Revision Notes: CAPS Physical Science Grade 12 – The Photoelectric Effect and Basic Nuclear Physics

Photoelectric Effect

Introduction

The photoelectric effect refers to the phenomenon where electrons are ejected from the surface of a material when it is exposed to light of a certain frequency. This concept is significant in understanding the dual nature of light, supporting the particle theory proposed by Albert Einstein.

Key Points

Definition:

The ejection of electrons from a material’s surface when illuminated by light of a certain frequency.

Supports the particle model of light over the wave model.

Einstein’s Postulate

Light consists of packets of energy called photons.

The energy of a photon ((E)) is proportional to its frequency ((f)): (E = hf), where (h) is Planck’s constant ((6.63 \times 10^{-34} \, \text{J·s})).

Key Equations:

Photon energy: (E = hf = \frac{hc}{\lambda}).

Work function ((W_0)): The minimum energy required to eject an electron from a metal’s surface.

Photoelectric equation: (hf = W_0 + \frac{1}{2}mv_{max}^2).


Threshold Frequency ((f_0)):

The minimum frequency of light required to eject electrons from a metal.

(W_0 = hf_0).Threshold Wavelength ((\lambda_0)):

The maximum wavelength of light required to eject electrons.

(\lambda_0 = \frac{c}{f_0}).

Real-World Applications

Solar Panels:

Convert light energy into electrical energy using the photoelectric effect.


Practical problem: Calculate the efficiency of a solar panel for given light intensity.Example Problem:
Given: Light of wavelength 280 nm on zinc (work function = (6.88 \times 10^{-19} \, \text{J})).
Find: Maximum kinetic energy of the emitted photoelectrons.


Solution:
[
hf = \frac{hc}{\lambda} = \frac{(6.63 \times 10^{-34} \, \text{J·s})(3.0 \times 10^8 \, \text{m/s})}{280 \times 10^{-9} \, \text{m}} = 7.10 \times 10^{-19} \, \text{J}
]
[
\text{KE}_{max} = hf – W_0 = 7.10 \times 10^{-19} \, \text{J} – 6.88 \times 10^{-19} \, \text{J} = 0.22 \times 10^{-19} \, \text{J}
]


Photoelectric Cells:

Used in light meters and automatic doors.

Common Misconceptions and Errors

Wave vs. Particle Nature:

Misconception: The photoelectric effect can be explained by the wave theory of light.

Strategy: Emphasize that only the particle theory can explain why light below the threshold frequency cannot eject electrons, regardless of its intensity.Intensity vs. Energy:

Misconception: Increased intensity increases the kinetic energy of photoelectrons.

Strategy: Clarify that intensity increases the number of photons, not the energy per photon.

Practice and Review

Practice Questions:

Calculate the threshold frequency for a metal with a given work function.

Determine the kinetic energy of electrons ejected by light of a given wavelength.Examination Tips:

Focus on keywords such as “threshold frequency,” “work function,” and “kinetic energy.”

Time management: Attempt easier questions first to secure marks before moving to complex calculations.

Connections and Extensions

Dual Nature of Light:

Links to quantum mechanics and the wave-particle duality of matter.


Explore how the photoelectric effect supports quantum theory.


Advanced Physics:

Investigate how the principles of the photoelectric effect are foundational in fields like spectroscopy and electronics.

Summary and Quick Review

  • Key Formula:
  • (hf = W_0 + \frac{1}{2}mv_{max}^2)
  • Key Concepts:
  • Photon energy, work function, threshold frequency, and kinetic energy of photoelectrons.

Additional Resources

Videos: Khan Academy: Photoelectric Effect

Articles: HyperPhysics: Photoelectric Effect

    By following these revision notes and actively engaging with the material, students should be well-prepared to understand and apply the concepts of the photoelectric effect in various contexts.


    If you have any specific questions or need further clarifications, feel free to ask!

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