The Photoelectric Effect Explanation

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Einstein's Photon Model

Albert Einstein came up with an explanation for the photoelectric effect. Einstein suggested that light was made up of little packets of energy called photons.

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One-on-one interaction

  • Einstein suggested that each photon had a one-on-one interaction with an electron.
    • The electron absorbs all the energy of one photon.
  • This explained why the maximum kinetic energy is independent of the intensity.
    • Intensity is the number of photons arriving per second.
    • It doesn't matter how many photons arrive per second because the electron only interacts with one.
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Energy depends on frequency

  • Einstein also suggested that the energy of a photon is proportional to its frequency. This relationship is described in the following equation:
    • The constant of proportionality is the Planck constant, h.
    • E=hf=hcλE = hf = \frac{hc}{\lambda}
  • This explained why the maximum kinetic energy of the emitted electrons is proportional to frequency.
    • The higher the frequency of a photon, the more energy is transferred to an electron.

Consequences of the Photon Model

Einstein's photon model helped explain the photoelectric effect.

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Work function

  • For an electron to leave a metal surface, it needs to overcome the bonds holding it down.
  • The energy needed to break these bonds is called the work function, φ.
  • The work function is different for different metals.
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Threshold frequency

  • We can use the threshold frequency to work out the work function of a particular metal and vice versa.
    • The energy of a photon at the threshold will equal the energy needed to break the electron free from the metal (i.e. the work function).
  • So ϕ=hf\phi = hf
  • And so threshold frequency, f=ϕhf = \frac{\phi}{h}
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Photoelectric equation

  • Consider the conservation of energy.
  • The electron absorbs energy hfhf from a photon.
  • The electron must lose at least the energy needed to break it free from the bonds in the metal, the work function ϕ\phi.
  • The maximum kinetic energy of an electron is given by the photoelectric equation:
    • hf=ϕ+Ek(max)hf = \phi + E_{k (max)}
  • Remember that the maximum velocity of the electron can be found from:
    • Ek(max)=12mv(max)2E_{k (max)} = \frac{1}{2}mv_{(max)}^{2}
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Stopping potential

  • Measuring the stopping potential, Vs, can help us work out the maximum kinetic energy, Ek(max)E_{k (max)}.
  • After the electrons are emitted, they pass through an electric potential.
  • As the electrons are charged, they must do work, e × Vs, to move through this potential.
  • The electrons will stop if all their kinetic energy is used up doing work against the potential.
  • So the equation for stopping potential is:
    • eVs=Ek(max)eV_s = E_{k (max)}

Jump to other topics

1Measurements & Errors

2Particles & Radiation


4Mechanics & Materials


6Further Mechanics & Thermal Physics (A2 only)

7Fields & Their Consequences (A2 only)

8Nuclear Physics (A2 only)

9Option: Astrophysics (A2 only)

10Option: Medical Physics (A2 only)

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12Option: Turning Points in Physics (A2 only)

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