4.5 Quantum Physics Flashcards
how does electromagnetic radiation travel through space
as a continuous wave
what happens when electromagnetic radiation interacts with matter
it interacts as discrete energy quanta (packets) called photons
what is the relationship between energy of a photon and the frequency of electromagnetic radation
energy of a photon is directly proportional to the frequency of the electromagnetic radiation
whats the equation for energy of electromagnetic radiation
E=hf or E=hc/λ where
E=energy of a photon [eV]
f=frequency of EM radiation [Hz]
h= Planck constant
c= speed of light [ms^-1]
λ=wavelength [m]
what is the value of the Planck constant
6.63x10^-34
what is an electronvolt defined as
the energy transferred when an electron travels through a potential difference of 1 volt
how can an electronvolt be substituted into a joule
using W=VQ
-> eV x e-= J so 1eV= 1.6x10^-19J
what is the photoelectric effect
when electromagnetic radiation is shone on to a metal, electron are released from the surface of the metal
what is the photoelectric equation
hf=ϕ + KE max
h= Plancks constant
f= frequency of electromagnetic radiation
ϕ= the work function of the metal
KE max= the maximum kinetic energy of the released electron
why is the maximum kinetic energy used in the photoelectric effect
because some electrons may be closer to the nucleus, requiring more energy than the work function amount to be released, leaving less energy left over as kinetic energy
what is the threshold frequency
the minimum frequency of incident radiation for electrons to be released
how does increasing the intensity of the incident radiation effect the rate of electron emission
the rate of electron emission is increased
why does the rate of electron emission increase when the intensity of radiation is increased
the increase in intensity increases the number of photons available to interact with the electrons
how can the kinetic energy of electrons released be increased
increase the frequency of radiation further above the threshold frequency, so there is more energy left over to be converted to kinetic energy
how can LEDs be used to determine the Planck constant experimentally
-LEDs only emit light when the pd across them exceeds the threshold pd required
-potential divider set up to vary the voltage
-place small black tube to make it more obvious when the LED has lit up
-vary the pd to determine the threshold pd
-as the LED produces light of a specific colour we know the wavelength of the light
-use eV=hc/λ to determine the Planck constant
how can the accuracy of the LED experiment to determine the Planck constant be increased
-use a variety of different coloured LEDs which each emit different wavelengths of light
-values of wavelengths and threshold pd for each can be recorded and a graph of V against 1/λ can be drawn
-the gradient is equal to hc/e -> as the speed of light and electron charge are known constants, h can be calculated
what can be used to demonstrate the photoelectric effect
a gold leaf electroscope
what is a gold leaf electroscope
a zinc plate o top of a negatively charged stem with a negatively charged piece of gold leaf attached to the stem
how does the gold leaf electroscope show the photoelectric effect
-initially the gold leaf and the stem have the same charge so they repel eachother
-if UV light is shone on to the zinc plate free electrons will be released from the surface of the plate
-the negative charge will slowly be lost
-the gold leaf will gradually fall back to the stem
what is the work function ϕ of a metal
the minimum energy required to free an electron from the surface of the metal
why is there a threshold frequency
each photon must have energy at least as great as the world function to release an electron and a photons energy is directly proportional to its frequency
what is wave-particle duality
diffraction and superposition of light relies on the radiation acting as a wave but the photoelectric effect relies on it acting as discrete photons - all matter can exhibit both wave and particle properties
what is the de Broglie equation
λ=h/p or λ=h/mv
λ=wavelength
h=plancks constant
p=momentum
m=mass
v=velocity
what did de Broglie realise
all matte can exhibit both wave and particle properties and that the wavelength associated with a particle is inversely proportional to its momentum p
how can electrons be used as evidence for wave-particle duality
-they can be accelerated and deflected by magnetic and electric fields which is behaviour associated with particles
-they can also be diffracted- when a beam of electrons is fired at a thin piece pf polycrystalline graphite the electrons are diffracted by the gaps between the atoms and produce a diffraction pattern when they hit a screen which is a wave property
