4.5 Quantum physics

What are photons?

discrete energy quanta of electromagnetic radiation

What is an electronvolt?

energy transferred when an electron travels through a potential difference of 1 volt - another way of measuring energy other than joules

How can you use LEDs to estimate the value of the Planck constant?

LEDs only emit light when the potential difference across them exceeds threshold p.d. required, so set up a potential divider to vary voltage through LED, place a small black tube over LED to make it obvious when it is lit up, we can determine threshold p.d., and we know wavelength of light of the specific colour of the LED, so we can use eV=hc/λ, using energy of an individual photon produced to determine Planck constant

use a variety of colours/wavelengths, draw a graph of V against 1/λ, gradient is hc/e, and as c and e are known, we can calculate h

What is the photoelectric effect?

when electromagnetic radiation is shone on to a metal and electrons are released from the surface, photons release electrons with a one-to-one interaction

How can you demonstrate the photoelectric effect?

use a gold leaf electroscope blah blah blah

What is Einstein’s photoelectric equation?

hf=Φ+KE, energy of incident photon is equal to the work function of a metal plus the maximum kinetic energy of the released electron, as energy must be conserved

What is hf in Einstein’s photoelectric equation?

energy of incident photon

What is the work function of a metal?

minimum energy to free an electron from the surface of the metal

What is threshold frequency?

minimum frequency required to free electrons from the surface of the metal

What is KE_max?

maximum kinetic energy of the released electron

What happens if you increase the intensity of radiation below threshold frequency?

nothing, no electron emission

What happens if you increase the intensity of radiation above threshold frequency?

it will increase the rate of electron emission, they are directly proportional

How can you demonstrate electron diffraction?

when a beam of electrons is fired at a thin piece of polycrystalline graphite, the electrons are diffravted by the gaps between atoms and produce a diffraction pattern when they hit a screen

What is the de Broglie equation?

an equation to work out the wavelength associated with a particle with mass, h/mv (wavelength is inversely proportional to particles momentum)

What is the evidence for wave-particle duality?

electrons:

particle - has mass and charge, can be accelerated and deflected by magnetic and electric fields

wave - they can diffract, e.g. fired at graphite they produce a diffraction pattern