AQA A-Level Physics Paper 1, November 2020: Question 12

✅ Correct Answer: D

The colliding electron transfers energy to the atomic electron.

During excitation by collision, the incident free electron transfers a discrete amount of its kinetic energy to an orbital electron within the atom, promoting it to a higher energy level. The colliding electron itself continues on with reduced kinetic energy.

💡 Key Knowledge

• Excitation: Process where an electron moves to a higher energy state by absorbing energy.
• Inelastic Collision: Collisions between free electrons and atoms where kinetic energy is not conserved because it is converted into internal atomic energy.
• Energy Transfer: The energy transferred must exactly equal the difference between two energy levels (ΔE = E₂ - E₁).

🧠 Exam Technique

Read through every option carefully before committing. Examiners often mix up the processes of excitation (absorbing energy from a colliding particle) and de-excitation (releasing energy as a photon). Eliminate options that describe photon emission during the upward transition.

❌ Common Errors

• Option B trap: Believing that a photon is emitted when an electron rises to a higher level. Photons are emitted during de-excitation (falling down), not excitation.
• Option C trap: Stating that an electron is emitted when returning to the ground state. De-excitation results in the emission of a photon, not another electron.
• Option A misconception: Assuming the colliding electron gets captured by the nucleus, which describes electron capture (a weak interaction process involving inner-shell protons and electrons), not atomic excitation.