Ionisation, excitation and the electronvoltAQA A-Level Physics: Subtopic test

1. 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)

   Which statement defines the electronvolt?

   [1 mark]

   • AThe energy needed to remove one electron from an atom
   • BThe energy transferred when 1 C of charge moves through a potential difference of 1 V
   • CThe energy of a photon of frequency 1 Hz
   • DThe kinetic energy gained by an electron accelerated from rest through a potential difference of 1 V

   (b)

   What is the ionisation energy of hydrogen in joules?

   [1 mark]

   • A8.5 × 10¹⁹ J
   • B2.18 × 10⁻¹⁸ J
   • C2.18 × 10⁻²⁰ J
   • D1.36 × 10⁻¹⁸ J

   (c)

   A photon has energy 4.8 × 10⁻¹⁹ J. Calculate its energy in electronvolts.

   [2 marks]

   Total for question 1: 4 marks

2. 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.

   (a)

   Which process produces the yellow light from a sodium lamp?

   [1 mark]

   • AExcited electrons in sodium atoms falling to lower energy levels
   • BFree electrons bouncing elastically off sodium atoms
   • CSodium atoms being ionised in collisions
   • DFree electrons being accelerated by the potential difference

   (b)

   A sodium atom de-excites to its ground state by emitting a photon of wavelength 589 nm. What is the energy of the photon in electronvolts?

   [1 mark]

   • A0.47 eV
   • B3.4 × 10⁻¹⁹ eV
   • C2.1 eV
   • D5.1 eV

   (c)

   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]

   Total for question 2: 4 marks

3. 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.

   (a)

   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]

   (b)

   The excited mercury atom returns directly to its ground state, emitting a photon. Calculate the wavelength of the photon and state which region of the electromagnetic spectrum it is in.

   [4 marks]

   Total for question 3: 7 marks

4. 4
   A fluorescent tube contains mercury vapour at low pressure. A high potential difference between the electrodes at the ends of the tube accelerates free electrons along it. The inside wall of the glass is coated with a phosphor powder. The lowest excitation energy of a mercury atom is 4.9 eV and its ionisation energy is 10.4 eV.

   (a)

   Free electrons with kinetic energies of 3.0 eV, 4.9 eV and 12 eV each collide with a mercury atom in its ground state. Discuss what happens in each case.

   [6 marks]

   (b)

   Explain how the tube produces visible light, referring to ionisation, excitation and the phosphor coating.

   [6 marks]

   Total for question 4: 12 marks

End of questions