Quantum Physics Flashcards
What is the elementary charge?
The smallest unit of charge that a particle or an object can have. (Size 1.6x10-19C)
Define electronvolt?
The energy transferred when an electron travels through a potential difference of one volt.
Define kinetic energy?
Energy of an object due to it’s motion.
Define threshold frequency?
The minimum frequency of the electromagnetic radiation that will eject electrons from the surface of a metal.
What particle models do we use to explain macroscopic phenomena?
Electricity - flow of electrons (current)
Gases - kinetic theory (pressure etc)
Solids - crystalline materials (mechanical properties)
What wave models are there?
Sound
Waves on strings (displacement)
Light (electric and magnetic field sure
What particle models of light are there?
A Geiger counter detects the particles alpha and beta. When gamma radiation is put next to a Geiger counter it clicks, meaning it also detects gamma. Gamma is part of the EM spectrum and alpha and beta are particles, but the clicking shows that gamma is acting like a particle and interacting with the counter the same way the alpha and beta particles do.
Also photoelectric effect.
What is a photon?
A ‘packet of energy’.
The energy (joules) of an electron = ?
Planck constant x frequency
E=hf
h = 6.63x10^-34 Js
J = Js x s^-1
Energy of a photon is directly proportional to frequency.
E(J) =
hc/wavelength
How do you convert eV to J?
Multiply by 1.6x10^-19
How do you covert J to eV?
Divide by 1.6x10-19
Energy change of electron travelling through a p.d. (W) =
QV = 1.6x10-19 x voltage
Equation for an electron accelerated from rest? What other particles does this equation account for? When can we not use this equation?
eV = 0.5mv^2 note that eV is electron charge multiplied by voltage
Accounts for any type of charged particles (protons and ions etc)
Can not use this equation when the charges particle is accelerated through large voltages to speeds approaching the speed of light because particles mass increases as it approaches speed of light.
Voltage = ?
Joules/coulombs(charge)
What factors of LEDs make them ideal for the experiment estimating plancks constant?
They conduct in only the forward direction
Requires min voltage (threshold voltage) to be applied before it allows a current.
LEDs of different colours require different threshold voltages before they conduct light.
Would a red or blue light need more energy to turn on? Why?
Red emits lower energy photons, therefore requires lower threshold voltage to turn on
Energy lost by electron to LED = ?
Energy of photon
eV = hc/lambda
V is threshold voltage
How can we determine h from the LED experiment once we have found V and lambda?
A graph of V against 1/lambda can be drawn. The gradient would then equal hc/e.
Explain step 1 of an experiment to view the photoelectric effect?
1) Get a gold leaf electroscope.
2) Transfer electrons to ES where they stay because it is electrically insulated.
3) Electrons repel each other and distribute themselves evenly throughout structure.
4) Gold leaf lifts because electrons repel each other and it is very light.
Step 2?
1) Irradiate with UV.
Gold leaf won’t move because each UV photon won’t have enough energy to remove electrons from the steel.
2) Fix a clean zinc plate (no oxide layer) to top of ES.
Gold leaf falls because UV has enough energy per photon to eject an electron from the surface of the metal.
1 photon = 1 electron
What are ejected electrons called?
Photoelectrons
Why, in the photoelectric effect experiment, will zinc lose an electron but steel won’t?
Because zinc is more reactive than steel.
What is photoelectric emission?
The ejection of electrons from the surface of a metal when they are irradiated with photons of high enough energy.
