How do you convert electronvolts to joules?

Multiply by 1.60 × 10⁻¹⁹.

An electron in an atom moving to a higher energy level.

The removal of an electron from an atom, leaving a positive ion.

What is the ionisation energy?

The minimum energy needed to remove an electron from an atom in its ground state.

What happens when an electron has less energy than the lowest excitation energy?

It collides elastically with the atom and keeps its energy.

What is de-excitation?

An electron falling to a lower level and emitting a photon of energy hf = E₁ − E₂.

Why is mercury vapour used in a fluorescent tube?

Its atoms are excited by electron collisions and emit ultraviolet photons when they de-excite.

What is the job of the phosphor coating?

It absorbs ultraviolet photons and emits visible photons.

Why are ionising collisions needed in a fluorescent tube?

They produce free electrons and ions so that the vapour conducts.

Ionisation, excitation and the electronvoltAQA A-Level Physics: Flashcards

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What is the electronvolt?

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What is the electronvolt?: The kinetic energy gained by an electron accelerated from rest through a potential difference of 1 V.
What is 1 eV in joules?: 1.60 × 10⁻¹⁹ J
How do you convert joules to electronvolts?: Divide by 1.60 × 10⁻¹⁹.
How do you convert electronvolts to joules?: Multiply by 1.60 × 10⁻¹⁹.
What is excitation?: An electron in an atom moving to a higher energy level.
What is ionisation?: The removal of an electron from an atom, leaving a positive ion.
What is the ionisation energy?: The minimum energy needed to remove an electron from an atom in its ground state.
What happens when an electron has less energy than the lowest excitation energy?: It collides elastically with the atom and keeps its energy.
What is de-excitation?: An electron falling to a lower level and emitting a photon of energy hf = E₁ − E₂.
Why is mercury vapour used in a fluorescent tube?: Its atoms are excited by electron collisions and emit ultraviolet photons when they de-excite.
What is the job of the phosphor coating?: It absorbs ultraviolet photons and emits visible photons.
Why are ionising collisions needed in a fluorescent tube?: They produce free electrons and ions so that the vapour conducts.

Exam questions on Ionisation, excitation and the electronvolt

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
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
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

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Written by the Exaim team, led by Shaun Daswani (Head of Upper Secondary, Improve ME Institute; MSc Financial Mathematics, Imperial College London; BSc, UCL) and Jason Daswani (operational lead, Improve ME Institute; LSE).