Module 4: Chapter 13 - Quantum Physics Flashcards
What is quantisation?
The availability of some quantities, such as energy and charge, only in certain discrete values
What is quantum mechanics?
The branch of physics dealing will phenomena on the very small scale, often smaller than the size of an atom
What is Einsteins photon theory of light?
Although light exhibits wave-like properties, it travels in particles called photons. Photons contain discrete “energy packets” called quanta, and that the energy of an individual quantum depends on the frequency of light
What is a photon?
A quantum of electromagnetic energy
What is the equation for photon energy?
E = hf or E = hc/λ
h = plancks constant
UNIT OF ENERGY = JOULES
What is the photoelectric effect?
The emission of photoelectrons from a metal surface when electromagnetic radiation above a threshold frequency is incident on the metal
What is the photoelectric effect equation?
hf = Φ + Ek
Photon energy = work function + maximum kinetic energy
UNIT OF ENERGY = JOULES
What are photoelectrons?
Electrons emitted fromt he surface of a metal by the photoelectric effect
What is an emission line spectrum?
A set of specific frequencies of electromagnetic radiation, visible as bright lines in spectroscopy, emitted by excited atoms as their electrons make transitions between higher and lower energy states, losing the corresponding amount of energy in the form of photons as they do so. Every element has a characteristic line spectrum.
How is an emission spectrum formed?
When excited electrons within atoms drop back down to their original energy levels, they emit a specific frequency of photon. This results in one line in an elements emission spectrum. Multiple electrons may be excited and drop back down, causing multiple lines in the emission spectrum
What is plancks constant?
6.63x10⁻³⁴ Js
What is an electronvolt (eV)?
A very small unit of energy, it is often used when dealing with quantum physics as a Joule is too large. An electronvolt is equal to the amount of energy transferred to a single electrons if it is accelerated through a potential differnce of 1V.
How do you convert from eV to J?
1 eV = 1.6x10⁻¹⁹ J
How do you convert from J to eV?
1J = 1/(1.6x10⁻¹⁹) eV
At what speed does a photon always travel?
The speed of light, 3x10⁸ ms⁻¹
How can you determine the wavelengths of light from an emission spectrum given the energies of each atomic energy level?
- E = photon energy, therefore the difference between 2 energy levels gives the energy of the photon
- hf = e
- λ = c/f
UNIT OF ENERGY = JOULES, YOU MUST CONVERT TO JOULES
Calculate the wavelength of light for the 1st electron
1.2x10⁻⁷ m
Calculate the wavelength of light for the 3rd electron
1.91x10⁻⁶ m
What is an absorption line spectrum?
A set of specific frequencies of electromagnetic radiation, visible as dark lines in an otherwise continuous spectrum on spectroscopy. They are absorbed by atoms as their electrons are excited between energy states by the absorbing the correspsonding amount of energy in the form of photons - every element has a characteristic line spectrum
How is an absorbtion spectrum formed?
When an atom absorbs a single photon of a specific energy, it can cause a single electron to jump up one or more energy levels. As a result of this, when a continuous spectrum of light is shone onto a transparent material, a few discrete frequencies will be absorbed whilst the rest are transmitted, forming an absorbtion spectrum. Only a single photon of the relevant energy can cause this, it is not possible for an electron to “store up” energy from smalle quanta.
What is a photocell and how does it work?
A photocell contains a separated cathode and anode within a vacuum, when light is shone onto the cathode of the photocell, electrons are released and attracted to the anode, causing current to flow. This is due to the photoelectric effect
Describe photoelectric emission with a gold leaf electroscope
If you briefly touch the top plate with a negative elctrode from a high-voltage power supply, the electroscope will be charged with excess electrons deposited on the palte stem, and gold leaf. This will cause the stem and leaf to repel each other as they are both engatively charged. If you then shine UV radiation onto the top plate, then the gold leaf will fall back towards the stem as the incident radiation allows the free electrons to be emitted from the metal through the photoelectric effect. Therefore the stem and leaf no longer repel each other as they are not like charged
What is the graph for stopping voltage against photon frequency?
(and derive the equation from the photoelectric equation)
hf = φ + QV
V = (hf - φ)/Q
V = (h/Q)f - φ/Q
V = (h/Q)f - hf₀/Q
Explain the obersvations seen in this diagram:
- The red laster has no effect on both a positive and negative plate as its frequency is beloq the threshold frequency and therefore no electrons will eb freed from the surface of the metal as there is insufficient energy.
- The UV light will have no effect on the positive plate as although it is above the threshold frequency, the palte is already positively charged meaning electrons have already been removed. Therefore, there are no electrons to be freed from the surface and therefore no effect
- However, the UV light will have an effect on the negative plate as it is above the threshold frequency and there are electrons available to be released
IMPORTANT QUESTION
How can you find an experimental value of plancks constant?
- Set up the circuit as shown in the diagram
- Increase the p.d until the LED first turns on (even if it is extremely dim), this is the threshold p.d
- Repeat this for many different colours of LED
- Plot a graph of threshold p.d against 1/λ
- eV = hc/λ => V = (hc/e)(1/λ), therefore the gradient = hc/e
- solve for h
