Quanta and Waves Flashcards
(38 cards)
What is the y value when calculating the maximum Kinetic energy?
y=0, Ekmax=1/2 m ω^2 A^2
Define Simple Harmonic motion
The oscillating motion of an object such that the acceleration of the object is directly proportional to and in the opposite direction to the displacement from its rest position.
a ∝ -x
Define Period (T) (in seconds)
The time it takes for one wave cycle to complete.
What mode must your calculator be in when doing SHM questions?
Radians.
What 4 experimental observations cannot be explained by classical physics, but can be explained using quantum theory?
The photoelectric effect.
Black body radiation curves (ultraviolet catastrophe)
The formation of emission and absorption spectra
Electron diffraction
Explain this relationship: ΔxΔp≥h/4π
(Heinsberg’s uncertainty principle)
The precise position and momentum of a particle cannot be known simultaneously.
Explain this relationship: ΔEΔt≥h/4π
(Quantum tunnelling)
It is not possible to know the lifetime of a quantum particle and the associated energy change simultaneously.
What does ‘quantum’ mean?
The minimum amount of any physical property involved in an interaction.
What is the lifetime of a quantum particle measured as?
An uncertainty in time
Explain what it is meant by Heisenberg’s ‘uncertainty principle’.
It is not possible to measure the position and the momentum of a quantum particle with absolute certainty.
What are 7 features of Bohr’s model of an atom?
- Positively charged central nucleus.
- Electrons (negatively charged) are in discrete energy levels and do not radiate energy.
- Electrons are not permitted to exist between these quantised states.
- When an electron makes a transition from one energy level to another a specific amount of energy is lost or gained.
- Energy is absorbed or released in the form of a photon of energy.
- Each line in a spectrum is produced when an electron moves from one energy level to another.
- The angular momentum of the electrons in these orbits are quantised in units of h/2π
How was de Broglie able to explain the stability of the electrons around the atom when classical physics suggested it should emit radiation and its orbit around the nucleus decrease?
When an electron is in one of the allowed orbits it behaves as if it is a standing wave and not a particle experiencing centripetal acceleration, therefore no radiation is emitted.
The radius of the orbit must be such that it allows for a standing wave to be created, hence it is quantised.
What did Louis de Broglie suggest in 1923?
Since light had particle-like properties, then maybe nature was dualistic and particles had wave-like properties.
λ=h/p , λ=h/mv
What is it meant by ‘quantum tunnelling?’
A quantum particle can exist in a position that, according to classical physics, has insufficient energy to occupy.
What is an example of quantum tunnelling?
Alpha decay.
According to classical physics there should not be enough energy available to cause an alpha emission from a radioactive nucleus
Quantum mechanics shows that an alpha particle can borrow enough energy to tunnel out of the unstable nucleus in a very short lifetime.
Explain Electron Diffraction
Electrons are accelerated by an electric field (high voltage)
The fact that they can be accelerated by an electric force shows that they are particles (F=ma)
Electrons then pass through a carbon grid and produce an interference pattern on a phosphor screen.
This shows that electrons are behaving like waves!
Describe the double slit experiment and it’s findings.
If electrons are fired at a double slit then an interference pattern is observed! Electrons are behaving like waves.
An interference pattern is still created over time when one electron is fired at a time.
By simply observing what slit the electron actually went through, the electron went back to behaving like a particle, and two fringes were produced!
The de Broglie interpretation is that the electron goes through both slits simultaneously as its wavelength is similar in size to the slit separation. (de Broglie wavelength)
Describe what is observed when the ‘photoelectric effect’ occurs.
Electrons are ejected or freed from a metal when it absorbs energy from photons of light.
Evidence for wave-particle duality.
What is the photoelectric effect?
The photoelectric effect is when electromagnetic radiation is directed at a metal surface and ejects electrons from the surface.
What is the ‘ultraviolet catastrophe’?
Predictions of the shape of the graph of black body radiation using classical theory worked at longer wavelengths but tended to infinity (infinite energy) at shorter wavelengths (ultraviolet).
It took Planck and Einstein using the idea of “quanta” of energy to explain the observed shape of the curve of the spectrum of light radiated by hot objects. (E=nhf)
What is the evidence that classical physics cannot account for the photoelectric effect?
If light was a continuous wave motion the electrons would absorb energy continuously and the effect would occur for all frequencies. There would be a time delay till the electrons had absorbed sufficient energy with lower frequencies. To deliver enough energy to the metal the light must occur as small bursts of wave energy.
Describe what is meant by ‘cosmic rays’.
Cosmic rays are energetic subatomic particles which arrive in the Earth’s atmosphere from outer space.
What is solar wind composed of?
Charged particles in the form of plasma.
Explain why charged particles entering Earth’s magnetic field follow a helical motion.
If a charged particle crosses the magnetic field at an angle then only a component of its velocity be perpendicular to the magnetic field. The perpendicular component, (v sin) provides the centripetal force which produces the circular motion. The component of velocity (v cos) parallel to the magnetic field causes no change in the particles motion but it continues to move in that direction in a helical path.
