Anonymous
Anonymous asked in Science & MathematicsPhysics · 2 months ago

# Need some help with solving this Physics exercise ? (Planks constant )?

If on a metal plate of Cesium is lighted with light which wavelenght is 400nm, the photoelectrons will stop if there's a voltage of 1V, and if it's used ultraviolet light with wavelenght of 200 nm, the stopping voltage is 4.11V. Calculate the value of Planks constant ?

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• 2 months ago

Assume the photon energy is given by E = h*f = hc/wl where f = frequency, wl = wavelength of photon, c = speed of light and h = unknown Planck constant.

Now when wl = wli1 = 400 nm, the electron has 1 eV of kinetic energy.  1eV = 1.6x10^-19 J so we can write

hc/wl1 = W + 1.6x10^-19 J  where W = work function of Cesium

For wl = wl2 = 200nm, the electron has 4.11 eV of kinetic energy  = 6.576x10^-19 J

and   hc/wl2 = W + 6.576x10^-10 J

Subtract the first energ balance from the second:

hc/wl2 - hc/wl1 = 4.976x10^-19 J  now simplify  let dE = 4.976x10^-19 J

use f2 = c/wl2 and f1 = c/wl1  then

h*(f2 - f1) = dE --->  h = dE/(f2-f1) = 6.634x10^-35 J-s