Quantum

Question 1

What is the photoelectric effect?

Answer 1

Light incident on a metal surface causes electrons to be emitted from the surface

Question 2

Why are electrons emitted from this surface by shining green and blue light on it? (not red)

Answer 2

Blue and green light are above the threshold frequency of this metal

So the photons of light have an energy > work function (φ)

Question 3

Why are no electrons emitted when red light shines on this metal?

Answer 3

The red light photons are below the threshold frequency

So the energy of each photon < work function (φ)

Question 4

Why does making the red light brighter not cause electrons to be emitted? (Photoelectric effect)

Answer 4

Electrons in the metal interact with photons in a 1-1 interaction

They only absorb photons which have an energy > work function (φ)

Question 5

1. Why do both light source cause electrons to be emitted? (from the surface)
2. What is different about the electrons emitted due to the blue light?

Answer 5

1. Both light sources have frequency above the threshold frequency (f₀) of the metal
2. The electrons emitted due to the blue light have a greater maximum kinetic energy (because blue photons have a greater energy from E=hf)

Question 6

What does threshold frequency (f₀) of a metal mean?

Answer 6

The minimum frequency of the incident light needed to cause electrons to be emitted from the surface

Question 7

1. What can you say about the green light incident on this metal?
2. What difference does the brighter lamp make?

Answer 7

1. The green light is above the threshold frequency so the photelectric effect happens
2. The brighter lamp causes more photons of light to collide with electrons so more photons are emitted per second (But the electrons have the same maximum kinetic energy)

Question 8

You are shining a light (above f₀) on a metal. How do you:

1. Increase the maximum kinetic energy of the emitted electrons?
2. Increase the number of emitted electrons per second?

Answer 8

1. Increase the frequency of the light source
2. Increase the brightness of the light source

Question 9

This is a graph for the photelectric effect. What information do the 3 features of the graph provide?

1. Y-intercept
2. X-intercept
3. Gradient

Answer 9

1. Y-intercept = - work function
2. X-intercept = threshold frequency
3. Plancks’ Constant

Question 10

This is the photoelectric effect graph for a metal

Plot a line on this graph for a metal with a higher threshold frequency

Answer 10

1. Y-intercept (φ) decreases
2. X-intercept (f₀) increases
3. But the gradient (h) is constant

Question 11

If you shine a really bright light on a metal but the light is below the threshold frequency why will electrons never be emitted?

Answer 11

Electrons interact with the photons in a 1-1 interaction

But only if the photon has an energy > work function

No red light photons have an energy > work function

So electron emission will never occur

Question 12

What is the definition of the work function (φ) of a metal?

Answer 12

The minimum energy required to liberate an electron from the surface of a metal

Question 13

How is the work function (φ) related to the threshold frequency (f₀) of a metal?

Answer 13

Question 14

When light (above f₀) is incident on a metal surface how is the maximum kinetic energy of emitted electrons calculated?

Answer 14

Difference between the energy of each photon and the work function (φ)

Question 15

For the gold leaf experiment (to show the photoelectric effect):

1. How do you make the gold leaf rise?
2. Why does the gold leaf fall?

Answer 15

1. A charged rod transfers additional electrons to the plate causing repulsion between the stem and gold leaf
2. Electrons are liberated from the metal surface (by light above f₀) so the stem and leaf become neutrally charged again

Question 16

Define the electron volt

Answer 16

The kinetic energy gained by 1 electron passing through a potential difference of 1 volt

Question 17

How do you convert between electron volts and Joules?

Answer 17

eV → J : multiply by 1.6x10^-19J

J → divide by 1.6x10^-19J

Question 18

How is the maximum kinetic energy of photoelectrons (emitted during the photoelectric effect) measured?

Answer 18

1. Connect the system to a circuit
2. Place a battery opposing the current produced by the emitted electrons
3. Measure the stopping potential when the total current = 0
4. E_kmax = eV_s

Question 19

During the Photoelectric effect why are electrons with a range of kinetic energies emitted?

Answer 19

Electrons deeper down require more energy to rise to the surface before being liberated

(Electrons at the very top of the surface are emitted with maximum kinetic energy)

Question 20

What are the 3 types of line spectra and how are they produced?

Answer 20

1. Continuous - Produced by blackbody
2. Emission - Produced by an excited gas
3. Absorption - Produced by a continuous spectrum passing through a cold gas

Question 21

What are the key ideas of the Bohr model of the atom?

Answer 21

• Electrons can only travel in allowed orbitals (energy levels)
• Electrons can emit or absorb energies to instantaneously transition between orbitals
• Electrons cannot exist between orbitals

Question 22

How could an electron excite from the n=2 → n=4 energy level?

Answer 22

It must absorb an energy = the difference between levels (By photon or electron collision)

Question 23

How could an electron de-excite from n=3 → n=1 energy level?

Answer 23

It must emit an photon of energy = the difference between levels

Question 24

How is the energy of a photon calculated?

Answer 24

Question 25

Why do different gases (made of different elements) have different emission spectra?

Answer 25

1. Each element has a different set of orbitals (with different energy levels)
2. So each element has a different set of electron de-excitation energies
3. The different de-excitation energies produce photons with different frequencies (E=hf)

Question 26

How would you show the 488nm hydrogen emission line corresponds to a de-excitation from n=4 → n=2?

Answer 26

1. Calculate then energy difference between the energy levels
2. Convert energy difference to Joules
3. Convert to f or λ (E=hf or E=hc/λ)

Question 27

What is the ionisation energy of an atom?

Answer 27

The energy required for an electron to to become liberated from an atom

Equal to the energy of the ground state

Question 28

What is wrong with this?

Answer 28

Never use work function when talking about energy levels. Ionisation and work function are different.

Question 29

How is excitation by photon different from excitation by an electron?

Answer 29

• Photon energy = Difference between energy levels
• Electron energy ≥ Difference between energy levels

Question 30

How many photons (of different wavelengths) can be emitted from this hydrogen atom?

Answer 30

6 possible transition so 6 different photons

Question 31

Why is this mercury vapor in the fluorescent tubes kept at low pressure?

Answer 31

So a large enough current (of incident electrons) can be sustained

Question 32

How does fluorescence work in a tube light?

Answer 32

1. Mercury atoms excite by absorbing electrons from the current
2. When the Mercury atoms de-excite they emit UV photons
3. UV photons are absorbed by and excite the phosphor coating
4. When the phosphor coating de-excites it emits visible light

Question 33

When do particles exhibit properties of waves? (refraction, diffraction and polarisation)

Answer 33

When their Debroglie Wavelength is similar to the size of the gap they are passing through

Question 34

What does this experiment show?

Answer 34

Wave-Particle duality

Electron diffraction through graphite to form maximas (bright rings) and minimas (dark rings)

Question 35

How is the Debroglie wavelength λ_db of a particle calculated?

Answer 35