Photons: (d) the electronvolt (eV) as a unit of energy

The idea

The electronvolt is a convenient energy unit for atomic and photon scales. One electronvolt is the kinetic energy gained by an electron accelerated through a potential difference of one volt, so 1 eV = 1.60 × 10⁻¹⁹ J. To convert, multiply eV by 1.60 × 10⁻¹⁹ to get joules, or divide joules by 1.60 × 10⁻¹⁹ to get eV. For example, a photon of energy 2.0 eV is 2.0 × 1.60 × 10⁻¹⁹ = 3.2 × 10⁻¹⁹ J. In MCQs, check whether the answer options are in eV or J and convert before comparing. The eV is a unit of energy, not potential difference or charge.

What earns the marks

• Defines 1 eV as the energy transferred when an electron moves through a potential difference of 1 V.
• States the conversion 1 eV = 1.60 × 10⁻¹⁹ J and applies it in either direction.
• Converts photon energies between eV and J correctly, for example 2.0 eV = 3.2 × 10⁻¹⁹ J.
• Recognises that the eV is an energy unit, not a unit of potential difference or charge.
• Chooses eV for atomic and photon-scale energies to avoid unwieldy powers of ten.

Common mistakes and how to put them right

• Confusing eV with volts.

  Put it right: The volt is potential difference, while the electronvolt is energy; convert using 1 eV = 1.60 × 10⁻¹⁹ J.

• Multiplying by 1.60 × 10⁻¹⁹ when converting joules to eV.

  Put it right: Divide joules by 1.60 × 10⁻¹⁹ instead.

• Assuming the eV applies only to electrons.

  Put it right: Any particle or photon energy can be expressed in eV.

Exam tips

• Write the conversion factor explicitly before calculating so the direction of conversion is clear.
• Check the unit requested in the answer options and convert your value to match before selecting.
• Use standard form throughout to keep track of the 10⁻¹⁹ factor.