Retrieval Practice
The Photoelectric Effect — AQA A-Level Physics
Q1. Define one electronvolt.
The energy gained by an electron travelling through a potential difference of 1 V. 1 eV = 1.60 × 10⁻¹⁹ J.
Q2. How do you convert (a) eV to J and (b) J to eV?
(a) Multiply by 1.60 × 10⁻¹⁹. (b) Divide by 1.60 × 10⁻¹⁹.
Q3. What is the photoelectric effect?
The emission of electrons from the surface of a metal when electromagnetic radiation of sufficiently high frequency is incident on it.
Q4. Why does the photoelectric effect provide evidence for the particle nature of light?
- Each electron absorbs only one photon.
- Below the threshold frequency, no electrons are emitted regardless of intensity.
- A wave model predicts that any frequency should eventually cause emission if given enough time — this does not happen.
Q5. Define threshold frequency.
The minimum frequency of incident electromagnetic radiation required to remove a photoelectron from the surface of a metal.
Q6. Define the work function.
- The minimum energy required to release a photoelectron from the surface of a metal.
- Related to threshold frequency by φ = hf₀.
Q7. State the photoelectric equation.
hf = φ + E_k(max), where hf = photon energy, φ = work function, E_k(max) = maximum kinetic energy of emitted photoelectrons.
Q8. Define stopping potential.
- The potential difference required to reduce the photoelectric current to zero.
- The maximum kinetic energy of the photoelectrons is E_k(max) = eV_s.
Q9. Why does increasing intensity not increase the maximum kinetic energy of photoelectrons?
- Each electron absorbs one photon.
- Intensity determines the number of photons per second (hence the number of photoelectrons), not the energy per photon.
- Photon energy depends only on frequency.
Q10. On a graph of E_k(max) against f, what do the gradient, y-intercept and x-intercept represent?
Gradient = Planck's constant (h). y-intercept = −φ (negative work function). x-intercept = f₀ (threshold frequency).
Q11. Why do most photoelectrons have kinetic energy less than E_k(max)?
- E_k(max) applies to surface electrons, which need only the work function energy to escape.
- Electrons deeper in the metal use extra energy reaching the surface, leaving less kinetic energy.
Q12. What happens to the photoelectric current if frequency increases but intensity stays constant?
- The current decreases.
- Higher-energy photons mean fewer photons per second at the same intensity, so fewer photoelectrons are emitted per second.