TOPIC TWO - Quantum Physics

Question 1

What is the photoelectric effect?

Answer 1

When e- are emitted from the surface of a metal when light shines on it

Question 2

What evidence is there for light being a particle in p.e.e?

Answer 2

~ e- are emitted immediately after light shines on the metal

~ increasing intenisty = increase in number of photoelectrons, but
the kinetic energy did not increase

~ red light will not cause electrons to be emitted regardless of
intensity

Question 3

What is threshold frequency?

Answer 3

The min frequency of a photon needed to eject photoelectrons from a metal surface

Question 4

What is work function?

Answer 4

Min E to move an e- from within the metal to the surface

Question 5

What is the equation for work function?

Answer 5

hf = Φ + Ek

Question 6

What is stopping potential?

Answer 6

The minimum potential needed to stop p.e.e emission

Question 7

What are the main factors affecting the number of photoelectrons emitted?

Answer 7

• nature of the material (i.e Φ)
• the frequency of incident radiation
• the intensity of incident radiation

Question 8

What is the order of magnitudes for the work function?

Answer 8

x 10 ^ -19 J

Question 9

When can an electron leave a metal?

Answer 9

If the E of the photon exceeds Φ

Question 10

What is the electron volt?

Answer 10

A unit of energy equal to the work done when an electron is moved through a p.d of 1V

Question 11

What is excitation?

Answer 11

When gas/metal atoms can absorb energy from colliding electrons without being ionised, it happens at certain energies (which are characteristic to the gas/metal)

Question 12

What happens if the colliding e- in excitation looses all its Ek?

Answer 12

Current due to flow of electrons is reduced

Question 13

What happens if the colliding electron doesn’t have enough kinetic energy to cause excitation?

Answer 13

It is deflected by the atom with no overall loss of kinetic energy

Question 14

What are excitation energies?

Answer 14

The energy values at which an atom absorbs energy

Question 15

What happens to the atomic electron when excitation occurs?

Answer 15

The colliding e- makes an electron inside the atom move up an energy level

Question 16

What’s is excitation energy always smaller than?

Answer 16

The ionisation energy, because the atomic electron is not removed completely from the atom when excitation occurs.

Question 17

What is the ground state?

Answer 17

The lowest E state of an atom (where it excites at standard conditions)

Question 18

What happens to the electron config. in an excited atom?

Answer 18

It becomes unstable because an electron that moves to an outer shell leaves a vacant in the shell it moves from, later the vacancy is filled by an electron from an outer shell transferring to it.

Question 19

What is de-excitation?

Answer 19

When the vacancy is filled by an electron from the outer shell, the electron emits a photon, the atom therefore moves to a lower energy level

Question 20

How do you work out the energy of the emitted photon?

Answer 20

hf = E1 - E2

Question 21

How does a fluorescent tube work? (6 marks)

Answer 21

1) A beam of free electrons flows through the tube from one end to the other.
2) These free electrons collide with orbital electrons in mercury atoms.
3) The orbital electrons gain energy and are excited to a higher energy level.
4) The electrons quickly return to their ground state and UV photons are released.
5) The electrons within the phosphorous coating on the inside layer of the tube absorb these photons and are excited to higher energy levels.
6) The excited electrons then return to a lower energy level, emitting visible light photons.

Question 22

On a line spectrum of an element, what are wavelengths specific to?

Answer 22

The wavelengths of the lines of a line spectrum of an element are characteristic of the atoms of that element. No other element produces the same pattern of light wavelengths.

Question 23

Why do no other element produce the same pattern of light wavelengths?

Answer 23

Because the same energy levels of each type of atom are unique to that atom, so the photons emitted are characteristic of the atom

Question 24

How are lines in the line spectrum formed?

Answer 24

When an atom de-excites a photon is emitted, each one has a specific wavelength so specific colour.

Question 25

What can you say about photons that produce each line?

Answer 25

They all have the same E, which is different from the E of the photons that produce any other line.

Question 26

When is the wave-like nature of light observed?

Answer 26

In diffraction

Question 27

What was de brois hypothesis?

Answer 27

• matter particles have a dual wave-particle nature

- the wave-like behaviour of a matter particle is characterised by its de brois wavelength