Particles 2

Question 1

Describe the photoelectric effect

Answer 1

the emission of photoelectrons from a metal when light above a threshold frequency is shone on it

Question 2

What does the photoelectric effect give evidence for?

Answer 2

evidence for the quantum model of light

Question 3

Why does the photoelectric effect give evidence for the quantum model of light?

Answer 3

• emission is instant (if at all)

- emission occurs above a threshold frequency (at any intensity)

Question 4

What is meant by intensity here?

Answer 4

the number of photons per second

Question 5

Explain what happens during the photoelectric effect?

Answer 5

• one electron absorbs one photon (one-to-one interaction)
• 100% of the photon energy is absorbed (or none of it)
• photon energy = hf

Question 6

What does ϕ equal in the equation: hf = ϕ + KE(max)? What is its definition?

Answer 6

• work function

- the minimum energy required for an electron to escape from the surface

Question 7

What does the work function depend on?

Answer 7

depends on the metal

Question 8

Why is is KE(max) in the equation hf = ϕ + KE(max)?

Answer 8

because it is the KE for an electron emitted from the surface

Question 9

What is the equation for ϕ?

Answer 9

ϕ = h(fo)

Question 10

What does fo equal?

Answer 10

the threshold frequency

Question 11

Define the threshold frequency

Answer 11

the frequency at which electron emission just starts

Question 12

What is the stopping potential?

Answer 12

the reverse p.d. needed to just stop electrons with maximum kinetic energy

Question 13

What does the work done on electrons equal?

Answer 13

• = KE(max)
• = QV
• = e(stopping potential)

Question 14

Define an excited atom

Answer 14

an atom is excited if its electrons occupy higher energy levels with vacancies below

Question 15

How much energy can orbital electrons have?

Answer 15

they can only have certain discrete energies when they orbit a nucleus (the energy levels are unique to each element)

Question 16

What are the two ways an orbital electron can become excited?

Answer 16

• if it absorbs a photon of exactly the energy difference between two levels
• if it collides with a free electron of sufficient energy

Question 17

Explain the first step of how fluorescent tubes works

Answer 17

• a high potential difference accelerates free electrons through a low pressure mercury vapour
• free electrons collide and excite mercury atoms
• excited electrons in mercury atoms fall down, de-excite, emitting photon(s)
• when electrons fall directly from higher energy levels down to lower we get UV photons

Question 18

Why in fluorescent tubes is a low pressure mercury vapour used?

Answer 18

it is low pressure so the electrons can still accelerate fast enough to have enough energy to collide with mercury atoms

Question 19

Explain the second step of how fluorescent tubes work

Answer 19

• the tube is coated
• the coating absorbs UV photons, exciting its electrons
• when electrons in the coating fall back down, they may emit several visible photons by falling via the intermediate levels

Question 20

What does the emission spectrum show?

Answer 20

the frequencies emitted by an excited atom, unique to that element, due to unique energy levels

Question 21

What does the absorption spectrum show?

Answer 21

the frequencies emitted are missing from the continuous spectrum

Question 22

Give evidence for light as a wave

Answer 22

it diffracts

Question 23

Give evidence for light as a particle

Answer 23

the photoelectric effect

Question 24

Give evidence for an electron as a wave

Answer 24

it diffracts

Question 25

Give evidence for an electron as a particle

Answer 25

• they are charged (e.g. they are deflected by magnetic fields)
• they have a rest energy (non-zero)

Question 26

What is meant by duality of electrons?

Answer 26

electrons behave like both waves and particles / electrons have characteristic of both waves and particles