Quantization

Question 1

What were Lenard’s Observations?

Answer 1

1. The current is directly proportional to the light intensity.
2. The current appears without delay.
3. Photoelectrons are emitted only if the light frequency exceeds a threshold frequency.
4. The value of the threshold frequency depends on the type of metal from which the cathode is made.
5. If the potential difference is more than about 1V positive the current does not change as V is increased. If V is made negative them the current decreases until V = stopping potential.
6. The value of the stopping potential is the same for both weak and intense light.

Question 2

How does a voltage across the plates effect the current?

Answer 2

• If V=0 : The photoelectrons leave the cathode in all directions. Only a few reach the anode.
• If V>0 : A positive anode attracts all the photoelectrons to the anode.
• If V<0 : A negative anode repels the electons, only the fastest make it to the anode.

Question 3

What were Einstein’s postulates?

Answer 3

1. Light of frequency f consists of discrete quanta, each of energy hf
2. Light can only be emitted or absorbed in these discrete quanta.
3. When absorbed by metal, each light quantum delivers its whole energy to one electron

Question 4

What is the proof for the de Broglie Wavelength?

Answer 4

Question 5

What equation describes the diffraction pattern of electrons through a pair of slits?

Answer 5

Question 6

Show how energy is quantized when modelled as a particle in a box.

Answer 6

Question 7

What causes absorbtion spectra?

Answer 7

The quantization of electrons means that electrons can only be at certain discrete energy levels.

This means only certain energies of photons can be absorbed.

Question 8

What causes emission spectra?

Answer 8

Energy is emitted as light when an electron changes energy level, this can only happen at discrete energies.

Question 9

Why do absorption spectras have less lines than emission spectras?

Answer 9

Usually, atoms exist in the lowest energy state, so that absorption only occurs from the lowest energy ground state whereas there are many possible energy states before emission.

Question 10

Describe Bohr’s model of an atom.

Answer 10

1. Negative electrons orbit a small positive nucleus
2. Electrons can only occupy certain orbits, which can be labelled with quantum numbers, n.
3. Each orbit has its own energy, E_n.
4. Atoms jump from one state to another by absorbing or emitting a photon with an energy E = Ef-Ei
5. Atoms can also change state through inelastic collisions with particles, or collisional excitation
6. Left alone, an atom will decay to the lowest energy ground state.

Question 11

How do you find the Bohr velocities?

Answer 11

Question 12

How do you use the bohr velocity and the centripetal force to solve for the bohr radius and the bohr velocity?

Answer 12

Question 13

How do you find the Bohr energies of an electron?

Answer 13

Question 14

What are the limitations of the Bohr model?

Answer 14

Too difficult to solve when there’s more than one electron.

Question 15

Explain or disprove the statement using the classical interpretation:

“The current is directly proportional to the light intensity”

Answer 15

Increased amount of heating will increase the flow of current.

Question 16

Explain or disprove the statement using the classical interpretation:

The current appears immediately when light turns on

Answer 16

Doesn’t explain.

Thermal emission takes some time to heat up the electrons to sufficient energy.

Question 17

Explain or disprove the statement using the classical interpretation:

“Current only flows if the light frequency exceeds a threshold frequency”

Answer 17

Can’t explain.

A low frequency but high intensity light could provide enough thermal energy to the electron to escape.

Question 18

Explain or disprove the statement using the classical interpretation:

“The value of the threshold frequency depends on the type of metal from which the cathode is made”

Answer 18

Threshold frequency cannot be explained.

Question 19

Explain or disprove the statement using the classical interpretation:

If the potential difference is positivem the current does not change as V is increased. If V is made negative, the current decreases until, at V= the stopping potential the current reaches zero.

Answer 19

This would just operate as normal attracting or repelling escaped electrons.

Question 20

Explain or disprove the statement using the classical interpretation:

The value of the stopping potential is the same for both weak and intense light.

Answer 20

Can’t explain.

More intense light would increase the electrons kinetic energy thus increasing the stopping potential.

Question 21

Explain or disprove the statement using Einstein’s interpretation.

“The current is directly proportional to the light intensity”

Answer 21

A more intense light means more quanta of the same energy. These quanta eject a larger number of photoelectrons and cause a larger current.

Question 22

Explain or disprove the statement using Einstein’s interpretation.

“The current appears immediately when light turns on”

Answer 22

Each light quantum transfers its energy to just one electron, that electron immediately has enough energy to escape.

Question 23

Explain or disprove the statement using Einstein’s interpretation.

“Current only flows if the light frequency exceeds a threshold frequency.”

Answer 23

An electron that has just absorbed a quantum of light energy has energy hf.

The work function E_0 is the minimum energy required to free an electron.

f0=E0/h

Question 24

Explain or disprove the statement using Einstein’s interpretation.

“The value of the threshold frequency depends on the type of metal from which the cathode is made.”

Answer 24

All metals are different.

Question 25

Explain or disprove the statement using Einstein’s interpretation.

“The value of Vstop is the same for both weak light and intense light”

Answer 25

There is a distribution of kinetic energies but the max is:

Kmax=hf-E0

Vstop=Kmax/e

This doesn’t depend on intensity.