Module 4.5 (Quantum Physics) Flashcards
What is the history of ideas about light?
- -Isaac Newton proposed light was a stream of tiny particles
-backed up by his own laws as light was reflected and refracted and travelled slower in denser mediums
2.-Christian huygen said light was a wave which explained all above and also that light would be diffracted when passing through a small slit - Thomas Youngs two slit experiment showed that light behaved like a wave as two coherent light sources produced dark and light fringes
What did Max Planck discover
-In 1900 he discovered that radiated energy doesn’t have continuous values and is instead released in packets or quantum of fixed energy
-Einstein used this idea to show that atoms absorb and emit these individual packets
-this opened up the idea that light could behave as both waves and particles
What does the photon model state?
light radiation consists of a stream of energy packets called photons
What is a photon?
-A quanta or packet of light or other electromagnetic radiation
What is the value of planck constant
6.626 *10^-34 Js
What is the equation for the energy of a single photon with and without wavelength?
How can we determine a value for plancks constant?
- Adjust a variable resistor to determine the current as the voltage increases for each LED when they start to emit light, get around six values of V and I
2.Plot a graph of V and I and use this to determine the activation voltage- maximum voltage that current increases linearly
3.Plot a graph of activation voltage against 1/wavelength of led light and determine gradient - Multiply by charge of electron and divide by speed of light to get a value for Planck constant
What is the gradient of an activation voltage and 1/wavelength graph equal to and the equation that supports this?
hc/e
Why can we use LEDs to determine plancks constant
-The colour or wavelength of light is related to photon energy, E=hc/wavelength
What is an electronVolt? and the equation for energy gained by an electron?
-An electron volt is the amount of kinetic energy gained by an electron when accelerated through 1V
-Energy gained=charge * Voltage
the charge of an electron is 1.602*10^-19 c
so an electron volt is that but with joules
-If we increase the particle’s charge or potential difference the kinetic
energy gained will increase proportionally
What is the photoelectric effect?
When EM radiation of a particular frequency is shone at the surface of a metal electrons are emitted (photoelectrons)
How do you set up a gold leaf electroscope?
-Place metal on cap and charge negatively with polythene rod, stem and gold leaf should repel
-Do the same but charge the metal positive and the stem and lead should repel
What are the observations from the gold leaf electroscope?
-When UV light and visible light is shone on the positively charged cap nothing happens and the stem and leaf still repel
-Same for when visible light is shone on negatively charged cap
-However when UV light is shone on the negatively charged cap the leaf falls which shows the cap has lost is charge due to emission of electrons.
-The loss of electrons can’t be due to ions as the plate is in a vaccum
What is Threshold frequency?
-The lowest frequency of radiation that causes electrons to be emitted from the particular metal. This is different for different metals but usually around UV radiation.
What is the work function of a metal?
-The minimum amount of energy required to release the electron from the surface of a metal and overcome the electrostatic attraction between the electron and the positive metal ion.
What was Einstein’s equation for photoelectric effect? why does this show that light can behave like a stream of particles
-This shows the photoelectric effect cannot be explained by a wave model as the energy of the wave depends on its amplitude not frequency
What happens to the electrons of a metal when the incident radiation frequency is below the threshold frequency or is equal to it
What happens to the electrons of a metal when the incident radiation frequency is above the threshold frequency and the intensity is increased
Explain an experiment for determining the maximum kinetic energy of a photoelectron?
1.Photocell is a vaccum so no collisions between electrons and gas in the air
2. Photoelectrons emitted cross the gap and register a current on the microammeter
3. Adjust the potential difference between the plates (increased) and electrons will be attracted back to the positive plate
4. Eventually no electrons will be able to cross the P.D. this occurs at is called the stopping potential
5.Work is done to decelerate the electrons which must be equal to the kinetic energy
so KEₘₐₓ =eV₀
v₀-stopping potential
Explain an experiment with factors that affect the rate of emission of photoelectrons?
1.Measure the current and p.d while changing the intensity and frequency of the incident radiation
2. You should find that for incident frequency above the threshold the number of emitted photoelectrons is proportional to intensity
3. The kinetic energy of the photoelectrons increases with an increase in frequency of incident radiation
What is Milikans graph and the two equations that you need to derive to understand it?
Explain Louis de Brogelie’s experiment and how this supports the wave-particle duality?
1.Slit used is much smaller than in light diffraction as electrons wavelengths are much smaller
2.Electrons are accelerated through the polycrystaline graphite and interfere constructively as they diffract through the atoms of the graphite creating a circular diffraction pattern
3.This shows electrons can behave as waves as they are showing wave like characteristics
How can we determine atomic spacing?
and if all objects have a wavelength how come we don’t show wave like properties?
-Electron diffraction can be used to determine the atomic spacing
- For macroscopic objects the wavelength is believed to be so low that the effects are negligible
What was Davison and Germers equation?
