Ch3: Quantum phenomena

Question 1

Define photoelectric effect

Answer 1

Electrons are emitted from a metal surface when electromagnetic radiation above the threshold frequency is directed at the metal.

Question 2

Define threshold frequency

Answer 2

The minimum value for incident radiation for the photoelectric effect to happen.

Question 3

Define work function

Answer 3

Minimum energy needed to escape surface of the metal.

Question 4

Ekmax=?

Answer 4

hf - work function

Question 5

Threshold frequency fmin=?

Answer 5

fmin= work function / h

Question 6

Define stopping voltage

Answer 6

The minimum energy needed to stop photoelectric emission

Question 7

In a vacuum photocell electrons are emitted at the ….. and attracted to the …..

Answer 7

Photocathode, anode

Question 8

Does intensity affect ekmax?

Answer 8

No, still one electron absorbing one photon

Question 9

Incident intensity is proportion to…

Answer 9

Photoelectric current

Question 10

Define ionisation

Answer 10

Process of creating ions

Question 11

How can you calculate the ionisation energy of a gas atom?

Answer 11

In a low pressure gas tube, increase pd between filament and anode, when current increases is the ionisation energy of the gas atoms, which = eV where e is charge of electron.

Question 12

Define excitation

Answer 12

Atoms can absorb energy from colliding electrons without being ionised.

Question 13

How can you calculate excitation energies of a gas?

Answer 13

Increase pd between filament and anode, measure pd when the anode current falls.

Question 14

What is an electron volt?

Answer 14

unit of energy equal to work done when an electron is moved through a pd of 1V

Question 15

An electron in a shell ….. to the nucleus has less energy.

Answer 15

Closer

Question 16

Define ground state

Answer 16

The lowest energy state of an atom.

Question 17

What conditions have to be met for photon absorption to cause excitation?

Answer 17

The energy of the photon is exactly equal to the gain in the electrons energy, or the difference in initial and final energy levels.

Question 18

Why are excited atoms unstable?

Answer 18

An electron that moves to an outer shell leaves a vacancy

Question 19

What is deexcitation?

Answer 19

Vacancy filled by outer shell electron transferring to inner shell, a photon is emitted by the electron and the atom moves to a lower energy state.

Question 20

energy of emitted photon hf=

Answer 20

E1 - E2
E1-higher energy state.
E2- lower energy state

Question 21

Describe how a fluorescent tube works

Answer 21

Glass tube with fluorescent coating with mercury vapour. Mercury atoms ionise and excite as they collide. Mercury atoms deexcite and emit UV photons, visible light, low energy photons. UV photons absorbed by coating, exciting atoms. Coating atoms deexcite in stages, releasing visible photons.

Question 22

Define a line spectrum

Answer 22

Spectrum of discrete lines of different colours, each of which corresponds to a certain wavelength. Each line spectrum specific to the element. Photons from one line have the same energies. Photon emitted due to deexcitation of atom.

Question 23

Which two equations could you use to calculate the energy of a photon?

Answer 23

hf= E1-E2
f=c / wavelength

Question 24

Wave nature of light: …..
Particle nature of light: ……..

Answer 24

Electron diffraction

Photoelectric effect

Question 25

State the de Broglie hypothesis

Answer 25

Matter waves have dual wave particle nature

Question 26

De Broglie equation

Answer 26

wavelength = h / mv

Question 27

Evidence of the de Broglie wavelength?

Answer 27

Electrons diffract into rings when passed through thin metal foil, and diffract less when the speed increases which makes the de Broglie wavelength smaller.