Particles, antiparticles and photons Flashcards – AQA A-level Physics (7408)

What counterpart does every particle type have?

A corresponding antiparticle.

How do a particle and its antiparticle compare in mass and rest energy?

They have the same mass and hence the same rest energy.

How do the electric charges of a particle and its antiparticle compare?

Their charges are opposite. If the particle is neutral, both have zero charge.

What are the antiparticles of the electron, proton, neutron and neutrino?

• Electron: positron
• Proton: antiproton
• Neutron: antineutron
• Neutrino: antineutrino

What does MeV stand for when expressing rest energy?

Megaelectronvolt, a unit of energy.

What is a photon?

A discrete packet of electromagnetic energy.

How does photon energy depend on frequency? Give the equation in terms of the Planck constant $h$.

Energy is proportional to frequency: $E = hf$.

What equation relates the frequency $f$ and wavelength $\lambda$ of electromagnetic radiation to its speed $c$ in a vacuum?

$c = f\lambda$

How does photon energy depend on wavelength? Give the equation in terms of the Planck constant $h$ and the speed $c$ of electromagnetic radiation in a vacuum.

Energy is inversely proportional to wavelength: $E = \frac{hc}{\lambda}$.

In $E = hf$, what units must $h$ and $f$ have to give photon energy in joules?

$h$ in joule-seconds and $f$ in hertz.

What is particle–antiparticle annihilation?

A particle meets its corresponding antiparticle, and their energy is transferred to radiation, typically photons.

Which energies contribute to the photons produced when an electron and positron annihilate?

Their rest energies and any kinetic energy they have.

Which two quantities are conserved in particle–antiparticle annihilation?

Energy and momentum.

Why must an electron and positron at rest produce at least two photons when they annihilate?

To allow the total photon momentum to balance.

What is pair production?

The creation of a particle and its antiparticle from photon energy.

What minimum photon energy is required for pair production?

The combined rest energies of the particle and antiparticle.

What can happen to photon energy above the minimum required for pair production?

It can appear as kinetic energy.

Why can pair production by a photon occur in the vicinity of a nucleus?

The nucleus can take up momentum, allowing both energy and momentum to be conserved.