M4 Quantum Physics Flashcards
what is the photoelectric effect
If you shine radiation of a high enough frequency onto the surface of a metal, it will eject electrons.
The free electrons near the surface will absorb energy from the radiation.
Before an electron can leave the surface of the metal it needs enough energy to break the bonds holding it there (work function energy)
what is threshold frequency
the minimum frequency of light needed to knock electrons off the plate
what does it mean if frequency is above the threshold frequency
the amount of electrons being knocked off is proportional to the intensity of the light
in the gold leaf electroscope experiment, what happens when red light is used
nothing because the red light doesn’t have a high enough frequency
what happens if glass is placed in the path of UV light
it gets absorbed
what is a photon
one quantum of energy of electromagnetic radiation
what is the difference between intensity and frequency
intensity = the amount of photons in a given area
frequency is proportional to energy so frequency is the energy of photons
what does this equation mean:
E = hf = φ + Ek
so what does it mean if frequency increases
energy of one photon = Planck’s constant X frequency = ‘energy used to dislodge electron’ + ‘energy left over which makes electron move’
so if frequency increases, the photon is carrying more energy
what does c = fλ mean
speed of light = frequency X wavelength
describe the process by which electrons are released from the cathode
The process is the photoelectric effect.
The EM radiation hits the cathode and the energy of the photons is used the dislodge the electrons and the excess energy which transfers to kinetic energy makes the electrons move. The energy of the photons comes from the frequency of the wave which are proportional to each other.
define planck’s constant
the amount of energy a photon has at a specific frequency
define volt
define electronvolt
volt - the work done to move one coulomb of charge
electronvolt - the kinetic energy gained by an electron when it’s accelerated through a pd of 1 volt
describe the quantum well
represents the energy needed to release a photoelectron from the surface of a metal
the photon has some energy and the electron that the photon hits absorbs this energy
It takes a certain amount of energy to release the electron and this energy is called the work function
what is 1 Joule in electronvolts
1 / 1.6 X 10^-19
= 6.25 X 10^18 eV
what is the photon energy in electronvolts of red light of wavelength 685nm
685nm = 685 x 10^-9 m
f = v / λ
3 X 10^8 / 685 X 10^-9 = 4.4 X 10^14 Hz
E = hf
= 6.63 X 10^-34 X 4.38 X 10^14
=2.9 X 10^-19 J
2.9 X 10^-19 X 6.25 X 10^18
=1.8 eV
what has greater energy, red or blue light
blue light because it has a higher frequency
what is wave-particle duality
All particles have both particle and wave properties
Waves can also show particle properties
when two crests meet what is it called
when a trough and a crest meet what is it called
two crests is constructive interference
crest and a trough is destructive interference
what is meant by the de Broglie wavelength of an electron
he associated waves and particles
wavelength associated with a particle = Planck’s constant / momentum
also written as wavelength = h / p = h / mv
what can you prove from electron diffraction
particle matter behaves as a wave
when you pass a beam of electrons through a thin graphite screen, the particles diffract so rings of light is formed because constructive and destructive interference is occurring
you only get diffraction if a particle interacts with an object of about the same size as its Broglie wavelength
in electron diffraction, what happens when the voltage increases
the speed of electrons increases so the rings become closer together
what is the kinetic energy of electrons with a voltage of 2000V?
2000 X 1.6 X10^-19
=3.2 X10^-16
what two proposals did Bohr make about how light interacts with matter
- like the photoelectric effect, light interacts with matter and light consists of photons
- when light interacts with matter, it’s the electrons that absorb the energy from the incoming photons. when electrons lose energy, light is emitted by matter in the form of photons
what happens if the frequency is below the threshold frequency
a single photon can still give up its energy to a single electron but this electron can’t escape from the attractive forces of the positive metal ions
the energy absorbed from the photons appears as Ek of the electrons
these electrons lose their Ek to the metal ions when they collide with them
this warms up the metal , why it gets hot
a material emits photoelectrons when the wavelength is 380nm
calculate its work function in eV
E = hc / λ
= 6.63 X 10^-34 X 3 X 10^8 / 380 X 10^-9
answer X 1 / 1.6 X 10^-19 which gives it in electronvolts
= 3.3 eV
what happens when you accelerate an electron in a potential difference?
hence, what equation can you use?
it transfers some electrical potential energy into kinetic energy
eV = 0.5mv^2
what experiment can you do to determine Planck’s constant
use the equation h = eVλ / c
set up circuit
adjust variable resistor until LED lights up
record the voltage across the LED (this is threshold voltage)
record the wavelength of the LED
Repeat the experiment to get an average voltage
Then repeat with other LEDs with different wavelengths
plot the graph of threshold voltage (y) against 1/wavelength (x) and Planck’s constant will be e times the gradient all over the speed of light
define work function
the minimum energy required to dislodge an electron from the surface of a metal
what is the gold-leaf electroscope
what does it prove
demonstrates the photoelectric effect
a zinc plate is attached to the top of an electroscope and it has a negative charge
the gold leaf also has a negative charge meaning they repel each other so the gold leaf rises up
UV light is then shone onto the zinc plate and the energy causes electrons to be lost from the zinc plate meaning the leaf is no longer repelled as the negative charge is lost from the plate so the leaf falls back down
what 3 conclusions can be made from the photoelectric effect
1) for a given metal, no photoelectrons are emitted if the threshold frequency isn’t met
2) photoelectrons are emitted with a variety of kinetic energies and the value of max kinetic energy increases with the frequency of the radiation (but it’s independent from intensity)
3) number of photoelectrons emitted per second is directly proportional to the intensity of the radiation
what equation could you use to calculate threshold frequency
work function / planck’s constant
what equation could you use to calculate the maximum kinetic energy of a photoelectron
0.5mv(max)^2
what is the intensity of the radiation
power transferred by the radiation over a given area
in the photon model it is the number of photons that hit a given area every second
what does it mean by one-to-one interaction in terms of photons
A photon either transfers all or none of its energy when interacting with another particle
