Electronvolt (eV)
Quantum Physics - OCR A-Level Physics
Key Definition
Electronvolt (eV)
The energy gained by an electron when it is accelerated through a potential difference of 1 volt. 1 eV = 1.6 × 10⁻¹⁹ J.
The energy gained by an electron when it is accelerated through a potential difference of 1 volt. 1 eV = 1.6 × 10⁻¹⁹ J.
- The electronvoltA unit of energyThe capacity to do work. Measured in joules (J). equal to the energyThe capacity to do work. Measured in joules (J). gained by one electron accelerated through a potential difference of 1 V. 1 eV = 1.6 x 10⁻¹⁹ J. is a convenient unit for atomic-scale energies because photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. energies and energy levelA discrete amount of energyThe capacity to do work. Measured in joules (J). that an electron in an atom can have. Electrons can only exist at specific energy levels, not between them. gaps are typically a few eV.
- To convert from eV to joules: multiply by 1.6 × 10⁻¹⁹.
- To convert from joules to eV: divide by 1.6 × 10⁻¹⁹.
- The relationship comes from W = qV: an electron (q = 1.6 × 10⁻¹⁹ C) through V = 1 V gains energy 1.6 × 10⁻¹⁹ J.
Worked Example [3 marks]
A photonA quantum (discrete packet) of electromagnetic radiation. Its energy is proportional to its frequency. has energy 3.1 eV. Calculate its energy in joules and its wavelengthThe minimum distance between two points on a wave that are in phase (e.g. crest to crest). Measured in metres (m)..
Show Solution
1
Convert to joules
$E = 3.1 \times 1.6 \times 10⁻¹⁹ = 4.96 \times 10⁻¹⁹ J$
[1]2
Use λ = hc/E = (6.63 × 10⁻³⁴ × 3.00 × 10⁸) / (4.96 × 10⁻¹⁹)
[1]3
$\lambda = 4.01 \times 10⁻⁷\;\text{m} = 401 nm (violet visible light)$
[1]Answer
$E = 4.96 \times 10⁻¹⁹ J; \lambda = 401 nm$
Common Mistake
MEDIUM
Students often: Substituting energy in eV directly into $E = hf$ or $E = hc/\lambda$.
Instead: These equations require energy in joules. Always convert eV to J first by multiplying by 1.6 × 10⁻¹⁹.
Instead: These equations require energy in joules. Always convert eV to J first by multiplying by 1.6 × 10⁻¹⁹.
Examiner Tips and Tricks
- The conversion 1 eV = 1.6 × 10⁻¹⁹ J is NOT on the OCR data sheet.
- You must memorise it.
- It appears in almost every quantum physics calculation.