Quantum Phenomena (topic 3)

Question 1

How do electrons inside a metal normally behave?

Answer 1

Move freely within the metal, colliding with each other and positive ions

Question 2

Photoelectric effect (definition)

Answer 2

electron are emitted from the surface of a metal when electromagnetic radiation above a certain frequency is directed at the metal

Question 3

What observations about the photoelectric effect suggest the particle nature of light?

Answer 3

• Only takes place above the threshold frequency - irrelevant of intensity
• Occurs without delay

Question 4

Explanation of the photoelectric effect (Einstein)

Answer 4

Electron at the surface absorbs a single photon and gains energy equal to hf. If the energy received exceeds the work function then the electron can escape.

Question 5

What happens to excess energy when an electron absorbs a photon (photoelectric effect)?

Answer 5

Becomes kinetic energy

Question 6

Stopping potential (definition)

Answer 6

The mimimum electric potential needed to stop photoelectric emission by attracting electrons back to the surface (as plate becomes positively charged so potential difference between cathode and anode grows)

Question 7

Maximum kinetic energy

Answer 7

= e x V(s)

which is charge of electron x stopping potential

Question 8

What did max Planck suggest about atoms?

Answer 8

Their energy is quantised and only appears in multiples of h

Question 9

What is the work function?

Answer 9

The minimum energy needed by a conduction electron to escape the metal’s surface when the metal is at zero potential (ground state)

Question 10

What happens if an electron absorbs a photon with energy less that the work function?

Answer 10

It collides with other electrons and positive ions until the extra kinetic energy is lost

Question 11

What is a vacuum photocell? How does it work?

Answer 11

Glass tube containing a metal plate (cathode) and smaller electrode (anode). EM radiation is emitted at the metal plate, which causes photoelectrons to be sent cathode > anode.

Question 12

How is the number of electrons released measured in a vacuum photocell?

Answer 12

Microammeter - current is proportional to electrons per second transferred ( divide by charge of electron)

Question 13

What is light intensity? How does it relate to current in a vacuum photocell?

Answer 13

Light intensity is a measure of energy per second carried by incident light, which is proportional to the photoelectric current and the number of photons per second emitted (as each electron absorbs one photon)

Question 14

Why is intensity of light not relevant to maximum kinetic energy?

Answer 14

Each electron only absorbs one photon

Question 15

On a graph of maximum kinetic energy against f, where is the threshold frequency?

Answer 15

X-intercept

Question 16

Practical to measure ionisation energy for a gas atom

Answer 16

• electrons released from a heated filament to a positive metal plate, in a tube with gas (low pressure so electrons reach anode).
• p.d. is increased s that electrons get faster
• when electrons move fast enough to ionise gases, the current hugely increases as ionised electrons also reach anode
• work done (ionisation energy of gas atom) = charge x volts = e x tube potential difference

Question 17

What is an electron volt?

Answer 17

The work done when an electron is moved through a p.d. of 1V (as E = QV and electrons have set charge)

Question 18

How does excitation by collision work?

Answer 18

Using gas filled tubes with a metal grid between the filament and the anode, we can observe that gas atoms absorb energy from colliding electrons without being ionised at certain energies

Question 19

Other than excitation, what happens to electrons in a gas filled cathode-anode tube?

Answer 19

• if a colliding electron does not have enough energy to cause excitation, it is deflected from the atom with no overall loss of E
• If the colliding electron loses all kinetic energy in the collision, the current due to flow of electrons is reduced

Question 20

How can a gas filled tube be used to find excitation energies?

Answer 20

Finding the p.d where the current at the anode falls (as the colliding electrons dont have enough kinetic energy to reach anode). The p.d. is the same magnitude as the excitation energy in eV

Question 21

Why is energy needed for excitation?

Answer 21

The electron moves further away from the positively charged nucleus

Question 22

What is the ground state?

Answer 22

All electrons at the lowest possible energy levels (each shell only holds a certain number of electron shells, so ground state != all on lowest level)

Question 23

Why does deexcitation happen?

Answer 23

an excited electron moves up a shell, which leaves a vacancy, which must be filled by an electron from an outer shell.

Question 24

How does excitation by photon occur?

Answer 24

A photon of exactly the correct energy is absorbed by an electron, which can move up a shell if there is a vacancy.

Question 25

What does it mean to fluoresce?

Answer 25

glow with visible light when absorbing UV radiation

Question 26

How does fluorescence occur?

Answer 26

Atoms in the substance absorb UV photons and emit visible photons

Question 27

How does a fluorescent tube light work?

Answer 27

• inner tube contains mercury at low pressure. Ionisation and excitation of the mercury atoms occur as they collide with each other and electrons from current in the tube
• Mercury atoms then emit UV photons (+ lower energy photons) when they deexcite
• UV photons are absorbed y atoms in the fluorescent coating, which emit visible photons when they deexcite

Question 28

What are the two wavelengths either side of visible light?

Answer 28

• 400nm = violet
• 650nm = red

Question 29

Why is a line spectra produced?

Answer 29

Photons of a specific energy are released, and energy is directly proportional to wavelength of a photon

Question 30

What is the “dual nature” of light?

Answer 30

It can behave as a wave or as a particle, according to circumstance

Question 31

What observations suggest light has a wave-like nature?

Answer 31

Diffraction, e.g. Young’s slits
(longer wavelengths diffract more)

Question 32

What observations suggest light has a particle-like nature?

Answer 32

The photoelectric effect, which begins more or less immediately rather than building up energy from a wave

Question 33

What observations suggest matter has a particle-like nature?

Answer 33

Electrons in a beam can be deflected by a magnetic field

Question 34

What is the de Broglie wavelength?

Answer 34

The wavelength of matter which is equal to h/momentum
(hence wavelength can be changed by changing velocity)

Question 35

What observations suggest matter has a wave-like nature?

Answer 35

When electrons in a vacuum tube are directed at a metal foil which acts as a diffraction grating due to its crystalline structure, they diffract and create a pattern of fluorescent rings

Question 36

How is a beam of electrons created for the electron diffraction experiment? How can this be used to dictate de Broglie wavelength?

Answer 36

Electrons are attracted to a metal plate with a small hole in the centre. By increasing the p.d. between the emitting filament and the plate, the electrons get faster.

de Broglie wavelengthis proportional to momentum, which is dictated by the electron’s velocity.