9702 · 22.2
Photoelectric effect flashcards
Revision flashcards for Cambridge 9702 Photoelectric effect (syllabus 22.2). Flip, recall, then mark a real past-paper question.
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What is the threshold frequency (f₀) in the photoelectric effect?
The minimum frequency of incident light required to cause electron emission from a specific metal surface.
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How does light intensity affect the maximum kinetic energy of photoelectrons?
It does not affect the maximum kinetic energy; KE_max is independent of light intensity.
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What is the formula relating photon energy, work function, and maximum kinetic energy of an emitted electron?
hf = Φ + KE_max (or hf = Φ + ½ mv_max_²)
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Define the work function (Φ) of a metal.
The minimum energy required to remove an electron from the surface of a metal.
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How does the photoelectric effect support the particle nature of light?
It demonstrates a one-to-one interaction where discrete photons transfer energy to individual electrons, and emission only occurs if the photon's energy exceeds a minimum value, regardless of intensity.
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What is a photon?
A discrete packet or 'quantum' of electromagnetic energy.
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What is Planck's constant (h)?
A fundamental constant in quantum mechanics that relates a photon's energy to its frequency (h = 6.63 × 10⁻³⁴ Js).
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Define the electronvolt (eV).
The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt (1 eV = 1.60 × 10⁻¹⁹ J).
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How does increasing light intensity affect the photocurrent, assuming frequency is above threshold?
Increasing intensity increases the number of photons hitting the metal per second, leading to more electrons being emitted and thus a larger photocurrent.
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What is the relationship between work function (Φ) and threshold frequency (f₀)?
The work function is equal to Planck's constant multiplied by the threshold frequency (Φ = hf₀).
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What happens if light frequency is below the threshold frequency (f₀)?
No electrons are emitted, regardless of how intense the light is, because individual photons do not have enough energy to overcome the work function.
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What is the stopping potential (V_s)?
The minimum negative (retarding) potential difference applied between the metal plate and a collector that is required to stop the photocurrent, by repelling even the most energetic photoelectrons. It's related to KE_max by KE_max = eV_s.
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What does the graph of maximum kinetic energy (KE_max) versus frequency (f) for the photoelectric effect look like?
A straight line with a positive gradient. The gradient is equal to Planck's constant (h), the y-intercept is the negative of the work function (-Φ), and the x-intercept is the threshold frequency (f₀).
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Why couldn't classical wave theory explain the instantaneous emission of photoelectrons?
Wave theory predicted that energy would be absorbed gradually over time. Therefore, there should be a time lag between the light hitting the metal and the electron gaining enough energy to escape. The observed instantaneous emission contradicted this.
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In the photoelectric effect, what is the relationship between the maximum kinetic energy of photoelectrons and the intensity of the incident light?
There is no relationship. The maximum kinetic energy (KE_max) of photoelectrons depends only on the frequency of the incident light and the work function of the metal, not on the light's intensity.