Quantum Physics

Question 1

What is the basic principle of Quantum Physics

Question 2

What phenomenon brought up the field of Modern Physics as it disputed Classical Physics theories?

Answer 2

The Photoelectric Effect

Question 3

What phenomenon provides evidence that particles can have wave-like properties?

Answer 3

Electron Diffraction

Electrons travelling through polycrystalline graphite will be diffracted by the atoms and the spacing between the atoms.

Question 4

What are the apparatus needed in an electron diffraction experiment?

Answer 4

A beam of electrons, a detection screen, and a graphite film

The layered-like structure of graphite allows it behave like a diffraction grating

Question 5

What can electron diffraction be used for?

Answer 5

To determine the arrangement of atoms in matter and the size of nuclei.

Question 6

How does

the Photoelectric Effect dispute Classical wave theory?

Answer 6

It proves that the emissionof electrons is dependent on the frequencyof the wave and the no. of e^- is dependent on the energy of the wave

Classical wave theory would have suggested that the reverse is the case

Question 7

What then is proportional to

the wave intensity in the Photoelectric Effect?

Answer 7

The Photoelectric current

This is the number of photoelectrons emitted per second

Question 8

What is

the Photoelectric Effect?

Answer 8

The emission of e^- from the surface of a metal when radiation is incident on it

Question 9

What is the prerequisite

for incident radiation to liberate photoelectrons from metals?

Answer 9

The frequency of the incident radiation must be ≥ the threshold frequency of the metal

Question 10

What is

Threshold frequency?

Answer 10

Theminimum frequency of the incident radiation on a metal’s surface needed to liberate photoelectrons from it

Question 11

What is

Threshold wavelength?

Answer 11

The maximum wavelength of the incident radiation on a metal’s surface needed to liberate photoelectrons from it

Question 12

Do all the emitted photoelectrons have the same KE when liberated?

Answer 12

No

Question 13

Which photoelectrons typically have the highest (maximum) KE?

Answer 13

The valence electrons of surface atoms

Question 14

What happens to the leaf of a gold-leaf electroscope when -vely charged metal plate is irradiated by light of a sufficient frequency to release photoelectrons?

Answer 14

The leaf falls

When photoelectrons are emitted, the -ve plate loses its -ve charge

Question 15

Will any change to the leaf of a gold-leaf electroscope be observed when the metal plate is given a +ve charge?

Answer 15

No

As photoelectrons are emitted, the +ve charge of the plate creates an attractive force between the electron and the metal plate, preventing them from being liberated

Question 16

What is Einstein’s photoelectric equation?

Answer 16

(Total) Σ = hf₀ + KE_max.

The total energy of the incident radiation is equal to the energy needed to liberate the electron from the metal and the maximum KE it can get on liberation

Question 17

What is a photon

Answer 17

A packet (or quantum) of energy possessed by an EM wave

Question 18

What is the implication of the Photon model of a wave

Answer 18

It implies that waves can have particle-like properties

Question 19

What are the particle-like properties of waves?

Answer 19

1. Elements of it (photons) collide elastically with e^- to liberate photoelectrons
2. Elements of it (photons) possess momentum

Question 20

What is the relationship between a photon and an electron

in the Photoelectric Effect under the Photon model of a wave?

Answer 20

It is a one-to-one relationship

One electron absorbs the energy of one photon and it is liberated if this Σ value is greater than hf₀

Question 21

What happens to the photon when it collides with an electron?

Answer 21

The electron absorbs its energy and it ceases to exist

Question 22

What happens when the Σ supplied by the photon is less than hf₀?

Answer 22

The electron absorbs its energy and it as KE

which it can lose upon collision with metal ions

Question 23

What is the work function (Φ) of a metal?

Answer 23

This is the energy required to liberate e^- from the surface of a metal

Each metal has their own w₀ (another way of representing work function) value

Question 24

Why is the value of the KE of photoelectrons up to a maximum?

Answer 24

For e^- that are not as loosely bounded, work is done against the attractive forces of the nucleus to bring the electron to the surface, hence its emission energy will be reduced

Question 25

What unit is used to better express photon energies?

Answer 25

The **electron-volt**

Question 26

What is electron-volt?

Answer 26

The work done in accelerating an electron through a PD of 1 volt

Question 27

What is the value of one electron-volt in Joules?

