The Photoelectric Equation
- Since energy is always conserved, the energy of an incident photon is equal to:
The work function + the maximum kinetic energy of the photoelectron
- The energy within a photon is equal to hf
- This energy is transferred to the electron to release it from a material (the work function) and the remaining amount is given as kinetic energy to the emitted photoelectron
- This equation is known as the photoelectric equation:
E = hf = Φ + ½ mv2max
- Where:
- h = Planck's constant (J s)
- f = the frequency of the incident radiation (Hz)
- Φ = the work function of the material (J)
- ½ mv2max = Ek(max) = the maximum kinetic energy of the photoelectrons (J)
- hf is equal to the energy of a single photon
- This equation demonstrates:
- If the incident photons do not have a high enough frequency and energy to overcome the work function (Φ), then no electrons will be emitted
hf0 = Φ
-
- Where f0 = threshold frequency, photoelectric emission only just occurs
- Ek(max) depends only on the frequency of the incident photon, and not the intensity of the radiation
- The majority of photoelectrons will have kinetic energies less than Ek(max)
Graphical Representation of Work Function
- The photoelectric equation can be rearranged into the straight line equation:
y = mx + c
- Comparing this to the photoelectric equation:
Ek(max) = hf - Φ
- A graph of maximum kinetic energy Ek(max) against frequency f can be obtained
- The key elements of the graph:
- The work function Φ is the y-intercept
- The threshold frequency f0 is the x-intercept
- The gradient is equal to Planck's constant h
- There are no electrons emitted below the threshold frequency f0
Worked example
The graph below shows how the maximum kinetic energy Ek of electrons emitted from the surface of sodium metal varies with the frequency f of the incident radiation.Calculate the work function of sodium in eV.
Step 1: Write out the photoelectric equation and rearrange to fit the equation of a
straight line
E = hf = Φ + ½ mv2max → Ek(max) = hf - Φ
y = mx + c
Step 2: Identify the threshold frequency from the x-axis of the graph
When Ek = 0, f = f0
Therefore, the threshold frequency is f0 = 4 × 1014 Hz
Step 3: Calculate the work function
From the graph at f0, ½ mvmax2 = 0
Φ = hf0 = (6.63 × 10-34) × (4 × 1014) = 2.652 × 10-19 J
Step 4: Convert the work function into eV
1 eV = 1.6 × 10-19 J J → eV: divide by 1.6 × 10-19
Exam Tip
When using the photoelectric effect equation, hf, Φ and Ek(max) must all have the same units (joules). Therefore make sure to convert any values given in eV into Joules (× (1.6 × 10-19))