4.5 Quantum Flashcards
What is a quantum?
a small discrete unit of energy
What is a photon?
A quantum associated with electromagnetic radiation
What is the Planck constant?
h, photon energies are always emitted at multiples of this
What is an electronvolt?
the kinetic energy gained by an electron when it is accelerated through a potential difference of 1 volt
What is threshold frequency?
the lowest frequency of radiation that will resul in the emission of electrons from a particular metal surface. For most metals = UV
Define work function
the minimum energy required to release an electron from its surface, overcoming the electrostatic attraction between the electron and positive metal ions
Describe quantum model
EM radiation emitted by hot objects could no have continuous values.
Had to be emitted in packets, that have fixed amounts of energy.
What did Newton say about light?
tiny corpuscles, reflection and refraction
move quicker through more dense medium
What did Huygens say about light?
Wave
Refraction and reflection
slow down in more dense
predict diffraction
What did Young’s double slit prove about light?
Wave
Interference to produce fringes
What did de Broglie do?
Both wave and particle
How do you calculate the energy of a single photon?
E = hf or E=hc/wavelength
How do we calculate the energy gained by a proton?
charge x potential difference
What is 1 eV equal to?
1.602 x 10^-19
Convert from eV to J?
multiply by 1.602x10^19
What did Einstein say about the work function
For electrons to be released,, the frequency of the incident radiation must exceed the threshold frequency for that energy
What happens when incident radiation is above threshold frequency?
Electrons emitted with some kinetic energy
Why can the photoelectric effect not be explained using a wave model?
energy of a wave depends on its amplitude (intensity), not its frequency
Why can we not notice the wave nature of large objects?
Diffraction needs to occur
Wavelength must be equal to gap of an object that radiation passes through
For big objects at slow speeds this is too small
What condition is necessary to produce observable electron diffraction when passing through matter?
Wavelength of electrons must be comparable to order of magnitude of atomic spacing
Whey in an electron gun, is the cathode connected to the negative terminal of the supply?
Electrons should be repelled by cathodes. Would be slowed down if cathode was positive
How can you demonstrate the photoelectric effect?
Clean zinc plate gold leaf electroscope Negatively charged UV light Gold leaf collapses and charge moves away Indicates emission of electrons
Photoelectric effect in terms of quantum behaviour
Individual photons absorbed by individual electrons
Overcome work function
Instantaneous emission
Number of electrons depends on light intensity
Einsteins photoelectric equation
Explain photoelectric effect
Only about threshold frequency Einsteins equation Work function is minimum energy Number of electrons is higher at higher intensity Emission is instantaneous
Explain what the de Broglie wavelength of an electron is?
Electrons are observed to behave as waves where wavelength depends on speed/momentum
What effect does doubling intensity have on the energy of a photon?
Photon energy does not change.
Only changes when frequency changes
Greater intensity = more photons incident
