Quantum - kerboodle Flashcards
3.1 The photoelectric effect
Discovery of the photoelectric effect
What is the photoelectric effect?
- A metal contains conduction electrons, which move about freely inside the metal. These electrons collide with each other and with the positive ions of the metal.
- When discovering radio Waves, Heinrich Hertz found that the sparks produced when radio waves were being transmitted were stronger when ultraviolet radiation was directed.
- further experimentation revealed that electrons were released from the surface of the metal when electromagnetic radiation above a certain frequency was directed at the metal.
- This effect became known as the photoelectric effect
Einstein’s explanation of the photoelectric effect
• The photon theory of light was put forward by Einstein in 1905 to explain the photoelectric effect
• he assumed that light is composed of wave packets or photons. each of energy equal to hf.
• To explain the photoelectric effect, Einstein said that:
When light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident light and therefore gains energy equal to hf, where hf is the energy of a light photon
An electron can leave the metal surface if the energy gained from a single photon exceeds the work function of the metal. This is the minimum energy needed by an electron to escape from the metal surface. Excess energy gained by the photoelectron becomes its kinetic energy
Stopping Potential
- Electrons that escape from the metal plate can be attracted back to it by giving the plate a sufficient positive charge
- The minimum potential needed to achieve this is called the stopping potential
- At this potential, the maximum kinetic energy of the emitted electron is 0 as it has to do extra work to leave the surface of the metal
3.2 - More about photoelectricity
Into the quantum world
- at the end of the 19th century it was suggested that the energy of each vibrating atom is quantised
- This meant that only certain levels of energy are allowed and that energy had to be in multiples of basic amounts
- This led to the development of the Planck constant (6.63e-34)
- Planck, the person who suggested that energy is quantised, imagined that each atom absorbed or emitted radiation as it moved up or down the energy levels.
More about conduction electrons
• conduction electrons move about at random, their average kinetic energy of a conduction electron depends on the temperature of the metal
• the work function of a metal is
the minimum energy required for a conduction electron to leave a metal surface
• when a conduction electron absorbs a photon, it’s kinetic energy increases by an amount equal to the energy of the photon
• if the energy of the photon exceeds the work function of the metal, the conduction electron can leave the metal surface, with the excess energy being transferred into kinetic energy
• if the electron does not leave the metal, it collides repeatedly with other electrons and positive ions and it quickly loses its extra kinetic energy
The Vacuum Photocell
- A vacuum photocell is a glass tube that contains a metal plate, referred to as the photocathode, and a smaller metal electrode referred to as the anode
- when light of a frequency greater than the threshold frequency for the metal is directed at the photocathode, electrons are emitted from the cathode and are attracted to the anode.
- A microammeter in a circuit with a vacuum photocell can be used to measure the photoelectric current
- the photoelectric current is proportional to the number of electrons per second that transfers from the cathode to the anode
- the photoelectric current is also proportional to the intensity of the light incident on the cathode, the light intensity is proportional to the number of photons per second incident on the cathode
- the intensity of the incident light does not affect the maximum kinetic energy of a photoelectron.
- the maximum kinetic energy of the photoelectrons emitted for a given frequency of light can be measured using a photocell
3.3 - Collisions of electrons with atoms
Ionisation
- An ion is a charged atom
- An ion is an atom where the number of electrons is not equal to the number of protons
- If it has more electrons than protons it is a negative ion, otherwise it is a positive ion
- The process of creating ions is known as ionisation
- Alpha, beta and gamma radiation create ions where they pass through substances and collide with the atoms of the surface
- Electrons passing through a fluorescent tube create ions when they collide with the atoms of the gas or vapour in the tube
The electron Volt
- Unit of energy equal to the work done when an electron is moved through a PD of 1 volt
- 1eV (electron volt) equals 1.6e-19J
Excitation by collision
- Using gas-filled tubes with a metal grid between the filament and the anode, you can see that gas atoms absorb energy from colliding energy without being ionised, this process is known as excitation.
- This only happens at certain energies, which differ according to which gas the atoms are in
- If a colliding electron looses all its kinetic energy when it causes excitation, the current due to the flow of electrons through the gas is reduced
- If the colliding electron does not have enough kinetic energy to cause excitation, it is deflected by the atom, with no overall loss of kinetic energy
- The energy values at which an atom absorbs energy are known as its excitation energies
- The excitation energies of the atoms can be determined by increasing the potential difference between the filament and the anode and measuring the pd when the anode current falls
- When excitation occurs, the colliding electrons make an electron inside the atom move from the inner shell from the outer shell
- As the atomic energy moves away from the nucleus of the atom. The excitation energy is always less than the ionisation energy, because the atomic electron is not removed completely from the atom when excitation occurs
3.4 - Energy Levels in atoms
Electrons in atoms
- The electron in an atom is trapped by the electrostatic force of attraction of the nucleus
- Because of this there are only certain areas where the electrons can be, which are the orbits, or shells surrounding the nucleus
- Each Shell or Nucleus has a different energy level, which can be seen in the energy level diagram
- Each shell can only hold a certain amount of electrons
- Each type of atom has several electrons
- The lowest energy state of an atom is called its ground state
- If an atom in the ground state absorbs energy, one of its electrons moves to a shell as higher energy, so the atom is now in an excited state
De-excitation
- When an electron moves upwards into an outer shell, the electron formation becomes unstable
- This is because when it moves upwards it leaves a vacancy in the shell it moves from, which needs to be filled to stabilise the atom
- For an electron to move down to a level it needs to emit a photon, lowering its energy level and allowing it to move down into lower energy.
- The energy of the photon is equal to the energy lost by the electron and therefore the energy lost by the atom.
Excitation using photons
- An electron in an atom can absorb a photon and move to an outer shell where a vacancy exists, but only if the energy of the photon is exactly equal to the gain in the electron’s energy
- In other words, the photon energy must be exactly equal to the difference between the atom’s final and initial energy levels.
- If the photon’s energy is smaller or larger than the difference between the two energy levels, it will not be absorbed.
Fluorescence
- An atom in an excited state can de-excite directly or indirectly to the ground state
- It can do this by absorbing photons of certain energies and then emitting photons of the same or lesser energies
- This overall process explains why certain substances fluoresce or glow with visible light when they absorb ultraviolet radiation
- When a substance absorbs ultraviolet radiation and excites, they then have to emit light photons from the ultraviolet radiation to de-excite
3.5 Energy levels and spectra
A colourful spectrum
• The wavelength of light photons that produce a continuous spectrum in between
a little less than 400nm (produces a deep violet) and 650nm (produces a deep red)
• If a tube of glowing gas is used instead of something like white light, a spectrum of discrete lines is formed instead, this is called a line spectrum
• The wavelength of the lines of a line spectrum of an element is a characteristic of the atoms of that element
• By measuring the wavelength of a line spectrum, theoretically we can deduce the element that produced the light
• This is because no other element produces the same pattern of light wavelengths and because the energy levels of each type of atom are unique to that atom, leading to the photons emitted also being a characteristic of the atom.
3.6 Wave-particle duality
The dual nature of light
- light can behave like waves as well as particles when certain events occur.
- Light can behave like a wave when diffraction takes place, this occurs when it is passing through a narrow slit and will diffract around it to spread past the gap.
- Light can also behave like a particle during the photoelectric effect.
