3.2.2.1
Intensity affects photoelectric current, not kinetic energy
The Photoelectric Effect — AQA A-Level Physics
- Each electron absorbs one photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency.. Increasing intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. means more photons per second, not more energetic photons.
- More photons per $second = more photoelectrons emitted per second = higher photoelectric currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).$.
- The kinetic energyThe capacity to do work. Measured in joules (J).The energyThe capacity to do work. Measured in joules (J). an object possesses due to its motion. of each photoelectron depends only on the photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). and the work functionThe minimum energyThe capacity to do work. Measured in joules (J). required to liberate an electron from the surface of a metal..
- Increasing intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. does not change the stopping potentialThe minimum potential difference required to stop the most energetic photoelectrons emitted in the photoelectric effect..
FrequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). vs intensityThe powerThe rate of energy transfer. Measured in watts (W). transmitted per unit area perpendicular to the wave direction. Measured in W m⁻². Proportional to amplitude squared. at constant intensity
- Increasing frequencyThe number of complete oscillations passing a point per unit time. Measured in hertz (Hz). while keeping intensity constant: each photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. has more energy, so fewer photons per second are needed to maintain the same intensity.
- Fewer photons per $second = fewer photoelectrons per second = lower photoelectric currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).$.
- But each photoelectron has higher kinetic energyThe energy an object possesses due to its motion. because each photon has more energy.
- This is because intensity $= nhf / (tA)$, where $n$ is the number of photons. If $f$ increases and $I$ stays constant, $n$ must decrease.
Common Mistake
MEDIUM
Students often: Don't say increased intensity increases the energy of the photoelectrons.
Instead: Intensity only affects the number of photoelectrons (and hence the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).). The maximum kinetic energyThe energy an object possesses due to its motion. depends only on frequency and work functionThe minimum energy required to liberate an electron from the surface of a metal..
Instead: Intensity only affects the number of photoelectrons (and hence the currentThe rate of flow of chargeA property of matter that causes it to experience a force in an electromagnetic field. Measured in coulombs (C).. Measured in amperes (A).). The maximum kinetic energyThe energy an object possesses due to its motion. depends only on frequency and work functionThe minimum energy required to liberate an electron from the surface of a metal..