maximum kinetic energy
Summary
Maximum kinetic energy is the largest kinetic energy of electrons ejected from a metal in the photoelectric effect. It equals the photon energy minus the work function, Kmax = hf − φ. It rises with light frequency, not intensity.
Detail
Meaning: In the photoelectric effect, each electron absorbs one photon of energy hf. Part of that energy pays the work function φ, the minimum energy needed to free an electron from the metal. The rest becomes kinetic energy. Electrons freed at the surface keep the most energy; those from deeper lose some on the way out, so Kmax is the upper limit.
Equation: Kmax = hf − φ, where Kmax is the maximum kinetic energy (J), h is Planck's constant (6.63 × 10⁻³⁴ J·s), f is the light frequency (Hz), and φ is the work function (J). It is also measured as Kmax = eV₀, where e = 1.60 × 10⁻¹⁹ C and V₀ is the stopping potential (V).
On the MCAT: A common graph plots Kmax against f. It is a straight line with slope h, x-intercept at the threshold frequency f₀ = φ/h, and y-intercept −φ. Different metals give parallel lines. Raising frequency raises Kmax. Raising intensity only increases the number of electrons ejected, so the current rises while Kmax stays the same.
Common confusion: Brighter light does not make faster electrons. Intensity sets photons per second, not energy per photon. Below the threshold frequency, no electrons are ejected at any intensity.
Sources
- Physics (Giancoli)
- Physics (Cutnell and Johnson)
- University Physics (Young and Freedman)
Written for the MCAT and checked before it was published. For study reference only. Spotted an error? Let us know.