3. Quantum Physics - short questions

Question 1

Describe the Thomson model.

Answer 1

Thomson model describes the atom as a sphericaly, positivly charged volume embedded with electrons.

Question 2

Describe the Planetary model. Who were the authors? What was the problem of this model?

Answer 2

The authors were Nagaoka and Rutherford.
There is a small positively charged volume called nucleus and the electrons are around it in a large volume.
Especially in the Nagaoka model, the frequency of the electron should vary continously leading to the collaps of the elctrons. That’s why this model could not explain the spectral emissions.

Question 3

Describe the 4 points of the Bohr’s model.

Answer 3

• Electrons move in circular orbits and only some of them are stable
• The allowed orbits are those for which the angular momentum of the orbital of the electron is quantized. Thus the for the stable orbits holds: mvr=nh_bar
• The centrifugally accelerated electrons cannot continously emit radiations since the enegy is quantized.
  An atom emits radiations when an electron performs a transition from a higer-energy state to a lower-energy state.
  The frequency of the emitted photon is related to the energy of the levels of the transition and it is not equal to the energy of the electron’s orbital motion
• The frequency v of the emitted radiation comes from the conservation of energy E_f - E_i = hv

h_bar = reduced Plank’s constant or Dirac’s constant AND v = frequency

Question 4

Bohr combined 3 theories to create his model. State them.

Answer 4

Plank’s quantum theory, photoelectric effect explained by Einstein in 1905, Rutherford planetary model.

Question 5

The formula mvr=nh_bar describes one of the 4 points of the Borh’s model. State it.

h_bar = reduced Plank’s constant or Dirac’s constant

Answer 5

The allowed orbits are those for which the angular momentum of the orbits of the electron is quatized.

Question 6

State the Schrodinger equation.

Question 7

In the Schrodinger equation, in which case the solution of the eq. depends only on one variable? Which variable is?

Answer 7

If there is no external magnetic field, the Schrodinger equation depends only on the quantum number n and we get the new formulation of energy:
~~~
E = (-13.6Z^2)/n^2
~~~

Question 8

The atomic models must study..

Answer 8

The discrete emission of light.

Question 9

The Stern-Gerlach experiments proved.. (2 results)

Answer 9

• the space-quantisation assosciated with the orbital angular momentum of the electron
• the anomalous Zeeman effect

Question 10

Zeeman measured..

Answer 10

The splitting spectral lines by a strong magnetic field

Question 11

Which is the difference between the normal and the anomalous Zeeman effect?

Answer 11

In the normal Zeeman effect the spectral lines appears with 2 other lines.
In the anomalous Zeeman effect, the spectral lines appears with several other lines

Question 12

The Bohr-Sommerfeld model predicted an odd number of ..(1).. because ..(2)..

Answer 12

1) components in the deflected beam
2) there should be 2l+1 orientations and l is a natural number

Question 13

What did the Bohr-Sommerfeld model predicted and in which occasion it has been found a contraddiction of it?

Answer 13

The Bohr-Sommerfeld model predicted an odd number of odd components in the deflected beam since there should be 2l+1 orientations and l is a natural number, but this is not always true because in the case of the Stern-Gerlach experiment.
An example of contraddiction is the anomalous Zeeman effect.

Question 14

Describe the Ster-Gerlach experiment and its result.

Answer 14

A beam of silver particles is sent towards a non-uniform magnetic field and the deflected beam is split into 2 discrete components.
If the magnetic field B is in the z-direction, the magnetic force will be proportional to the magnetic moment mu_z.

Question 15

What do we expect as a result of the Stern-Gerlach experiment from a classical point of view? What did the experimental result showes?

Answer 15

• From classical pov: since mu can have any orientation, we expect the deflected beam to be spread out continously.
• From experimental results and according to quantum mechanics, the deflected beam is splitted in a number of discrete components, so space-quantisation is proved.

Question 16

What did the Stern-Gerlach experiment showed about a beam of elementary particles? What can we deduce from this result about elementary particles?

Answer 16

That the deflected beam was spit into 2 directions: up and down. This means that the particles has an intrinsic magnetic moment and an intrinsic angular moment that was later called spin.
Later, a fourth quantum number was added to the already existing once (n,m,l) and it is m_s that can have value +0.5 or -0.5

Question 17

State the formula for the computation of the modulus of the spin.

Answer 17

sqrt(s) * (s+1) * h_bar

h_bar = reduced Plank’s constant or Dirac’s constant

Question 18

In the photoelectric effect, how are the emitted electrons called?

Answer 18

Photoelectrons

Question 19

Esplain the classical pov, the exerimental results and the explanation given by Einstein about the DEPENDENCE OF THE EJECTION OF THE ELECTRONS ON LIGHT FREQUENCY.

Question 20

Esplain the classical pov, the exerimental results and the explanation given by Einstein about the INDEPENDENCE OF THE PHOTOELECTRON KINETIC ENERGY ON THE LIGHT INTENSITY.

Question 21

Esplain the classical pov, the exerimental results and the explanation given by Einstein about the DEPENDENCE OF THE PHOTOELECTRON KINETIC ENERGY ON THE LIGHT FREQUENCY.

Question 22

Esplain the classical pov, the exerimental results and the explanation given by Einstein about the FACT THAT IT DOES NOT EXIST A TIME INTERVAL BETWEEN THE INCIDENCE OF LIGHT AND THE EJECTION OF THE PHOTOELECTRONS.

Question 23

State the 4 points of the photoelectric effect.