Photoelectric Current and Collector Potential
The photocurrent-voltage curve rises toward saturation under accelerating voltage and falls to zero at the stopping potential under reverse voltage. At fixed suitable frequency, saturation current scales with incident intensity.
Why this shows up in the exam
Reading photocell current-voltage curves · Comparing light intensities with a photocathode · Choosing collector bias in photoelectric detectors
Learn the idea
Collector voltage controls collection, intensity controls saturation current, and frequency controls stopping voltage. A positive collector gathers more emitted electrons until nearly all are collected. A reverse voltage rejects electrons, with the fastest electron stopped only at the stopping potential.
🧠 Memory hook: Intensity lifts the plateau; frequency moves the cutoff.
Get this one clearly and it pays off every single time it shows up in the paper. 🎯
Formulas & facts to keep ready
- I_s proportional to photon arrival rate — saturation-current dependence when quantum efficiency is unchanged
- e V_s = K_max — reverse potential required to stop the fastest photoelectrons
How to approach it
- 1Identify the saturation plateau and zero-current cutoff
- 2Compare intensities using plateau currents
- 3Compare frequencies using stopping-potential magnitudes
Common slip-ups that cost marks
- •Changing stopping potential when only intensity changes
- •Treating saturation current as unlimited growth with collector voltage
- •Using reverse-voltage sign without checking the stated convention
🌟 That's the whole idea — you've got this. Try the practice set below; every question you attempt makes it stick a little harder.
Original chapter practice
Original questions for this chapter, not past-paper questions or an exact mapping to this individual concept.
Photons of energy 5 eV illuminate a metal of work function 2 eV. Find the stopping potential.
More from Dual Nature of Matter and Radiation
Photoelectric Effect
The photoelectric effect describes the emission of electrons from a material when it is exposed to light of sufficient frequency, governed by concepts such as threshold frequency, work function, stopping potential, and the Einstein photoelectric equation.
de Broglie Wavelength and Matter Waves
All matter exhibits wave-like properties, with the de Broglie wavelength inversely proportional to momentum and dependent on factors like velocity, temperature, and particle type.
Photon Properties and Energy-Momentum Relations
Photons are massless particles of light characterized by their energy, frequency, momentum, and charge neutrality, and their interactions obey conservation laws.
Photon Energy and Momentum
For a photon in vacuum, energy is proportional to frequency and momentum is energy divided by c. Frequency and wavelength obey c = nu lambda, so shorter-wavelength photons have larger energy and momentum.
Photon Rate, Power, and Energy Density
For monochromatic radiation, total energy is the number of photons times h nu. Power is energy per unit time, so the photon emission rate equals power divided by single-photon energy.
Radiation Pressure and Photon Momentum Transfer
Radiation force is the rate of photon momentum transfer. For normal incidence on an ideal absorber the pressure is intensity divided by c; for an ideal reflector it is twice that value.