- Each line on the graph has the same slope. The slope = 6.63 X 10-34 J.s. This value is called Planck's constant.
- To explain the effect, Einstein made the radical proposal that the energy of light is not transmitted in a continuous wave, but rather in concentrated bundles called photons.
- He also proposed that the energy in each bundle was given by Planck's equation:
E = hf
- Einstein was able to use this concept of the photon to explain the photoelectric effect. He also predicted new undetected effects from this photon model.
- The intensity of light was no longer related to wave amplitude, but to the rate of arrival of photons.
- If an electron gains energy from a photon (E=hf), and if the work required to remove the electron from the material is W, then the remaining energy (kinetic energy) will be given by:
qV(stop) = Ek(max) = hf - W
- W = the work function of the surface
- From the above equation, the larger the frequency, the larger the kinetic energy.
- If the photon has just enough energy to cause emission, then the photoelectrons will have zero kinetic energy after emission. Therefore we can write:
0 = hf - W (solving for W)
W = h(fo) (fo = the threshold frequency)
- Many things can happen at the surface of the metal.
- Electrons get knocked back into the surface.
- Electrons get knocked out of the surface at angles not equal to 90 degrees.
- Electrons get knocked out of the surface at angles equal to 90 degrees.
- This means that not all electrons will be emitted with the same kinetic energy.
- When the electrons have to move against the force due to the electric field, only the most energetic electrons reach the anode.
- As the electric field is increased by increasing the potential difference, fewer and fewer electrons reach the anode until V = Vo (The stopping voltage) and then all electrons are turned back.
- It is for the above reason that the stopping voltage measures the maximum kinetic energy for the photoelectrons.
- Einstein's equation of the photoelectric effect explains the graph shown above, because his equation is the equation of a straight line with slope = h and y-intercept equal to -W.
- The x-intercepts for the different metals is the threshold frequency for that metal. Each metal has a different threshold frequency and work function.
- Millikan, In 1916 showed that Einstein's equation was quantitatively correct, and also he showed that Planck's constant had the value previously calculated by Planck.
- Note that there is another unit of energy called the electron volt (eV). It is the quantity of energy that an electron gains when it accelerates through a potential difference of 1 V.
eV = 1.6 X 10-19J
Thermionic Effect
- There are limitations on electron flow through an evacuated tube. They are:
- electrode size
- electric potential difference.
- EDISON EFFECT - A plate positively charged inside of a turned on light bulb collects electrons but not if negatively charged.
- O.W. Richardson conducted research on the Edison effect. He suggested that heated surfaces in a vacuum evaporate electrons like molecules leaving a hot liquid. The positive plate then attracts electrons.
- THERMIONIC EMISSION - The process whereby electrons are emitted by a hot filament.
- DIODE - An electron tube which contains two electrodes.
- A heated cathode is surrounded by a cylindrical anode or plate enclosed in an evacuated tube.
- The current flows only from cathode to anode.
- It was first used for detecting radio waves and converting alternating current to pulsating direct current.
- By Introducing other charged plates:
- current can be controlled.
- current can be amplified.
- current can be shut off.
- VACUUM TUBE - A device such as a diode.
- Vacuum tubes are used in devices such as radios, televisions, sound amplifiers, rectifiers, radar equipment, and computers.
- Transistors and integrated circuits now replace these vacuum tubes.
January 20, 2014