ONE-EIGHTH STAGE 2026
Aburi Girl’s SHS: 38 points
St. Joseph’s Seminary SHS: 20 points
Fafraha Community SHS: 12 points
St. John's Grammar School: 40 points
Mpraeso SHS: 23 points
Tema Secondary School: 13 points
St. Augustine’s College
Anlo SHS
Kumasi High School
ROUND 2 - SPEED RACE
QUESTION
In a photoelectric effect experiment conducted in vacuum with a suitable monochromatic light source, what primary factor determines the photoelectric current?
ANSWER: Incident photon flux, light intensity, or light power (the number of photons striking the surface per second determines the number of photoelectrons emitted per second, i.e. the current — not the photon energy/frequency, which instead determines the maximum kinetic energy of the emitted electrons).
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PRACTICE QUESTIONS
1. QUESTION: What determines the maximum kinetic energy of the photoelectrons emitted from a given metal?
ANSWER: The frequency of the incident light
SOLUTION:
Einstein's equation gives $KE_{\text{max}} = hf - W$.
For a given metal the work function is fixed, so only the frequency changes the maximum kinetic energy.
2. QUESTION: What happens to the photoelectric current when the intensity of the light is doubled while its frequency stays above the threshold?
ANSWER: The current doubles
SOLUTION:
Twice as many photons strike the surface each second.
Each photon can release at most one electron, so twice as many electrons are emitted each second.
3. QUESTION: What happens to the maximum kinetic energy of the photoelectrons when the intensity of the light is doubled at a constant frequency?
ANSWER: It stays the same
SOLUTION:
Each photon still carries the same energy $hf$.
$KE_{\text{max}} = hf - W$ does not depend on the number of photons.
4. QUESTION: What name is given to the minimum frequency of light that can eject electrons from a metal surface?
ANSWER: The threshold frequency
SOLUTION:
At the threshold frequency $f_0$, the photon energy just equals the work function: $hf_0 = W$.
5. QUESTION: What name is given to the minimum energy needed to remove an electron from the surface of a metal?
ANSWER: The work function
SOLUTION:
The work function depends on the metal and on the state of its surface.
6. QUESTION: What name is given to the reverse potential that just stops the most energetic photoelectrons from reaching the anode?
ANSWER: The stopping potential
SOLUTION:
$eV_s = KE_{\text{max}}$.
7. QUESTION: Why does very intense red light fail to eject electrons from zinc, while weak ultraviolet light does?
ANSWER: Each red photon has less energy than the work function of zinc
SOLUTION:
An electron absorbs one photon at a time.
Red photons each carry too little energy, however many arrive; each ultraviolet photon carries more than the work function.
8. QUESTION: What happens to the stopping potential when the frequency of the incident light is increased?
ANSWER: It increases
SOLUTION:
$eV_s = hf - W$, so a higher frequency gives faster photoelectrons that need a larger reverse potential to stop them.
9. QUESTION: What does the gradient of a graph of stopping potential against frequency of the incident light represent?
ANSWER: The Planck constant divided by the electronic charge
SOLUTION:
$V_s = \dfrac{h}{e}f - \dfrac{W}{e}$.
The graph is a straight line of gradient $\dfrac{h}{e}$.
10. QUESTION: What does the photoelectric effect show about the nature of light?
ANSWER: Light behaves as a stream of particles (photons)
SOLUTION:
The instant emission, the threshold frequency and the intensity-independent maximum kinetic energy cannot be explained by a continuous wave.
They are explained if light energy comes in packets of $hf$.
11. QUESTION: Why does the photoelectric current reach a saturation value as the anode potential is made more positive?
ANSWER: All the emitted electrons are already being collected
SOLUTION:
Once every photoelectron reaches the anode, a larger potential cannot increase the number collected per second.
Only more photons per second can raise the current further.
12. QUESTION: What happens to the photoelectric current if the frequency of the light is increased while the number of photons arriving per second stays the same?
ANSWER: It stays about the same
SOLUTION:
The number of electrons emitted per second is set by the number of photons per second.
The higher frequency only gives the electrons more kinetic energy.