Quantum Physics Flashcards
Define energy of a photon
The energy, E, of a photon is directly proportional to the frequency, f, of the electromagnetic radiation
What is the equation for energy of a photon?
E = hf
since v = fλ, this can also be written as
E = hc/λ
Define the electronvolt
1 electronvolt is defined as the energy transferred when an electron travels through a potential difference of 1 volt
What is the value in joules of an electronvolt?
1.6x10^-19J
State the photoelectric effect
When electromagnetic radiation of high enough frequency is shone onto a metal, electrons are released (emitted) from the surface of the metal
Define the work function (Φ) of a metal
The work function is the minimum energy required to remove an electron from the surface of the metal
Define threshold frequency of a metal
The threshold frequency is the minimum frequency of electromagnetic radiation required to free electrons from the surface of the metal
What is Einstein’s photoelectric equation?
hf = Φ + KEmax
where Φ is the work function of the metal, and KEmax is the maximum kinetic energy of the released electron
or hf = Φ + 1/2mv^2max
Explain what effect the intensity of radiation has on the rate of electron emission, with regard to the threshold frequency
If the incident radiation has frequency below the threshold frequency, then no electrons will be released, regardless of the intensity. However, if the frequency is above the threshold frequency, then increasing the intensity of radiation will increase the rate of electron emission
Explain how we know electromagnetic waves show both wave and particle behaviour
Diffraction and superposition of light relies on the radiation acting as a wave, but the photoelectric effect relies on it acting as discrete photons.
What is the de Broglie equation?
λ = h/p
since p = mv, we can also write this as
λ = h/mv
Explain how electrons show wave-particle duality
Electrons are classified as particles, with mass and charge. They can be accelerated and deflected by magnetic and electric fields, particle behaviour. However, they can also be made to diffract, which is a property of waves
State the characteristics of the particle model (photon model) of light
- light and all forms of electromagnetic radiation, is emitted in brief bursts of energy. It is quantised
- the bursts of energy, photons, travel in one direction and only in straight lines
- when an atom emits a photon, it’s energy changes by an amount equal to the photon energy
- the amount of energy, E, contained in each quantum = hf
State the value of Planck’s constant (h)
6.63x10^-34
Define the photon
A photon is a quantum of energy of electromagnetic radiation
Describe a demonstration of the photoelectric effect
- zinc connected to a negatively charged electroscope
- gold leaf with a negative charge attached to the electroscope is repelled
- when UV light is shone onto the zinc, photoelectrons are emitted and the electroscope gradually discharges
- the gold leaf is no longer repelled and falls
Explain how an experiment using LEDs can be used to estimate Planck’s constant
- LEDs require a threshold voltage to conduct and emit light
- assuming that when an LED conducts, all of the electrical energy lost by an electron becomes the energy of a single emitted photon
eV = hc/λ, where eV is the electron energy and hc/λ is the photon energy - we know the value of e, we can read off the value of V, we know the value of λ (depending on LED colour), and we know the value of c
Equation for threshold frequency
f₀ = Φ/h
How can Planck’s constant (h) be worked out graphically?
The values of wavelength and threshold p.d. for the wavelength of light can be recorded, and a graph drawn of V against 1/λ. The gradient of this graph will be equal to hc/e. As the speed of light and the electron charge are known constants, we can calculate the value of h from this.
