# AS AQA Phys Unit 1- Energy Levels and Photon Emission

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Collisions of electrons with atoms

Ionisation

An ion is a charged atom where the number of electrons are not equal to the number of protons. An ion is formed from an uncharged atom by adding or removing electrons. Adding electrons forms a negative ion, removing electrons forms a positive ion.

Any ion creating process is called ionisation, e.g.

-Alpha, beta and gamma radiation create ions when passing through substances and colliding with atoms of that substance.

-Electrons passing through a fluorescent tube create ions when they collide with atoms of gas or vapour.

By measuring p.d. (V) ionisation energy of a gas atom can be calculated;

ionisation energy of a gas atom = eV (where e is charge)

Electron volt

This is a unit of energy equal to work done when an electron is moved through a p.d. of one volt. Work done = qV (where q is charge) for example:

An ion of charge +2e moves through a p.d. of 10V = 20eV

The definition of an electronvolt is; The kinetic energy carried by an electron after it has been accelerated through a potential difference of 1 volt.

energy gained by an electron (eV) = accelerating voltage (V)

1eV = 1.6*10^-19 J

Excitation

Excitation is where an atom absorbs energy from colliding electrons without being ionised. With atoms in a gas, if a colliding electron looses all its kinetic energy when causing excitation, the current, due to the flow of electrons through the gas, will be reduced. If the colliding electron doesn't have enough energy to cause excitation it will be deflected by the atom and no overall loss of kinetic energy will occur.

The energy values at which an atom absorbs energy are known as the atom's excitation energies. When excitation occurs, the colliding electron makes an electron inside the atom move to an outer shell. Energy is needed for the process because an atomic electron is moving away from the nucleus. Excitation energy is…

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