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Electron Properties & Blackbody Radiation: Discoveries by Thompson, Wien, Planck, Einstein, Study notes of Physics

The groundbreaking discoveries in physics by j.j. Thompson, max planck, and albert einstein. Thompson measured the charge to mass ratio of the electron, leading to the discovery of electrons as negative charges much smaller than atoms. Blackbody radiation, as described by stefan's law and wien's law, explains how hot objects emit electromagnetic radiation. Planck's quantum hypothesis introduced the concept of quantized energy, and einstein's photoelectric effect demonstrated that light behaves as both a particle and a wave.

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Uploaded on 08/30/2013

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Properties of the Electron
J.J. Thompson (1897) measured the charge to mass
ratio of the electron (cathode rays)
C/kg1.76x10
r
B
E
m
e11
2
E-electric field, B-magnetic field, r-radius of curvature
Electrons are negative charges but are much smaller in
mass than atoms
Millikan’s oil drop experiment: found e=1.6x10-19 C
Electric charge in atoms are quantized
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Download Electron Properties & Blackbody Radiation: Discoveries by Thompson, Wien, Planck, Einstein and more Study notes Physics in PDF only on Docsity!

Properties of the Electron

J.J. Thompson (1897) measured the charge to mass ratio of the electron (cathode rays)

1.76x10 C/kg B r

E

m

e (^11)  

E-electric field, B-magnetic field, r-radius of curvature Electrons are negative charges but are much smaller in mass than atoms

Millikan’s oil drop experiment: found e=1.6x10-19^ C Electric charge in atoms are quantized

Blackbody Radiation, Wien’s Law

Hot objects emit electromagnetic (EM) radiation (waves) with EM Intensity~T^4 – Stefan’s Law Blackbody: Absorbs all EM radiation incident on it Blackbody radiation: EM radiation emitted by a blackbody resulting from its interior temperature Examples: stars (sun), red hot metal block, fire embers

Wien’s Law for Blackbody Radiation: Peak wavelength in blackbody radiation spectrum inversely proportional to temperature:

p T 2.90x10-^3 mK

OR peak frequency is proportional to temperature:

f (^) peak =(5.88x10^10 s -1^  K-1^ ) T

Radiant energy from intensity spectrum of blackbody equals area under curve.

Blackbody EM radiation results from oscillating electric charges on molecules within blackbody.

The Photoelectric Effect

Einstein’s Photoelectric Effect (Nobel Prize-1921)

  1. Increasing intensity of light increases the number of photons in beam. Thus, more photoelectrons are emitted from a metal electrode. However, increasing the intensity of light does NOT increase the energy of emitted photoelectrons.
  2. Photoemission occurs when an electron in an electrode absorbs a photon of incident light. If the frequency of light is increased, the maximum KE of the electron increases linearly: KEmax^2 = hf - W  (photoelectric energy conservation).
  3. Minimum energy needed to emit a photoelectron from an electrode: hf (^) o = W  where the value f (^) o is known as the threshold frequency and W  is work function of the electrode (constant in units of energy which is different for different metals). No electrons are emitted if f<f (^) o regardless of light intensity.
  4. The Stopping Potential, Vo, required to stop the electron flow completely is: e Vo = KEmax^2 This can also be determined by: (^) o o

hf V e e

W