Answer 27

1.6 x 10^-19 J

Question 28

How does KE_max. relate to stopping potential?

Answer 28

The value of Voltage that gives the maximum KE is the PD required to stop emitted photoelectrons from leaving the surface of the metal

Question 29

# What are Energy Levels?

Answer 29

**Specified regions of space** around an atom where its e^-*can be found* ## Footnote they are **quantised energy states** within the atom

Question 30

# What is the **ground state** of an element?

Answer 30

The state at which the atom possesses the least amount of energy with all the e^- in their **lowest energy state**

Question 31

# What are the 2 main characteristics of Energy Levels?

Answer 31

They are **fixed** and are **peculiar** to each element

Question 32

# What is required for an e^- to transit between 2 energy levels?

Answer 32

It has to absorb the **exact amount of energy** needed for a specific transition ## Footnote E.g. if an electron wanted to move btw Σ levels 2 and 3, it has to absorb Σ equivalent to the energy difference between the 2 levels

Question 33

# What happens if an e^- absorbs less or more Σ than what is required?

Answer 33

**No transition will occur**

Question 34

# Why do have energy levels -ve values?

Answer 34

Because **external energy is supplied** to the atom in order to make the transition

Question 35

# What is the value in eV of the ionisation level?

Answer 35

**0 eV** ## Footnote At ionisation the e^- has escaped from the attractive pull of the atom's nucleus

Question 36

What are ***all other energy levels*** apart from the ground state called?

Answer 36

**Excited states**

Question 37

What happens when an electron 'falls' from a higher energy level to a lower one?

Answer 37

**A photon is emitted** ; its frequency is equal to the frequency of the energy difference between the 2 levels

Question 38

What equation helps find the energy difference btw 2 energy levels?

Answer 38

ΔΣ = hf_**higher** - hf_**lower**

Question 39

How many types of emission spectra are there?

Answer 39

**Two** : **Line** emission spectrum and **Continuous** emission spectrum

Question 40

# What is the Line emission spectrum?

Answer 40

It is a spectrum that consists of bold, distinct coloured lines, that **show the specific wavelengths** absorbed (and thus emitted) by electrons in gaseous atoms

Question 41

# What is the **nature** of the gases that are used to produce the Line emission spectrum?

Answer 41

They must be **hot** ## Footnote Hot gases are raised past their ground states. e^- can absorb energy from the heat to make transitions btw Σ levels

Question 42

# What is the Continuous emission spectrum?

Answer 42

It is a spectrum that consists of a wide, continuous range of wavelengths with no clear-cut distinction between the colours.

Question 43

# What **type of light** produces the Continuous emission spectrum?

Answer 43

**White Light**

Question 44

# What is an **absorption** spectrum?

Answer 44

It is a spectrum that consists of dark lines **against the background of a continuous spectrum** to show the absence of light of particular wavelengths.

Question 45

# How is an absorption spectrum produced?

Answer 45

When white light passes through a substance (such as a **cool gas**) before being diffracted

Question 46

# What do the dark lines in an absorption spectrum show?

Answer 46

The wavelengths absorbed by the electrons in the atoms of a substance when white light is incident on it

Question 47

Why are **cool gases at low pressure** used when trying to produce an absorption spectrum?

Answer 47

When gases are cool, its atoms are more likely to be in their ground state. When gases are at low pressure, its atoms are duly separated from each other ## Footnote This allows photons interact with electrons on an atom-by-atom basis

Question 48

What does the line emission spectrum of an atom tell us about it?

Answer 48

Since there are sharp distinct, coloured lines, electrons in the atom *can only exist* in specific energy levels.

Question 49

Why do the line emission spectra of different atoms differ?

Answer 49

Because they have **different spacings** between their energy levels

Question 50

Is a photon a particle?

Answer 50

**No**, it is an **element** of an electromagnetic wave

Question 51

Do photons have mass?

Answer 51

**No**

Question 52

Who suggested that matter can have wave-like properties?

Answer 52

**Louis de Broglie**

Question 53

Who suggested that waves can have matter-like properties?

Answer 53

**Albert Einstein**

Question 54

What is the equation for the de Broglie wavelength?

Answer 54

λ = h/p h = Planck's constant p = momentum of the **photon** | where λ = wavelength ## Footnote Make sure "de Broglie wavelength" is specified before you use this formula - Mr Onanuga