Quantum Phenomena

Question 1

What is the photoelectric effect?

Answer 1

The photoelectric effect is where the surface of a metal emits photoelectrons after being exposed to a light above a certain frequency

Question 2

What criteria must be met for the photoelectric effect to occur

Answer 2

The light must meet the threshold frequency

Question 3

What did the photoelectric effect prove?

Answer 3

• EM waves travel in photons which have energy proportional to frequency
• each electron absorbs a single photon, therefore is only emitted when frequency threshold is met
• If the intensity of light is increased, more electrons are emitted per second

Question 4

What is work function?

Answer 4

The minimum amount of energy required for electrons to be emitted from the surface of a metal

Question 5

What is stopping potential

Answer 5

It is the potential difference needed across the metal to stop photoelectrons with maximum kinetic energy
Ek max = e * stopping potential (V)

Question 6

What is excitation

Answer 6

When electrons can gain energy from collisions with free electrons to move up energy levels

Question 7

What is ionisation

Answer 7

When electrons gain energy from collision with free electrons to be able to leave the atom entirely (only occurs when the free electrons energy is greater than the ionisation energy)

Question 8

What happens after excitation

Answer 8

An electron would return to its original energy level (the ground state) by releasing a photon

Question 9

Give an example of a practical use of excitation (no need to explain)

Answer 9

Excitation in a fluorescent tube to produce light

Question 10

Explain what is going on in a fluorescent tube?

Answer 10

A high voltage is applied which accelerates free electrons
- free electrons collide with mercury vapour atoms that become excited
- this releases more free electrons
- when they dexcite they release UV photons
- The fluorescent coating absorbs the UV photons and the electrons in the coating become excited and when de-excited they release photons of visible light.

Question 11

How do you convert electron volts to joules

Answer 11

Multiply by 1.6*10^-19

Question 12

On a line spectrum what does each line represent

Answer 12

A wavelength of light emitted from a tube (fluorescent tube emitting Visible light photon), wavelength values are discrete proving that electrons in atoms can only transition between discrete energy levels.

Question 13

What experiments prove the wave-particle duality of light

Answer 13

Young Double experiment - proves light can diffract and interfere like a wave
Photoelectric effect - proves light can act like particles

Question 14

How can we prove electrons having both wave and particle properties

Answer 14

Via electron diffraction (as only waves can diffract)

Question 15

WIth electron diffraction - how can you make the concentric ring pattern closer and why does this happen

Answer 15

Increasing the momentum of electrons as their wavelength will decrease therefore their diffraction amount would decrease (using de broglie rule)

Question 16

What does it mean if a metal has a greater work function

Answer 16

This means more energy is required to free the photoelectrons.

The gradient on a ek-f graph is planks constant

Question 17

what is the mass of a photoelectron?

Answer 17

9.11*10^-31

Question 18

what is de broglie wavelength equation

Answer 18

lander = h / m * v

Question 19

Formulate an equation involving kinetic energy and de Broglie wavelength (do on paper)

Answer 19

must input p^2 in kinetic formula

Question 20

How does UV photons convert into the visible part of the electromagnetic spectrum

Answer 20

UV photons is absorbed by the fluoresecent coating which excites their electrons

These electrons would dexcite and emit visible light photons

Question 21

Explain how excited mercury atoms emit photons

Answer 21

Excited atoms deexcite

Emitting a photon of energy equal to the energy difference between levels

Question 22

Explain how mercury atoms in a fluorescent tube are excited

Answer 22

Electrons through the tube from a potential difference collide with mercury atoms

This transfers energy from the collision

electrons in the mercury atom moves to higher energy levels

Question 23

Describes what occurs during the photoelectric effect

Answer 23

Photons of light incident to the metal surface causes an emission of electrons

The electrons emitted are those near the surface

Question 24

What does increasing intensity to do the current of a system

Answer 24

Increasing intensity results in an increase in photons incident per second

As a result current increases

Question 25

Formula of stopping potential?

Answer 25

eV = Ek max

e = charge of electron
V = stopping potential

Question 26

How do you work out the minimum p.d needed to ionise an electron

Answer 26

p.d = ground state energy / charge of electron

Question 27

What does increasing intensity do (2 points)

Answer 27

More electrons released per second

More photons striking the metal surface per second

Question 28

How does allowing air molecules into a system affect photoelectrons from reaching the anode

Answer 28

Increase collision of electrons and air molecules

fall in Ek

therefore less electrons reach the anode per second

Question 29

Why would you use crystals or graphite as a material to given an observable diffraction pattern with electrons?

Answer 29

Their atomic spacing is similar wavelength to electrons

Question 30

What is meant by ionisation energy

Answer 30

Minimum energy required to release an electron from an atom from its ground state