Quantum Phenomena

Quantum Phenomena

1. The Photoelectric effect

   • Idea
     • Electrons can escape from a metal if given enough energy
       • Energy from
         • Heating
         • Light (photon)
       • Demonstrated by
         • Gold leaf electroscope
           • When UV light is shone onto the zinc plate leaf falls rapidly
             • Shows
               • Negatively charged rod and gold leaf loosing charge
                 • Therefore
                   • UV light can rip electrons from zinc
                     • Exceptions
                       • UV light cannot allow emission of electrons on a postive version of this model
       • Will only happen if energy meets or exceeds
         • Threshold Frequency
           • Min. frequency of light below no photoelectron emission occurs
           • Different frequency for each metal
         • Work Function
           • Min. energy needed to rip an electron from surface of metal
           • work function = plank's constant x threshold frequency
       • Emitted electrons called 'photoelectrons'
       • Varients
         • No. of photoelectrons
           • Determined by intensity of light
             • Closer light=increase in emission rate
         • Photoelectron kinetic energy
           • Determined by frequency of light
           • From 0 to a maximum
             • Maximum kinetic energy=
               • planks contant x threshold frequency - work function
   • Photoelectric equation
     • (plank's)x(threshold)=(work func.)x(kinetic energy)

2. Spectra

   • Idea
     • When an electric current passes through a low pressure gas light is produced
       • Each element gives off a different coloured light
       • Light can be split up
         • into
           • Wavelenghs/ frequencies
         • Using a diffraction grating
       • Explained
         • Electrons in the lowest shell are in 'ground state'
         • Electric discharge allows electrons to jump from ground state to an excited state
           • Ground to excited state
             • Absorbsion of photon
               • Excited electrons almost alway return to ground state
                 • via
           • Excited to ground state
             • emission of photon
               • accounts for produced light
         • Electrons in higher shells are in an 'excited state'
           • Electrons excited past all shells are liberated
             • Ionisation
             • Needs energy to surpass 0ev

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