Ionisation, excitation and the electronvoltAQA A-Level Physics: Mind map

Exam questions on Ionisation, excitation and the electronvolt

1. Atomic physicists quote the energies of atoms in electronvolts, but calculations with the Planck constant use joules. The energy needed to ionise a hydrogen atom in its ground state is 13.6 eV.
   A photon has energy 4.8 × 10⁻¹⁹ J. Calculate its energy in electronvolts.2 marks
2. A sodium vapour street lamp contains sodium atoms at low pressure. A potential difference across the lamp accelerates free electrons, which collide with the sodium atoms. The lowest excitation energy of a sodium atom is 2.1 eV and its ionisation energy is 5.1 eV.
   An electron with a kinetic energy of 1.5 eV collides with a sodium atom in its ground state. Explain why the atom is not excited.2 marks
3. In a demonstration tube, electrons are accelerated from rest through a potential difference and then collide with mercury atoms in a vapour at low pressure. The lowest excitation energy of a mercury atom is 4.9 eV.
   An electron is accelerated from rest through a potential difference of 5.0 V and then excites a mercury atom by transferring the minimum energy possible. Calculate the kinetic energy of the electron before the collision in joules, and the kinetic energy it has after the collision in electronvolts.3 marks