# AQA A Level Physics复习笔记2.4.1 The Electronvolt

### 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 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.

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 #### Exam Tip

• To convert between eV and J:
• eV → J: multiply by 1.6 × 10-19
• J → eV: divide by 1.6 × 10-19 