The Electronvolt

• What is the electronvolt used to express?

  Very small energies
• Why is the electronvolt commonly used?

  Quantum energies are smaller than 1 Joule
• What is the formula for potential difference?

  V = E / Q
• What do the variables in the potential difference formula represent?

  V = potential difference, E = energy, Q = charge
• What happens when an electron travels through a potential difference?

  Energy is transferred between two points
• If an electron with a charge of $1.6 \times 10^{-19} \text{ C}$ travels through a potential difference of 1 V, what is the energy transferred?


  $1.60 \times 10^{-19} \text{ J}$
• How is an electronvolt defined?

  Energy gained by an electron at 1 V
• What is the value of 1 eV in joules?

  $1.60 \times 10^{-19} \text{ J}$
• How do you convert eV to J?

  Multiply by $1.6 \times 10^{-19}$
• How do you convert J to eV?

  Divide by $1.6 \times 10^{-19}$
• What happens to a charged particle when it is accelerated through a potential difference?

  It gains kinetic energy
• What is the equation relating eV to kinetic energy?

  $eV =$$\frac{1}{2} mv^2$
• What do the variables in the kinetic energy equation represent?

  e = charge, V = potential difference, m = mass, v = velocity
• How can you rearrange the kinetic energy equation to find the speed of the electron?

  $v =$$\sqrt{\frac{2eV}{m}}$
• If an electron has an energy of 2.4 eV, what is its energy in joules?

  $3.84 \times 10^{-19} \text{ J}$
• If a photon has an energy of $4.9 \times 10^{-19} \text{ J}$, what is its energy in electronvolts?


  $3.06 \text{ eV}$
• What is the equation for photon energy?

  $E =$$\frac{hc}{\lambda}$
• What is the photon energy in joules for light with a wavelength of 700 nm?

  $2.84 \times 10^{-19} \text{ J}$
• How do you convert photon energy from joules to electronvolts?

  Divide by $1.60 \times 10^{-19}$
• What is a key skill needed for exam questions on electronvolts and joules?

  Converting between electronvolts and joules
• How do you calculate photon energy in joules using wavelength?

  $E =$$\frac{(6.63 \times 10^{-34})(3.0 \times 10^{8})}{700 \times 10^{-9}}$
• Do you need to remember the value of 1 eV for exams?

  No, it will be given on the datasheet
• Why is confidence in conversion important during exams?

  To avoid slowing down during questions