Module 4.5 - Quantum Physics Flashcards
In 1672, what did Isaac Newton believe light to be made of? Why?
> Light was made of corpuscles
> Supported his laws of motion; and reflection and refraction
In 1901, what did Max Planck hypothesise?
> Realised radiated energy couldn’t have continuous values
> Hypothesised that the total radiated energy were emitted in PACKETS with each having a fixed amount.
What did Einstein and De Broglie find in 1905?
> Einstein showed that atoms absorb and emit light
> De Broglie showed that light behaves as a wave and a particle.
Define Quantum (Pl - Quanta)
Small DISCRETE unit of ENERGY
Define Photon
A QUANTUM associated with EM radiation
What are the two equations to find the energy of a photon?
E = hf E = hc ÷ lambda
What is E in E=hf?
E - energy - J
What is h in E=hf?
h - Planck’s constant - J/s
What is f in E=hf?
f - frequency - Hz
What is c in E=hc÷lambda?
c - wave speed - m/s
What is lambda in E=hc÷lambda?
lambda - wavelength - m
How do you work out the energy of n number of photons?
E = nhf
What is an Electron volt (eV)?
The KINETIC energy gained by ONE electron when it is accelerated through a p.d. of 1V
How do you work out the energy gained by an electron?
E = charge x p.d.
Charge of electron is 1.602x10^-19
What is 1eV equal to?
1.602x10^-19J
E=QV = 1.602x10^-19 x 1 = 1.602x10^-19J
J —> eV?
DIVIDE by 1.602x10^-19
eV—>J?
MULTIPLY by 1.602x10^-19
How much kinetic energy (in eV) does an electron of charge e have through a p.d. of 100v?
100eV
- 602x10^-19 x 100 = 1.602x10^-17J
- 602x10^-17 ÷ 1.602x10^-19 = 100eV
Derive eV = hc ÷ lambda
energy of a photon = energy of electrons that are excited in the semiconductor material of the LED.
E = hc ÷ lambda
Therefore
eV = hc ÷ lambda
What is the photoelectric effect?
When EM radiation of a particular frequency is shone on the surface of a metal, electrons are emitted. The photons from the radiation causes electrons to be released as photoelectrons.
How is the photoelectric effect demonstrated on an electroscope?
> Shining small amount of UV on zinc plate, causes electrons to be emitted from the negative zinc plate causing the angle between the gold leaf and the plate to decrease
But it doesn’t change when large amounts of VISIBLE light is shone
Define Threshold frequency
The LOWEST frequency of radiation that will result in the emission of electrons from a particular metal surface
Define Work Function
MINIMUM energy required to release an electron from the surface of a metal - energy to overcome the attraction between the electron and metal ions
What happens if the incident radiation has a frequency below the threshold frequency?
The photons DO NOT have enough energy to overcome the work function of the metal
What happens if the incident radiation has a frequency equal to the threshold frequency?
Causes the electrons to be OMITTED but with NO KE
What happens if the incident radiation has a frequency above the threshold frequency?
Causes the electrons to OMITTED with SOME KE
What happens if the incident radiation has a higher intensity whilst above the threshold frequency?
MORE electrons are omitted PER SECOND but WILL NOT affect KE
What is Einstein’s Photoelectric equation?
hf = ø + KE max
What is hf in Einstein’s Photoelectric equation?
hf is the energy of the incident photon where h is Planck’s constant
What is ø (phi) in Einstein’s Photoelectric equation?
Ø is the work function of the metal
What is KE max in Einstein’s Photoelectric equation?
KE max is the MAXIMUM kinetic energy of an electron once it has been ejected from the metal
What were Einstein’s two main points on the Photoelectric effect?
> Each single photon could only eject one single electron
> Either the photon energy was LARGER than the work function; or SMALLER so the electron would remain on the metal.
How do you determine the minimum frequency at which electrons are released?
hf = ø + KE max
At minimum frequency, only enough energy to release electron
KE max = 0
f0 = Threshold frequency
hf0 = ø
f0 = ø ÷ h
What is KE max independent of in the photoelectric effect?
Intensity - it is not affected by the amount of photons coming in
What is the rate of emission of photoelectrons (above threshold frequency) proportional to?
Intensity - the larger the number of photons coming in, the greater the rate of emission of photoelectrons.
What portrays light to be a wave?
> Refraction
> Interference
What portrays light to be a particle?
> The photoelectric effect
What indicates waves acting like a particle?
hf = E - Frequency of a wave can give certain photons a certain energy
What indicates particles acting as waves?
Electron Diffraction - an electron is a particle yet it can diffract like a wave. This is shown by firing electrons at a thin layer of graphite in a vacuum which creates a circular electron diffraction pattern
Why must particles have a wavelength?
For electrons and other particles travel through space as a wave, they must have a wavelength
How do you work out the wavelength of a particle?
lambda = h ÷ p = h ÷ mv
What are the terms of the De Broglie Equation?
lambda - wavelength h - Planck’s constant p - momentum m - relativistic mass (as v—>c , m increases) v - velocity of particle
What does the intensity of a wave at a point represent?
The probability of a particle being there
How do electrons act like waves?
The way they travel through space
How do electrons act like particles?
The way they interact, such as attraction.
