3 Quark Phenomena

Question 1

what is the photoelectric effect?

Answer 1

emission of electrons from a metal surface when the surface is illuminated by light of frequency greater than a minimum value known as the threshold frequency

Question 2

what is threshold frequency?

Answer 2

the minimum frequency of light that can cause the photoelectric effect

Question 3

Explain Einstein’s theory for the photoelectric effect:

Answer 3

• when light is incident on a metal surface, an electron at the surface absorbs a single photon from the incident ray and gains energy (equal to hf)
• the electron can then leave the metal surface if the energy received from the photon exceeds the work function

Question 4

what is the work function of a metal?

Answer 4

the minimum energy needed by an electron to escape from the metal surface

Question 5

what happens to excess energy of a photoelectron when it leaves the surface?

Answer 5

it becomes kinetic energy

Question 6

what is the equation of the maximum kinetic energy of an emitted electron?

Answer 6

Eₖ = hf - 𝜙

Question 7

what is the stopping potential of a material?

Answer 7

the minimum potential needed to stop photoelectric emission

Question 8

why is the kinetic energy of an emitted electron reduced to zero when the metal is at stopping potential?

Answer 8

each emitted electron must do extra work to leave the metal surface

Question 9

why does increased intensity increase photoelectric emission?

Answer 9

intensity - energy per second which is proportional to the number of photons per second
each photoelectron absorbs one photon so the number of photoelectrons emitted is proportional to the intensity of the light

Question 10

what is ionisation?

Answer 10

any process of creating ions

Question 10

what is an ion?

Answer 10

a charged atom

Question 10

what does not affect the maximum kinetic energy of a photoelectron?

Answer 10

the intensity of the incident light

Question 10

what are some ways ions are created?

Answer 10

• radiation
• electrons passing through fluorescent tubes

Question 10

what is the electron volt?

Answer 10

unit of energy
equal to work done when an electron is moved through a pd of 1 V.
equal to 1.60 x 10⁻¹⁹ J

Question 11

what is excitation?

Answer 11

process in which an atom absorbs energy without becoming ionised as a result of an electron inside the atom moving from an inner shell to an outer shell

Question 12

what are excitation energies?

Answer 12

energy values at which an atom absorbs energy

Question 13

what happens in excitation by collision?

Answer 13

atoms absorb energy from colliding electrons
the colliding electron makes an electron inside the atom move from an inner shell to an outer shell

Question 14

what is the ground state of an atom?

Answer 14

the lowest energy state of an atom

Question 15

what is de-excitation?

Answer 15

process in which an atom loses energy by photon emission, as a result of an electron inside an atom moving from an outer shell to an inner shell or in which an excited nucleus emits a gamma photon

Question 16

what does fluorescence mean?

Answer 16

glow of light from a substance exposed to UV radiation: the atoms de-excite in stages and emit visible photons in the process

Question 17

How do fluorescent tubes work?

Answer 17

• ionisation and excitation of metal vapour atoms occur as they collide with each other and electrons in tube
• metal vapour atoms emit UV photons as well as visible photons when the atoms de-excite
• UV photons are absorbed by atoms of the fluorescent coating causing excitation of these atoms
• coating atoms de-excite in steps and emit visible photons

Question 18

how are line spectra formed?

Answer 18

• each line is due to light of a certain colour (so certain wavelength)
• the photons that produce each line all have the same energy which is different from the energy of the photons which produce other lines
• each photon is emitted when an atom de-excites due to one of its electrons moving to an inner shell

Question 19

why can line spectra be used to identify elements?

Answer 19

the wavelengths of the lines of a line spectrum of an element are characteristic of the atoms on that element

Question 20

what are some properties of light which show it’s wave-like nature?

Answer 20

• diffraction
• interference (shown with interference patterns)

Question 21

What are some properties of light which show the particle-like nature of light?

Answer 21

• the photoelectric effect

Question 22

What is the de Broglie hypothesis?

Answer 22

• matter particles have a dual wave-particle nature
• wave-like behaviour of particles is characterised by its de Broglie wavelength

Question 23

what is de Broglie wavelength related to?

Answer 23

the particle’s momentum

Question 24

what is the formula for de Broglie wavelength?

Answer 24

λ = h / mv

Question 25

what is the evidence for the de Broglie hypothesis?

Answer 25

electron diffraction

Question 26

explain electron diffraction?

Answer 26

• beam of electrons is directed to metal foil
• rows of atoms cause the electrons to be diffracted in certain directions as the beam passes through the foil
• a pattern of rings is formed on a fluorescent screen

Question 27

what changes the size of the rings in electron diffraction?

Answer 27

increased speed decreases the de Broglie wavelength which decreases the size of the rings