Are mass and energy the same thing?
 Mass and energy can be converted to one
another
 Energy required to break apart the nucleus
 Overcome strong nuclear force
 Binding energy
 Energy required to break apart nucleus
converted to mass !!!
 Ebinding = (Dm)c2

28. Summary of Quantum Physics
 Everything is made of waves; also particles !
 Quantum physics is discrete
 Quantum physics is probabilistic
 Quantum physics is non-local
 Quantum physic is very small
 Quantum physics is not magic
 Matter can exist in multiple states simultaneously!
 Schrodinger's Cat
 See this article for more information

29. Wave-Particle Duality
Double-slit experiment
 The light passing through two slits, a, forms an
interference pattern, b, shown graphically in c.