Quantum Phenomena Flashcards
What is the photoelectric effect?
The photoelectric effect is where the surface of a metal emits photoelectrons after being exposed to a light above a certain frequency
What criteria must be met for the photoelectric effect to occur
The light must meet the threshold frequency
What did the photoelectric effect prove?
- EM waves travel in photons which have energy proportional to frequency
- each electron absorbs a single photon, therefore is only emitted when frequency threshold is met
- If the intensity of light is increased, more electrons are emitted per second
What is work function?
The minimum amount of energy required for electrons to be emitted from the surface of a metal
What is stopping potential
It is the potential difference needed across the metal to stop photoelectrons with maximum kinetic energy
Ek max = e * stopping potential (V)
What is excitation
When electrons can gain energy from collisions with free electrons to move up energy levels
What is ionisation
When electrons gain energy from collision with free electrons to be able to leave the atom entirely (only occurs when the free electrons energy is greater than the ionisation energy)
What happens after excitation
An electron would return to its original energy level (the ground state) by releasing a photon
Give an example of a practical use of excitation (no need to explain)
Excitation in a fluorescent tube to produce light
Explain what is going on in a fluorescent tube?
A high voltage is applied which accelerates free electrons
- free electrons collide with mercury vapour atoms that become excited
- this releases more free electrons
- when they dexcite they release UV photons
- The fluorescent coating absorbs the UV photons and the electrons in the coating become excited and when de-excited they release photons of visible light.
How do you convert electron volts to joules
Multiply by 1.6*10^-19
On a line spectrum what does each line represent
A wavelength of light emitted from a tube (fluorescent tube emitting Visible light photon), wavelength values are discrete proving that electrons in atoms can only transition between discrete energy levels.
What experiments prove the wave-particle duality of light
Young Double experiment - proves light can diffract and interfere like a wave
Photoelectric effect - proves light can act like particles
How can we prove electrons having both wave and particle properties
Via electron diffraction (as only waves can diffract)
WIth electron diffraction - how can you make the concentric ring pattern closer and why does this happen
Increasing the momentum of electrons as their wavelength will decrease therefore their diffraction amount would decrease (using de broglie rule)
What does it mean if a metal has a greater work function
This means more energy is required to free the photoelectrons.
The gradient on a ek-f graph is planks constant
what is the mass of a photoelectron?
9.11*10^-31
what is de broglie wavelength equation
lander = h / m * v
Formulate an equation involving kinetic energy and de Broglie wavelength (do on paper)
must input p^2 in kinetic formula
How does UV photons convert into the visible part of the electromagnetic spectrum
UV photons is absorbed by the fluoresecent coating which excites their electrons
These electrons would dexcite and emit visible light photons
Explain how excited mercury atoms emit photons
Excited atoms deexcite
Emitting a photon of energy equal to the energy difference between levels
Explain how mercury atoms in a fluorescent tube are excited
Electrons through the tube from a potential difference collide with mercury atoms
This transfers energy from the collision
electrons in the mercury atom moves to higher energy levels
Describes what occurs during the photoelectric effect
Photons of light incident to the metal surface causes an emission of electrons
The electrons emitted are those near the surface
What does increasing intensity to do the current of a system
Increasing intensity results in an increase in photons incident per second
As a result current increases
Formula of stopping potential?
eV = Ek max
e = charge of electron
V = stopping potential
How do you work out the minimum p.d needed to ionise an electron
p.d = ground state energy / charge of electron
What does increasing intensity do (2 points)
More electrons released per second
More photons striking the metal surface per second
How does allowing air molecules into a system affect photoelectrons from reaching the anode
Increase collision of electrons and air molecules
fall in Ek
therefore less electrons reach the anode per second
Why would you use crystals or graphite as a material to given an observable diffraction pattern with electrons?
Their atomic spacing is similar wavelength to electrons
What is meant by ionisation energy
Minimum energy required to release an electron from an atom from its ground state
