Lesson 8.3.1

8.3.1 Conservation of charge, energy and momentum in interactions Quiz: Pearson Edexcel Physics, Unit 8

20 questions

Written by Revision Ninja · Last reviewed 11 October 2026

In partnership with Revision Ninja

Lesson 8.3.1, Conservation of charge, energy and momentum in interactions: 20 multiple choice questions for the Pearson Edexcel Physics (9PH0), Unit 8: Nuclear and Particle Physics, written with Revision Ninja.

Host it live on the board and students join with a game code on their own devices, or revise alone with Free Play. The answers are revealed in the game.

Host this setFree Play

The 20 questions

  1. Which quantities must be conserved in every particle interaction?

    • Electric charge, total energy including rest mass energy, and momentum
    • Electric charge and speed only, with no requirement on energy or momentum
    • Kinetic energy alone, with charge and momentum allowed to change freely
    • Number of particles and the mass of the heaviest particle only

  2. What does the total charge before a particle interaction equal compared with the total charge after it?

    • It is always larger than the total charge after the interaction
    • It is equal to the total charge after the interaction
    • It is zero in every interaction, so charge is not conserved
    • It is always twice the total charge after the interaction

  3. What does the curvature of a charged particle's track in a magnetic field indicate about that particle?

    • Its momentum and the sign of its charge
    • Its position at the moment of creation only
    • Its rest mass and its lifetime only
    • Its spin and the number of quarks it contains

  4. A neutron decays into a proton, an electron and an antineutrino. What is the total charge of the products?

    • +2, the combined charge of the proton and the electron
    • -1, which is the charge of the electron alone
    • +1, which is the charge of the proton alone
    • Zero, the same as the neutron

  5. What is the minimum total energy of the two photons produced when an electron and a positron at rest annihilate?

    • 0.511 MeV
    • 2.04 MeV
    • 0 MeV
    • 1.02 MeV

  6. An electron and a positron at rest annihilate to produce two photons travelling in opposite directions. What is the total momentum of the two photons?

    • Zero
    • Equal to twice the momentum of one photon
    • Equal to the mass energy of the electron divided by c
    • Equal to the momentum of one photon

  7. A stationary particle of rest energy 135 MeV decays into two photons of equal energy. What is the energy of each photon?

    • 67.5 MeV
    • 270 MeV
    • 33.8 MeV
    • 135 MeV

  8. A stationary unstable particle decays into two particles of equal mass. How are their momenta related?

    • They are equal in magnitude and opposite in direction
    • They are zero, because the parent particle was at rest
    • They are unequal, because one particle always carries all the momentum
    • They are equal in magnitude and in the same direction

  9. In pair production, a photon creates an electron and a positron. Why can a single electron not be produced instead?

    • A single electron would have too much mass to be produced by a photon of any energy
    • A single electron would not conserve charge, because the photon has no charge and the total charge would become -1
    • A single electron would be produced, but it would immediately decay back into a photon
    • A single electron would violate the conservation of momentum only, not of charge

  10. In the annihilation of a stationary electron and a stationary positron, how does the total lepton number before the interaction compare with the total lepton number after it?

    • It is +2 before and 0 after, because both particles count as leptons
    • It is +1 before and +1 after, because the positron carries no lepton number
    • It is zero both before and after, because the electron (+1) and positron (-1) cancel and photons carry no lepton number
    • It is -2 before and +2 after, because the photons carry lepton number

  11. A 2.0 MeV photon creates an electron-positron pair in the presence of a nucleus that absorbs the recoil momentum. What total kinetic energy is shared between the electron and positron?

    • 2.0 MeV
    • 0.51 MeV
    • About 0.98 MeV
    • 1.02 MeV

  12. Why can a photon not create an electron-positron pair in empty space without another body present?

    • Energy could never be conserved, because photons always carry zero energy
    • Charge could never be conserved, because positrons cannot exist in a vacuum
    • The photon would have to change its speed, which is forbidden by the laws of physics
    • Momentum could not be conserved, because the photon's momentum must be shared with a third body such as a nucleus

  13. A proton and an antiproton, both at rest, annihilate. Approximately what total energy is released? Each has rest energy of about 0.94 GeV.

    • About 0.94 GeV
    • About 1.9 GeV
    • About 3.8 GeV
    • About 0.51 GeV

  14. A track in a bubble chamber has radius 0.50 m in a uniform magnetic field of flux density 0.30 T, for a particle of charge 1.6 x 10^-19 C. What is the momentum of the particle?

    • 2.4 x 10^-20 kg m s^-1
    • 3.2 x 10^-19 kg m s^-1
    • 1.2 x 10^-20 kg m s^-1
    • 4.8 x 10^-20 kg m s^-1

  15. A student claims that an interaction is allowed whenever total energy is conserved. Which response is correct?

    • The student is wrong because particle interactions do not conserve energy at any stage
    • The student is wrong: charge, momentum and other quantities such as lepton number must also be conserved
    • The student is right, but only when the particles involved have no charge at all
    • The student is right: energy conservation alone decides whether any particle interaction is allowed

  16. Why is the total momentum of the decay products of a particle that was at rest equal to zero?

    • Decay products always move at the speed of light, so their momenta cancel exactly
    • Momentum is never conserved in particle decays, so the products must be at rest
    • Each decay product has zero mass, so each has zero momentum
    • Momentum is conserved, and the parent particle had zero momentum before it decayed

  17. Why is the reaction e^- -> gamma (a single electron decaying into a single photon) forbidden?

    • The photon would travel faster than light, which is forbidden by the reaction
    • The photon has too much energy to be created from an electron of any mass
    • The electron's charge of -1 cannot be matched by a photon, which has zero charge, so charge would not be conserved
    • The reaction would require a neutron to be present, which is never available

  18. A pi-zero meson (charge 0) is produced in the collision p + p -> p + p + pi0. What is the total charge after the collision?

    • 0
    • +3
    • +1
    • +2

  19. A particle at rest with rest energy 1000 MeV decays into two particles. One of them has energy 600 MeV. What is the energy of the other?

    • 300 MeV
    • 1600 MeV
    • 600 MeV
    • 400 MeV

  20. Which conservation law forbids the decay of a proton into a positron and a neutral pion?

    • Conservation of mass, because the mass of the products is always smaller than the proton
    • Conservation of momentum, because the neutral pion carries no momentum
    • Conservation of baryon number, because the proton has baryon number 1 but the products have none
    • Conservation of electric charge, because the total charge changes from +1 to zero

All Pearson Edexcel Physics quizzes