Physics 1202: Lecture 30 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like...
-
Upload
rachel-webb -
Category
Documents
-
view
217 -
download
0
Transcript of Physics 1202: Lecture 30 Today’s Agenda Announcements: Extra creditsExtra credits –Final-like...
Physics 1202: Lecture 30Today’s Agenda
• Announcements:
• Extra creditsExtra credits–Final-like problemsFinal-like problems
–Team in class Team in class
• HW 9 next FridayHW 9 next Friday
• Modern physics
Quantization• Physical quantities come in small but finite quantities
– Quantum (or quanta for many of them)
– Not continuous
• Atomic Spectra:a) Emission line spectra for hydrogen, mercury,
and neon;b) Absorption spectrum for hydrogen.
Blackbody radiation• Heating an object
– Motion of closely spaced atoms/molecules produce E&M waves in a continuous range of or
• Blackbody radiation– The hotter an object the more white it appears
» E.g. the sun
– Not all wavelength are emitted equally though
» Very hot appears more blue (e.g. Sirius)
» Colder means less blue: object appears more red (e.g. fire)
» Earth at 300K: mainly infrared waves.
Blackbody radiation• In late 19th century
– Study of relationship ot temperature T and
– Wilhelm Wien (1893):
maxT = 2.898 X 10-3 mK
max
(emissivity) = 1 for perfect blackbody (absorbs all: appears black) = 0 for perfect reflective surface
• Stefan-Boltzmann law:– Power radiated by a surface of
area A and temperature T
P = A T4
5.67 X 10-8 W/(m2K4)
Black Body Radiation
Intensity of blackbody radiationClassical Rayleigh-Jeans law forradiation emission (1905)
Ultraviolet catastrophe
Black Body Radiation
Intensity of blackbody radiation
Planck’s expression
h = 6.626 10-34 J · s : Planck’s constant
Assumptions: 1. Molecules can have only discrete values of energy En;
2. The molecules emit or absorb energy by discrete packets - photons
Max Planck (1899):
Photoelectric effect
• In 1887, Heinrich Hertz– shining ultra-violet light on metal in
vacuum
– If V not large enough, no current
Photoelectric effect
Kinetic energy of liberated electrons is
where is the work function of the metal
Photoelectric effect
• Explained by Einstein in 1905– Based on quantum of light (Planck)
– Nobel Prize in 1914