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Flashcards in 4.5 Quantum Deck (26)
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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?

Refraction and reflection
slow down in more dense
predict diffraction


What did Young's double slit prove about light?

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