The Electronvolt (AQA A Level Physics): Revision Note
Exam code: 7408
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:
Where:
V = potential difference (V)
E = energy (J)
Q = charge (C)
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:
Therefore, an electronvolt is defined as:
The energy gained by an electron travelling through a potential difference of one volt
To convert 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:
Where:
e = charge of an electron (1.60 × 10–19 C)
V = potential difference (V)
m = mass of the particle (kg)
v = velocity of the particle (m s–1)
Rearranging the equation gives the speed of the electron:
Worked Example
(a) An electron has an energy of 2.4 eV.
Give the energy of the electron in joules.
(b) A photon has an energy of 4.9 × 10−19 J.
Give the energy of the photon in electronvolts.
Answer:
Part (a)
Part (b)
Worked Example
Show that the photon energy of light with wavelength 700nm is about 1.8 eV.
Answer:
Step 1: Write the equation for photon energy
Step 2: Calculate the photon energy in Joules
Step 3: Convert the photon energy into electronvolts
J → eV: divide by 1.60 × 10-19
Examiner Tips and Tricks
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.
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