TOPIC 19: QUANTUM PHYSICS (COMPLETE)

Question 1

[Definition + Formula] Photon

Answer 1

A photon is discrete packet of energy of electromagnetic radiation

E = hf = hc/λ
E: Energy of 1 photon
f: frequency of the EM radiation

Question 2

[Definition] Photoelectric Effect

Answer 2

Photoelectric effect is the emission of electrons when electromagnetic radiation of high-enough frequency is incident on a metal surface

Question 3

[Formula] Photoelectric Equation

Answer 3

hf = Φ + KEmax

Question 4

[Definition] Work function energy

Answer 4

It is the minimum energy needed to cause an emission of electron from the surface of a metal

Question 5

[Definition] Threshold frequency

Answer 5

It is the minimum frequency of electromagnetic radiation to cause emission of electron from the surface of a metal

Question 6

[Definition] Threshold wavelength

Answer 6

It is the maximum wavelength of electromagnetic radiation to cause emission of electron from the surface of a metal

Question 7

[Explanation] Why for any wavelength of electromagnetic radiation, most of the electrons are emitted with KE less than the max KE?

Answer 7

The photoelectrons only gain max KE when they are at the surface of the metal, losing the minimum amount of energy before emission.

For electrons that are below the surface of the metal, additional energy is required to bring the electron up to the surface before emission.

Question 8

[Definition] Stopping potential, Vs

Answer 8

Stopping potential is the minimum potential difference between the emitting metal and the collector that prevents the most energetic photoelectrons from reaching the collector plate, resulting in zero photoelectric current

Question 9

[Formula] Stopping potential, Vs

Answer 9

KEmax = eVs

Question 10

[Graph] Photoelectric current, I - V

Answer 10

Positive V: horizontal line (saturation current)

Negative V: Curve (from increasing gradient to decreasing gradient)

Question 11

Key concept of how the photoelectric current - V graph would change when the variables are changed

Answer 11

Intensity = Power / Area
(aka Energy per unit Time per unit Area)

Energy = no. of photons x Energy of 1 photon

Frequency –> Energy of each photon
Intensity –> Number of photons
Power –> Both Frequency & Intensity

So if constant power, either lots of electrons, but each of low energy OR less electrons, but each with high energy

Question 12

[Explanation] Increasing intensity on photoelectric current - V graph

Answer 12

If frequency remain constant, energy per photon constant, so max KE of photoelectrons constant, stopping potential same

Since energy per photon constant, with increasing intensity, more photons reaching emitter. So more photoelectrons emitted per second, saturation current increases

Question 13

[Explanation] Increasing frequency on photoelectric current - V graph, with same number of photons.

Answer 13

Increasing f, energy per photon increases, max KE increases, stopping potential increases

No. of photons same, so no. of photoelectrons emitted same, photoelectric current same

Question 14

[Explanation] Increasing frequency on photoelectric current - V graph, with same power output

Answer 14

Increasing f, energy per photon increases, max KE increases, stopping potential increases

Since P same, with increasing f, the number of photons decrease, so no. of photoelectrons decrease, saturation current decreases

Question 15

[Definition] Absorption line spectra

Answer 15

Continuous spectrum crossed by dark lines

Question 16

[Definition] Emission line spectra

Answer 16

Discrete bright lines of different colours on a dark background

Question 17

[Understanding] How does the absorption and emission lines differ on the energy level diagram?

Answer 17

Absorption lines will be upwards, because when the electrons absorb the photons, it causes the photons of that energy level to be ‘gone’. so it will be continuous spectra crossed by dark lines

Emission spectra will be downwards, because when the electrons de-excite back to ground state, it emits photons. so it will be different coloured lines on a dark background

Question 18

[Understanding] What causes the emission and absorption line spectra to occur in v simple terms?

Answer 18

Emission: electrons de-excite. simple

Absorption: by absorbing photon, OR through high speed collisions with other particles

Question 19

[Explanation] How is absorption spectrum formed?

Answer 19

1. Atoms in the cool hydrogen absorbs photons of specific energy levels that are equal to the difference between the discrete energy levels
2. The specific energy of the absorbed photons shows up as dark lines of specific frequencies against the white / continuous spectrum
3. As the electrons de-excite, the absorbed photons are re-emitted in all directions. So only a small negligible fraction is observed on the spectrum, so its still dark lines

Question 20

[Explanation] How is emission spectrum formed?

Answer 20

1. When the cool hydrogen atoms absorb photons, they are excited and unstable
2. Thus, they will shortly de-excite and re-emit photons in all directions.
3. These photons have specific energy values that are equals to the difference in discrete energy levels, hence showing up as coloured lines of specific frequencies on a dark background

Question 21

[Understanding] What can the x-ray production process be compared to?

Answer 21

Opposite of photoelectric effect!

Photoelectric effect –> Photons of high frequency results in emission of electrons

X-ray production –> Electrons strike metal, resulting in emission of photons when electrons lose their energy

Question 22

[Explanation] How is the continuous spectrum of the X-ray spectrum formed?

Answer 22

When electrons with very high kinetic energies bombard with the nuclei in the metal, they lose energy and emit photons in the X-ray energy range aka X-ray photons

Question 23

[Explanation] Why is there a continuous distribution of wavelengths for the x-ray continuous spectrum?

Answer 23

X-ray photons are produced when the bombarding electron loses energy when it interacts with a nucleus

Larger acceleration of electron results in larger photon energy

Since the electrons hitting the metal target have a distribution of accelerations, there will be a distribution of wavelengths.

Question 24

[Formula] Main formula for the formation of x-ray photons

Answer 24

Electrons accelerated through potential difference to gain KE. KE is then lost to produce photons

qV = 1/2mv^2 = E(photon) = hf

Question 25

[Explanation] How are characteristic peaks of the X-ray spectrum formed?

Answer 25

1. Inner shell electron is being knocked out by the bombarding electrons, leaving a vacancy
2. Electrons from the nearby shells with higher energy de-excites and fills up the vacancy, producing an x-ray photon

Question 26

[Understanding] What is the similarity / difference between emission line spectrum and x-ray characteristic peaks?

Answer 26

Both processes involves the electrons transiting from a higher energy level to a lower energy level

However, the electrons involved in the emission line spectrum are VALENCE ELECTRONS, while electrons involved in x-ray production are those CLOSEST TO NUCLEUS

because the difference in discrete energy levels closer to the nucleus are larger, so more energy released, resulting in high-energy x-ray photons produced

Question 27

[Explanation] What is the difference between Kα and Kβ of the characteristic peaks in the X-ray spectrum?

Answer 27

Inner most shell: K, L, M
Kβ: M –> K ; Kα: L –> K

Energy difference between M and K LARGER than energy difference between L and K, so X-ray photons corresponding to Kβ is of higher energy (shorter wavelength) than Kα

L shell closer to K, so higher probability of electron de-exciting to fill up vacancy in K shell, so Kα peak is higher

Question 28

[Formula] de Broglie wavelength

Answer 28

λ = h/p

Question 29

[Formula] Heisenberg Uncertainty Principle

Answer 29

∆p∆x ≥ h

Question 30

[Understanding] What is the key thing to take note of regarding the heisenberg Uncertainty Principle

Answer 30

p and x, meaning momentum and position MUST both be along the same direction