4.5 - Quantum Physics

Question 1

What is the photon model?

Answer 1

Proposes that electromagnetic radiation has a particulate nature

States that light exists as tiny packets of energy, rather than a continuous wave

Can be used to explain the interactions between light and matter

Question 3

What is a photon?

Answer 3

• a quantum (discrete packet) of electromagnetic radiation energy

Question 4

What is the equation for the energy of a photon?

Answer 4

E = hf

E - energy of a photon
h - Planck constant
f - frequency of the em radiation

OR

E = hc/λ

E = energy
c = speed of light
λ = wavelength

Question 5

What is the electron volt?

Answer 5

• symbol eV
• the energy change of an electron when it moves through a potential difference of 1V
• 1eV = 1.6x10^-19 J

This is derived from W=QV

Question 6

How do you convert between joules and eV?

Answer 6

JOULE TO ELECTRON VOLT:
÷ 1.6x10^-19

ELECTRON VOLT TO JOULE:
x 1.6x10^-19

Question 7

How can you use a single LED to estimate the value of h?

Answer 7

• LEDs convert electrical energy into light energy
• they will emit photons (visible light) when the pd is above the threshold pd

• at the threshold frequency, the energy transferred by the electron is approximately equal to the energy of the single photon emitted
W = hf
eV = hf

• this can be expressed as eV = hc/λ

Question 8

How can you experimentally determine the value of h?

Answer 8

Kesfuh

Question 9

What is the photoelectric effect?

Answer 9

• The emission of photoelectrons from a metal surface when electromagnetic radiation is incident on the metal
• provides evidence for the particulate nature of em radiation

Question 10

What are photoelectrons?

Answer 10

Electrons emitted from the surface of a metal by the photoelectric effect

Question 11

What is the value of Planck’s constant?

Answer 11

6.63 x 10^-34 Js

Question 12

How can the photoelectric effect be demonstrated?

Answer 12

A gold-leaf electroscope

• briefly touching the top plate with a negative electrode will deposit excess electrons onto the plate, charging the electroscope
• the charge spreads across the electroscope, including the stem and the gold leaf
• because the leaf and the stem have the same charge, they repel and the leaf lifts away from the stem
• if a zinc plate is placed on top of a negativly charged electroscope and UV light is shone on the surface, the leaf will fall back down
• this is because the UV light causes photoelectrons to be emitted from the electrode, meaning it loses its negative charge

Question 13

What are the key observations of the photoelectric effect?

Answer 13

• light below a threshold frequency will not cause electrons to be emitted
• increasing the frequency of light increases the energy of photoelectrons, but won’t increase number of photoelectrons emitted
• increasing the intensity of light won’t increase the energy of photoelectrons, but will increase number of photoelectrons emitted (maximum KE is independent of intensity)
• if the frequency of light is above the threshold frequency, emission of photoelectrons is instantaneous

Question 14

Why doesn’t the wave theory explain the photoelectric effect?

Answer 14

According to the wave theory:

• at a high enough intensity, any frequency of light should be able to release electrons from the surface
• increasing intensity should result in higher KE of the emitted electrons

Question 15

What is threshold frequency?

Answer 15

• the minimum frequency of electromagnetic radiation that will cause the emission of an electron from the surface of a particular metal
• symbol f0
• measured in Hz

Question 16

How to electrons and photons interact?

Answer 16

Einstein proposed the idea the there is a one-to one interaction between a photon and an electron, meaning an electron can only absorb one photon.

Question 18

How does the photon model explain the concept of threshold frequency?

Answer 18

• electrons can only absorb 1 photon
• E = hf, meaning if a photon doesn’t have a high enough frequency (ie if it is below the threshold frequency), it won’t have enough energy for the electron to escape from the surface

Question 19

How does the photon model explain the relationship between frequency of a light source and the electrons emitted?

Answer 19

Increasing the frequency increases the energy of photons

Any energy not used by electrons to escape the surface becomes KE of the electron

Therefore, an increase in frequency leads to increased KE of the electrons (for the same metal)

Question 20

How does the photon model explain the relationship between intensity of a light source and the electrons emitted?

