P13 Flashcards
What is a photon
A quantum of EM radiation, where EM can be seen as tiny packets of energy - quanta - of a particulate nature, one quantum is one photon using this model
thus photons are quantised (e.g by multiples of plancks constant?)
Photon energy
dependent on frequency where E = hf where h is plancks constant = 6.63x10-34
or rewritten as E = hc/λ
electronvolts
eV same units as J
eV as in 1.6 x 10-19 x W/Q where W and Q is 1 - as 1 eV is the energy transferred to/from an e- as it passes through a pd of 1V
J to eV divide by 1.6x10-19
and multiply for eV to J
finding plancks constant
use different colour leds with known wavelength and cover with tube to remove ambient light, slowly increase V until threshold to turn on, then do a graph of V against 1/λ
as eV = hc x 1/λ
gradient = hc/e
h = gradient x e / c
photoelectric effect
single photon reacts with single electron with an energy conserved reaction
if above work function and has enough energy then will excite the e- to escape metallic bonding and emit from surface
Gold leaf electroscope
touch top plate with -i’ve electrode
excess e- dropped on top, then negative charge spreads to rod and leaf so they repel each other
when you add zinc plate on top shined on by UV photoemission occurs and photoelectrons leave so charge neutralises and leaf and rod return to normal
Observations of photoelectric effect
1 - only emitted if incident radiation is above threshold frequency (thus photons energy is above work function)
2 - photoelectron emission is instantaneous as only 1 to 1 interactions so instant
3 - increasing intensity doesnt increase ke max but just rate of electron emission, energy only increases when radiation frequency increases - rate of emission proportional to intensity
photoelectric equation
Einstein realised the energy of photon must be conserved - so it only frees the e- and the rest transferred to ke
so hf = min energy to free + ke(max)
all in J or all in eV
ke max because most e- have less energy so move slower e.g slightly further into the metal
a photon at threshold frequency will have no remaining aiming energy so e- freed and that’s it
photoelectric graph
graph of ke max against f
y = 0 until x reaches threshold frequency, then photoelectrons start to be emitted and line rises with constant gradient
gradient = h
if rising line extrapolated beneath x axis then y intercept = work function
because hf = φ + ke max
so ke max = hf - φ
gradient = h, intercept = -φ
and y = ke x = f
