What is photoelectricity?
Metals have free electrons swimming casually within. When light is shone at the surface of the metal, electrons are given off.
Electrons are knocked off by kinetic energy of photons. We'll see this in detail in a later card.
The photoelectric effect puzzled early scientists, who only knew of light travelling as a wave. It took Einstein to suggest light could travel as particals (photons).
If a wave of light hit the metal, electrons would be given energy until they left the metal. But that doesn't happen. Photons HAVE to be above a certain frequency (the threshold frequency) to knock off electrons.
How can we work stuff out?
To calculate the minimum frequency of a photon that is needed to knock off an electron from a metal (which is the ______ frequency) can be calculated through:
F=φ/h Where 'F' is minimum frequency; 'φ' is work function; and 'h' is Plancks constant. What is this mysterious (φ) work function? It's the energy needed for electrons to leave an atom.
Because of ths, the maximum kinetic energy of the electron can be worked out. That is, the most energy it is possible for the electron to have. It if found by E(max kinetic) = hF-φ.
This is because, hf is the energy given by the photon. φ is the energy used to make the electron leave the atom. So what ever is left over is the kinetic energy.
Ions are atoms with a different number of electrons, to protons. they have more or less electrons that make the atom positive or negative.
They can be created through electrons colliding with atoms. They can knock the electron out of orbit, Ionising the atom.
An electron volt is the unit of energy, equal to the work done when an electron is moved through 1 volt.
The energy of an electron is 1.6x10-19. so multiplying this by the voltage gives electron volts.
flying stingray (drag picture)
Energy levels within atoms
Electrons 'orbit' at different distances above the atom. anyone doing chemistry will know this in more detail. If an electron is given energy, it moves from the lowest energy it can be (ground state) to a higher energy shell.
This is excitation.
Photons are able to raise electrons to these energy levels by supplying the energy. When an electron returns to ground state (de-excitiation) it emits a photon with wavelength corresponding to the distance it dropped.
This explains how when a substance is burned, it gives out unique colours.
Because the energy levels that are changing are unique to that atom.