Chapter 13 - Quantum Flashcards
Define a quanta
A quantum is the smallest indivisible amount of a physical quantity
Define the term Quantised
A physical quantity whose magnitude is limited to a set of discrete values e.g. charge
Define a photon
A quantum of Electromagnetic radiation I.e. the smallest indivisible unit of light energy
What are energy levels
They are shells at set distances from a nucleus
Atoms can orbit in the energy levels but never in between
What happens when an electron becomes excited
It gains energy, moves up an energy level
They can then emit the energy as a photon when they drop down
What is the photon equation
What is the different form of the equation
E = hf E = energy h = 6.63x10^-34 f= frequency
I.e the energy of a photon is directly proportional to its frequency
E = hc/lambda
What is an electronvolt
How to calculate it
An electron volt is the energy transferred when one electron travels through a potential difference of 1 V
Given V = E/Q
E = VQ = 1.6x10^-19 J
Describe the method of the LED experiment
1) set up a circuit with a variable resistor and a voltmeter and LED in parallel
2) place a black tube over the LED and gradually increase R (or V) until the LED just starts to emit light, record the voltage
3) plot a graph of P.D. against 1/lambda
How to analyse the results of the LED experiment
Given W = VQ and E = hc/lambda This means VQ = hc/lambda V=(1/lambda)(hc/e)
Thus our gradient = hc/e so we can calculate h
Summarise the wave model of light
- the energy delivered by the wave depends on the intensity
- energy arrives continuously and can build up over time
Summarise the photon model of light
- light is made of many individual quanta called photons
- each photon has an energy given by E = hf
- the intensity of the light is the number of photons per second
- Each photon can be absorbed by each electron
Briefly explain the photoelectric effect experiment
- a gold leaf electroscope is negatively charged (by induction) both the stem and the leaf become negatively charged and the lead rises
- if electrons are emitted from the plate’s surface, the lead falls down
What were the findings of the photoelectric effect experiment
- threshold frequency: no matter the intensity, if the incident radiation was below the threshold frequency no photoelectrons are emitted
- time : providing the correct frequency, the emission of electrons was instantaneous
- effect of light intensity : if the frequency is correct, increasing the intensity doesn’t affect KE but does affect number of electrons emitted
- MAX KE: increasing the frequency increases KE
What is the photon explanation of the photoelectric effect
- light is a stream of photons
- each photon has a set energy
- each electron needs a certain energy to escape the material (work function)
- photons and electrons interact one-to-one
- this explains why there is no time delay, electrons cannot accumulate energy from different photons
What is Einstein’s photoelectric effect equation
And a real terms explanation
E(Kmax) = Hf - (work function)
I.e. the energy of an incident photon will
- free an electron
- any leftover energy goes to the KE of the electron
Why is it KE(max) and not just KE
Electrons at different points of the metal structure will have different work functions
How to calculate threshold frequency
Occurs where E(k) = 0
This means
HF(0) = work function
What are the important features of an Ek(max) against f graph
- the y-intercept = -(work function)
- gradient = h
- where y<0 no electrons are emitted
What is the electron gun diffraction experiment
- a thin piece of wire is heated by a very high P.D. where work done by pd = E
- electrons are emitted and the electron ‘gun’ fires at a thin piece of crystalline graphite
- the gaps between the atoms is similar to the do Broglie wavelength of the electrons
- thus they diffract and interfere and and form interference patterns on a fluorescent screen
What are some notable observations from the electron gun experiment
- when the electrons are accelerated by the pd they act as particles
- when they diffract, they act as waves
- they act as particles upon impact with the screen
What is the de Broglie equation in terms of p, hmv, and Ek
Lambda = h/p P = momentum
Lambda = h/mv
Lambda is inversely proportional to sqrt(Ek)
What is threshold frequency (definition)
The minimum frequency of light required to produce electrons from a metal
What is the effect of increasing frequency on the photoelectrons produced
They will have a greater kinetic energy
what are some key points to mention when describing the photoelectric effect
- Photoelectric effect is the removal of electrons (from metals) when exposed to EM radiation
- Surface electrons are involved
- energy is conserved
what to always mention if explaining why different wavelengths of light have different effects in the photoelectric effect
- energy of one wavelength of photon < work function
- energy of the other wavelength of photon > work function
- intensity is independent of energy
- one-to-one interaction
- e = hf
what improvement can you make to the LED practical to get a more accurate reading for when the LED first starts to emit light
- place a sensitive ammeter in series with the LED
- when it starts to register a reading you know you have reached the threshold P.D.
