Not given in the exam, recall it.
What it is for
Work function is used in Quantum Physics (SPM Physics Form 5). Keep the SI units in every line of working. Quantum Physics
Symbols and SI units
| Symbol | Meaning | SI unit |
|---|---|---|
| W | work function | J |
| h | Planck constant | J s |
| f₀ | threshold frequency | Hz |
Rearrangements
f₀ = W / hh = W / f₀
Worked example
Common trap
Understanding the work function
The equation W = hf₀ gives the minimum energy needed to release an electron from a metal surface. Here W is the work function in joules (J), h is the Planck constant in joule-seconds (J s), and f₀ is the threshold frequency in hertz (Hz).
Each metal holds its surface electrons with a certain binding energy. A photon must supply at least this much to release one electron.
The threshold frequency f₀ is simply the lowest frequency of light that carries exactly this energy per photon.
Rearrange it as f₀ = W / h to find the threshold frequency from a known work function, or h = W / f₀ if an experiment measures both. The work function is a fixed property of the metal, so it does not change with the brightness of the light.
A worked example, step by step
A clean metal surface has a threshold frequency f₀ = 5.5 × 10¹⁴ Hz. Find its work function.
Take h = 6.63 × 10⁻³⁴ J s.
Start from the formula:
W = hf₀
Substitute with units:
W = 6.63 × 10⁻³⁴ J s × 5.5 × 10¹⁴ Hz
Multiply the numbers and add the powers of ten:
W = 3.6465 × 10⁻¹⁹ J
Rounded to three significant figures, the work function is W ≈ 3.65 × 10⁻¹⁹ J. In electronvolts this is 3.65 × 10⁻¹⁹ ÷ 1.6 × 10⁻¹⁹ ≈ 2.28 eV.
Common mistakes and how this is tested
The key idea to remember is that f₀ is the threshold, or minimum, frequency. Below f₀ no photoelectrons are emitted at all, no matter how bright or how long the light shines.
Errors that lose marks:
- Treating brightness (intensity) as able to compensate for a low frequency.
- Confusing the work function W with the incident photon energy hf.
- Leaving out the negative exponent in 10⁻³⁴.
Under define, state that the work function is the minimum energy required to release an electron from the metal surface. Under calculate, show W = hf₀ with the substitution and units.
Source: DSKP KSSM Physics Form 4 and 5 (Versi English), Sijil Pelajaran Malaysia: Format Pentaksiran mulai 2021, Fizik (4531) (Bahagian Pembangunan Kurikulum (BPK), KPM)