Quantum Physics Flashcards
What is a photon
Is a quantum of energy of electromagnetic radiation
How to calculate energy of one photon
E=hf=hc/λ
H=planks constant 6.63x10⁻³⁴
C=speed of light 3x10⁸
What is an electron volt eV
Is kinetic energy gained by an electron when it is accelerated through a potential difference of 1volt.
1ev=1.6x10⁻¹⁹
eV=0.5mv² V=voltage v=velocity
How to estimate planks constant
- Connect an LED of known wavelength into a circuit with an ammeter, voltmeter and a variable resistor.
- Start off with no current and slowly increases current with the variable resistor until the LED lights up.
- Record the voltage and repeat this experiment with different colour LEDs.
- Plot a graph of threshold voltages against 1/λ
- You will get a straight line with the gradient being hc/e
What is the photoelectric effect
The emission of photoelectrons from a metal surface when electromagnetic radiation above a threshold frequency is incident on the metal.
What happens in then photoelectric effect
- Free electrons on the surface of the metal absorb energy of light.
- If an electron absorbs enough energy the bonds holding into to the metal breaks and it is emitted from the surface.
What is the gold leaf electroscope
- The electroscope plate is originally negatively charged so the gold leaf is repelled.
- The zinc plate is the exposed to ultraviolet light and the photoelectric effect causes free electrons to become ejected.
- This causes the gold leaf to lose its negative charge and the leaf is no longer repelled.
What is work function φ
Minimum energy required to free an electron from the metal surface
In many question they will give you power and time to work out the total energy from photons
P=w/t
You can the divide this by the energy of a single photon to find the number of photons
How to work out maximum energy given
hf=φ + KEmax
KEmax=0.5m(v²max)
Maximum kinetic energy is independent to the intensity of incident radiation.
What is threshold frequency
Is the minimum frequency of light which can cause photoelectric emission.
Threshold frequency is directly proportional to intensity of the incident of radiation
What is a wave partial duality
That light is both a wave and a particle
What is electron diffraction
Is when they diffract electrons. Interference patterns where observed when accelerated electrons in a vacuum tube diffract between the spaces between carbon atoms in polycrystalline graphite.
This shows electrons have wave like properties.
What is the de Broglie equation
λ=h/p
P=momentum P=mv
λ=h/mv
Equation for stopping potential (v)
KE=VQ