Answer 20

If the intensity of light increases, more photons fall on the surface of the metal so more electrons can be liberated at once.

rate of emission of photoelectrons ∝ intensity of incident radiation

Question 21

What is Einstein’s photoelectric effect equation?

Answer 21

hf = φ + KE (max)

• hf - energy of incident photon
• φ - work function of the surface
• KE (max) - maximum kinetic energy of freed electron

Question 22

What is work function?

Answer 22

• the minimum energy requires to release a single electron from the surface of a particular metal
• symbol φ
• measured in joules

Question 23

How does the photon model explain the instantaneous release of electrons?

Answer 23

As long as the incident radiation has a frequency ≥ the threshold frequency, as soon as photons hit the surfaces of the metal, photoelectrons are emitted

Electrons cannot accumulate energy from multiple photons

Question 24

Which principle helped Einstein develop his equation?

Answer 24

Principle of conservation of energy

He realised that the energy of each photon must be conserved. This energy does 2 things:
• frees a single electron from the surface in a
one-to one interaction
• any remainder is transferred into the kinetic
energy of the photo electron

Question 25

Why is the kinetic energy calculated in Einsteins photoelectric equation only the maximum value?

Answer 25

• some KE is lost through collisions between particles as they are deep below the surface
• some electrons are ‘tightly held’ and require more energy to break free

Question 26

What happens if a photon strikes the surface of a metal at the threshold frequency?

Answer 26

The photon will only have enough energy to free a surface electron, with none left over to be transferred into kinetic energy

In this case, Einsteins photoelectric equation becomes:

hf0 = φ

Question 27

What is photoelectric current?

Answer 27

EM radiation releases electrons from a metal cathode (negative)

These electrons are attracted to the anode (positive) and complete a circuit, allowing a current to flow

Question 28

Describe a stopping potential circuit.

Answer 28

A circuit is set up with an anode and a cathode, with a varying pd across them.

Light is shone on the anode (+ve).

At low potential differences, high energy electrons would still be able to cross the tube to the negative anode, causing a current

The p.d. is increased until even the most energetic electrons are unable to cross.

Question 29

What is the stopping potential?

Answer 29

The minimum pd which stops every the most energetic electrons leaving the anode.

At this point, work done on the electron by the p.d. is equal to the kinetic energy the electron is given by the photon

Since W=eV,

eV = 1/2mv²

Question 30

What is wave-particle duality?

Answer 30

The theory that states that matter has both Particle and wave like properties, and that EM radiation has both wave and particulate nature.

Question 31

How can electrons be thought of both a wave and a particle?

Answer 31

• Electron diffraction
• electrons have mass and charge, meaning they can be accelerated and deflected by magnetic and electric fields, behaviour only associated with particles
• However, under certain conditions they can be diffracted, and they can even form diffraction patterns like light, meaning electrons sometimes behave like waves

Question 32

Describe the process of electron diffraction

Answer 32

• an electron gun fires electrons at a thin slice of polycrystalline graphite, which has carbon atoms arranged in many different layers
• the electrons pass between the individual carbon atoms
• the gaps between atoms are so small (approx 10^-10m) that they are similar to the wavelength of the electrons
• this means electrons diffract as waves and form a diffraction pattern at the end of the tube

Question 33

How does electron diffraction provide evidence for the wave-particle duality theory?

Answer 33

• electrons behave as particles when they are accelerated through the high potential difference
• they behave as waves when they are diffracted
• they behave as particles again when they hit the screen with discrete impacts

Question 34

What are de Broglie waves?

Answer 34

All particles move through space as waves

Anything with mass that is moving has wave like properties

These waves are known as matter waves or de Broglie waves

Question 35

What is the de Broglie equation?

Answer 35

λ = h/p

• λ - wavelength in m
• h = Planck’s constant
• p = momentum of particle in kgms^-1

NOTE: this equation can only be used for particles, not waves

Question 36

Why is it hard to observe the wave-like properties of all particles?

Answer 36

As particles become larger, their wave like properties become harder to observe

Eg the mass of a proton is greater than electrons, so when they are travelling at the same speed, a protons momentum is much greater than the momentum of an electron

This means the wavelength of the proton would be much smaller, making it harder to observe.