A-Level Physics - Quantum Physics (13)
What piece of apparatus is used to diffract electrons?
An electron gun with a thin piece of polycrystalline graphite after the aperture (anode), with a screen at the end
Describe how you can demonstrate the photoelectric effect with a gold leaf electroscope
Briefly touch the top plate with a negative electrode to give it a negative charge. The gold leaf will repel from the stem running through the centre as the negative charge will spread to the gold leaf too and cause an electrostatic repulsion. Place a piece of zinc onto the surface
What is the largest object demonstrated to have a wave nature?
Buckminsterfullerene
What fundamental principle did Einstein use in his photoelectric effect equation
Conservation of energy
State the equations for the energy of a photon
E = hf E = hc/λ
What did Einstein propose on the photoelectric effect in terms of the particulate model of electromagnetic radiation?
Each photon could transfer its exact energy to one surface electron in a one-to-one interaction. Each electron must require a certain amount of energy in order to escape from the metal
What were gold leaf electroscopes originally used as?
Early voltmeters
What are the SI units of the work function?
Electronvolts (eV)
What two things is the energy of a photon used for in the photoelectric effect?
Freeing an electron from the surface of a metal, giving the electron a kinetic energy
What and when was the first discovery of quantum physics?
In 1900 Max Plank discovered that electromagnetic radiation had a particulate nature such that it was a continuous flow of packets of energy rather than continuous energy
State the relationship between incident intensity and emission rate in the photoelectric effect
Incident intensity is directly proportional to emission rate
Who is responsbile for the discovery of wave-particle duality?
Louis de Broglie
What are the waves associated with wave-particle duality referred to as?
Matter waves or de Broglie waves
What graph is conventionally plotted in experiments of the photoelectric effect?
Maximum kinetic energy against frequency
What did Max Planck call the 'packets of energy' he was responsible for discovering in 1900?
Quanta of energy
Define photoelectric effect
The emission of electrons due to electromagnetic radiation of a particular frequency being incident on the surface of a metal
Define the unit electronvolt (eV)
The energy transferred to or from an electron when it moves through a potential difference of 1V
Define work function and state the symbol for this
The minimum energy required to free an electron from the surface of the metal, Φ (italic)
What can be said about the observations made from the photoelectric effect?
They cannot be explained using the wave model of electromagnetic radiation
When is the photon model of electromagnetic radiation used?
To explain how electromagnetic radiation interacts with matter
When is the wave model of electromagnetic radiation used?
To explain how electromagnetic radiation propagates through space and how different sources interact
State the quantum mechanical equation related to the threshold p.d. of a light-emitting diode
eV = hc/λ
State the equation for the threshold frequency in the photoelectric effect
f₀ = Φ/h
State the value and units of the Planck constant
h = 6.63 x 10⁻³⁴ Js
State Einstein's photelectric effect equation
hf = Φ + KEmax
What is the distance of the gaps between atoms in polycrystalline graphite?
~10⁻¹⁰ m
State the de Broglie equation
λ = h/p
What is used to demonstrate the photoelectric effect?
A gold-leaf electroscope
Define wave-particle duality
A model used to describe how all matter with mass has both wave and particle properties
State an application of the photoelectric effect
Night vision goggles
What are the emitted electrons in the photoelectric effect called?
Photoelectrons
What were the three observations of the experiments concerning the photoelectric effect
Photoelectrons were emitted only if the incident frequency was above the threshold frequency for the metal. This was not effected by the intensity of the source. Emission of photoelectrons is instantaneous provided the previous condition is present The intensity of the incident radiation only increases the rate of emission and not the maximum kinetic energy of the photoelectrons
What did Einstein call the 'packets of energy' Max Plank was responsible for discovering in 1900?
Photons
Describe how you can experimentally determine the value of the planck constant using different light-emitting diodes
Plot a graph of V against 1/λ ; the gradient will be equal to hc/e
What is used to diffract electrons?
Polycrystalline graphite
Why is the 'maximum kinetic energy' used in Einstein's photelectric effect equation?
Some electrons in the surface of the metal are closer to the positive metallic ions than others. Their relative positions affect how much energy is required to free them. Therefore, most photoelectrons have less than the maximum kinetic energy
State an inconvenience caused by the photoelectric effect
Surface charging - The surface of spacecrafts can become positively charged due to the emission of photoelectrons by the photoelectric effect due to high-energy electromagnetic radiation present in space This can lead to the damage of the flow of charge through key electronic components in the spacecraft