4.5 Quantum physics Flashcards
What is the particulate nature (photon model) of electromagnetic radiation?
Acts like a wave and particle
What is a single packet of EM radiation called?
Quantum
What is the equation to find the energy carried by one wave packet (Photon)?
E=hf=hc/λ
What is the equation to find the energy carried by one wave packet (Photon)?
If frequency increases what happens?
E=hf=hc/λ
Increase in frequency, wave packet carries more energy
When a photo interacts with another particle how much energy does it transfer?
All or None
What is the charge on a photon?
Neutral
What happens when you accelerate an electron between 2 electrodes?
It transfers some of its energy (eV) into kinetic energy
What is the definition of an electron volt?
The kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt
eV=1/2mv^2
Define the threshold voltage (Vo)
The voltage needed to give electrons the same energy as a photon (emitted by an LED)
Current will only pass through an LED after what?
after a minimum voltage is placed across it - the threshold voltage
Describe an experiment to measure the Planck Constant
- Connect an LED of know wavelength in the electrical circuit shown
- Start off with no current flowing through the circuit, then adjust the variable resistor until a current just begins to flow through the circuit and the LED lights up
- Record the voltage (Vo) across the LED, and the wavelength of light the LED emits.
- Repeat the experiment with a number of LEDs of different colours that emit light at different wavelengths
- Plot a graph of threshold voltages (Vo) against 1/λ
- You should get a straight line graph with a gradient of hc/e-which you use to find h
Grad=hc/e e=charge of electron, c=speed of light
What is 1 eV in joules?
e x V = 1.6x10^-19C x 1JC^-1= 1.6X10^-19 J
What is the photoelectric effect?
When you shine EM waves of high frequency onto a surface of a metal, it will instantaneously eject electrons.
Describe the Photo electric effect
- Free electrons on the surface of the metal absorb energy from light
- If an electron absorbs enough energy, the bonds holding it to the metal break and it is emitted from the surface
- The electrons emitted are called photo electrons
How do you demonstrate the photoelectric effect?
The photoelectric effect can be demonstrated using a gold leaf electroscope – a zinc plate on top
of a negatively charged stem, with a negatively charged piece of gold leaf attached to the stem.
Initially, the gold lead and the stem have the same charge, so they repel each other.
If UV light is shone on to the zinc plate, free electrons will be released from the surface of the plate, and the
negative charge will slowly be lost, so the gold leaf will gradually fall back to the stem.
What 3 conclusions were reached from the gold leaf experiment?
- For a given metal, no photo electrons are emitted if the radiation has a frequency below a certain value-Threshold frequency
- The photo electrons are emitted with a variety of kinetic energies ranging from zero to some maximum value. This value of maximum kinetic energy increases with the frequency of the radiation, and is unaffected by the intensity of the radiation.
- The number of photo electrons emitted per second is proportional to the intensity of the radiation
Why can’t the photoelectric effect be explained by wave theory?
According to wave theory:
- For a particular frequency of light, the energy carried is proportional to the intensity of the beam
- The energy carried by the light would be spread evenly over the wave front
- Each electron on the surface would gain a bit of energy from each incoming wave
- Gradually, each electron would gain enough energy to leave the metal
So high intensity should mean more energy transferred to each electron so KE increase with intensity. There’s no explanation for KE being dependent on only frequency
No explanation for threshold frequency, electrons should be emitted eventually no matter what frequency according to wave theory
According to the photon model
- When light hits the surface, the metal is bombarded by photons
- If one of these photons is absorbed by a free electron the electron will gain energy equal to hf
What is the work function (⌀)?
minimum energy required to free an electron from the surface of the metal
What’s the equation for threshold frequency?
f=⌀/h
What is hf?
The energy transferred to an electron
Is the kinetic energy of an electron effected by intensity?
No, because they only absorb one photon at a time.
What is the rate of photo electron emission proportional to?
Intensity of radiation, provided its above the threshold frequency-more photons per second means more collisions
What’s Einstein’s photoelectric equation?
hf=⌀ + KEmax
What shows light behaving as a wave?
Interference and diffraction patterns (Young’s double slit experiment)
What shows light behaving as a particle?
Photoelectric effect, beam of light as a series of particle like photons
If a photon of light is a discrete bundle of energy, then it can interact with an electron in a one to one way
All the energy in the photon given to one electron
What is the equation for DeBroglie wave particle duality theory?
λ=h/p
p=momentum
What confirms electrons wave like properties
-Electrons behave as waves/diffract
-As gaps between atoms are of similar wavelength to
electrons
-The ordered pattern of atoms/atoms act as a grating
allowing interference to produce pattern on screen
rings occur because atomic ‘crystals’ at all possible orientations to beam
If momentum is higher, how does it effect wavelength and the spread of lines?
Wavelength shorter
spread of lines smaller
Wavelength for electrons accelerated through a vacuum tube is about the same size as what?
EM waves in the x-ray part of the spectrum
What wavelength would an electron need to be diffracted by the atoms in poly-crystaline structures?
1x10^-10m
What fact is used in an electron microscope about diffraction?
Shorter wavelength gives less diffraction effect. Diffraction effects blur detail on an image. For tiny details shorter wavelength is best
Define a photon
A quantum of electromagnetic energy
Explain what energy levels are and how they can be used to explain the emission of photons from atoms. (4 marks)
energy levels explanation: electrons have discrete energies in atom
each photon produced by electron moving between levels
photon energy equal to energy difference between levels
electron loses energy/making transition in correct direction
Define threshold frequency
The minimum frequency of the EM waves / light / uv /
photon for the removal of (surface) electron(s)
