The Electronvolt (OCR AS Physics): Revision Note
Exam code: H156
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
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
Show that the photon energy of light with wavelength 700nm is about 1.8 eV.
Answer:
Step 1: Write the equations for wave speed and photon energy
Step 2: Calculate the photon energy in Joules
Step 3: Convert the photon energy into electronvolts
1 eV = 1.6 × 10-19 J J → eV: divide by 1.6 × 10-19
Examiner Tips and Tricks
To convert between eV and J:
eV → J: multiply by 1.6 × 10-19
J → eV: divide by 1.6 × 10-19
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