Pysähdyskäyrä

Pysähdyskäyrä refers to a concept in physics, particularly in the study of the photoelectric effect. It is a graphical representation of the relationship between the stopping potential and the frequency of incident light. The stopping potential, also known as the cutoff potential, is the minimum negative voltage applied to the collector electrode that prevents any photoelectrons from reaching it, effectively stopping the photocurrent.

When light of a certain frequency strikes a photosensitive material, electrons are emitted. If a reverse voltage is applied between the emitter and the collector, the emitted electrons are repelled. As the reverse voltage increases, fewer electrons reach the collector. The stopping potential is reached when the kinetic energy of the most energetic electrons is just enough to overcome the potential barrier. The photoelectric equation relates the maximum kinetic energy of the emitted electrons ($K_{max}$) to the energy of the incident photons ($hf$) and the work function of the material ($\phi$) as $K_{max} = hf - \phi$. Since $K_{max}$ is related to the stopping potential ($V_s$) by $K_{max} = eV_s$, where $e$ is the elementary charge, the equation becomes $eV_s = hf - \phi$.

Plotting the stopping potential ($V_s$) on the y-axis against the frequency ($f$) on the x-axis results in a straight line. The slope of this line is given by $h/e$, where $h$ is Planck's constant and $e$ is the elementary charge. The y-intercept of the graph corresponds to $-\phi/e$. Therefore, the pysähdyskäyrä provides an experimental method for determining Planck's constant and the work function of the material. This graphical method was crucial in confirming the quantum nature of light and the validity of Einstein's explanation of the photoelectric effect.