Stopping Potential

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In a photoelectric experiment the fastest electrons emitted from a cathode are found to have a maximum kinetic energy of 1.6 eV, and a technician measures the stopping potential.

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The stopping potential is the reverse voltage that just halts the most energetic photoelectrons, and NCERT relates it to their kinetic energy through $eV_0=K_{max}$. Because the electron charge magnitude is $e$ and kinetic energy here is quoted in electron-volts, the numerical value of the stopping potential in volts equals the kinetic energy in electron-volts. Thus with $K_{max}=1.6$ eV, we have $V_0=\frac{K_{max}}{e}=\frac{1.6\ 	ext{eV}}{e}=1.6$ V. This neat correspondence is exactly why the electron-volt is such a convenient unit here. The value 3.2 V is wrong because it doubles the kinetic energy, perhaps confusing charge with a factor of two. The value 0.8 V is wrong because it halves the correct result without justification. The value $1.6	imes10^{-19}$ V is wrong because it confuses the electron charge in coulombs with the potential, mixing units incorrectly. As stated in NCERT Class 12, Chapter 11, the stopping potential directly measures the maximum kinetic energy of photoelectrons. A unit check confirms that dividing an energy in electron-volts by the electronic charge yields a potential in volts of the same numerical value, matching 1.6 V.

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