The Electronvolt
- The electronvolt is a unit which is commonly used to express very small energies
- This is because quantum energies tend to be much smaller than 1 joule
- The electronvolt is derived from the definition of potential difference:
- When an electron travels through a potential difference, energy is transferred between two points in a circuit, or electric field
- If an electron, with a charge of 1.6 × 10-19 C, travels through a potential difference of 1 V, the energy transferred is equal to:
E = QV = 1.6 × 10-19 C × 1 V = 1.6 × 10-19 J
- Therefore, an electronvolt is defined as:
The energy gained by an electron travelling, from rest, through a potential difference of one volt
1 eV = 1.6 × 10-19 J
- To covert between eV and J:
- eV → J: multiply by 1.6 × 10-19
- J → eV: divide by 1.6 × 10-19
Relation to kinetic energy
- When a charged particle is accelerated through a potential difference, it gains kinetic energy
- If an electron accelerates from rest, an electronvolt is equal to the kinetic energy gained:
eV = ½ mv2
- Rearranging the equation gives the speed of the electron:
Worked example
An electron has an energy of 2.4 eV.
Give the energy of the electron in joules.
Answer:
Worked example
A photon has an energy of 4.9 × 10−19 J.
Give the energy of the photon in electronvolts.
Answer:
Exam Tip
Converting between electronvolts and joules is a skill that you will use a lot in exam questions on this topic. You do not need to remember the value of 1 eV as this will be given on your datasheet, but you do need to be confident in the conversion so that it doesn't slow you down.