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VICPhysicsQuick questions
Unit 4: How have new ideas and ways of thinking developed our understanding of the physical world?
Quick questions on Photoelectric effect and the photon model of light: VCE Physics Unit 4
9short Q&A pairs drawn directly from our worked dot-point answer. For full context and worked exam questions, read the parent dot-point page.
What is example 1. Threshold from work function?Show answer
Caesium has work function $2.1$ eV. Find the threshold frequency and threshold wavelength.
What is example 2. Kinetic energy above threshold?Show answer
Sodium has work function 2.3 eV. Light of wavelength 400 nm shines on a clean sodium surface. Find $E_{k,\max}$.
What is example 3. Stopping voltage?Show answer
Same sodium experiment, $E_{k,\max} = 0.8$ eV. Stopping voltage: $V_0 = 0.8$ V.
What is confusing photon energy with kinetic energy?Show answer
$E = hf$ is the photon energy. $E_{k,\max} = hf - \phi$ is the max kinetic energy of the ejected electron. Different quantities.
What is using J instead of eV?Show answer
Both work, but be consistent. $h$ in J s with energy in J, or $h$ in eV s with energy in eV. Mixing units gives wrong answers.
What is forgetting $\phi$ when computing $E_{k,\max}$?Show answer
A common slip is to report $hf$ as $E_{k,\max}$. The work function must be subtracted.
What is treating below-threshold light as producing slow electrons?Show answer
Below threshold, no electrons are emitted at all, not slow electrons.
What is confusing intensity and frequency dependence?Show answer
Intensity affects how many electrons per second. Frequency affects how energetic each electron is.
What is sign of $V_0$?Show answer
The stopping voltage is positive (a magnitude). The plot $V_0$ vs $f$ has a positive gradient $h/e$ and negative $y$-intercept $-\phi/e$.