In experiments on the photoelectric effect, several different metals were expose
ID: 1999461 • Letter: I
Question
In experiments on the photoelectric effect, several different metals were exposed to ultraviolet light of a single (unknown) wavelength to eject electrons. For each metal the stopping voltage was measured (in volts). The ultraviolet light source was then replaced by a tunable light source and the cut-off frequencies Vo (light frequency below which no photoelectrons were emitted) were measured (in THz) for the same metals in separate experiments.
(a) Explain the meaning of the stopping potential.
(b) From the values given below, estimate Planck’s constant.
Metal Vo/THz Stopping voltage/V
Cs 460 4.8
Be 940 2.8
Hg 1100 2.2
Ni 1210 1.7
Pt 1530 0.4
Explanation / Answer
a) Stopping Potential is the Voltage required to stop the photoelectrons which are ejected from the photometals.
b) h = e*V/V0 = e*Stopping potential / cutoff frequency
Cs --- h = 1.6*10^-19* 4.8/ 460 *10^12 = 16.6*10^-34 J.s
Be --- h = 1.6*10^-19* 2.8/ 940 *10^12 = 4.76*10^-34 J.s
Hg --- h = 1.6*10^-19* 2.2/ 1100 *10^12 = 3.2*10^-34 J.s
Ni --- h = 1.6*10^-19*1.7/ 1210 *10^12 = 2.24*10^-34 J.s
Pt--- h = 1.6*10^-19*0.4/ 1530 *10^12 = 0.4*10^-34 J.s