Chapter 13 - Quantum

Question 1

Define a quanta

Answer 1

A quantum is the smallest indivisible amount of a physical quantity

Question 2

Define the term Quantised

Answer 2

A physical quantity whose magnitude is limited to a set of discrete values e.g. charge

Question 3

Define a photon

Answer 3

A quantum of Electromagnetic radiation I.e. the smallest indivisible unit of light energy

Question 4

What are energy levels

Answer 4

They are shells at set distances from a nucleus

Atoms can orbit in the energy levels but never in between

Question 5

What happens when an electron becomes excited

Answer 5

It gains energy, moves up an energy level

They can then emit the energy as a photon when they drop down

Question 6

What is the photon equation

What is the different form of the equation

Answer 6

E = hf 
E = energy 
h = 6.63x10^-34
f= frequency

I.e the energy of a photon is directly proportional to its frequency

E = hc/lambda

Question 7

What is an electronvolt

How to calculate it

Answer 7

An electron volt is the energy transferred when one electron travels through a potential difference of 1 V

Given V = E/Q

E = VQ = 1.6x10^-19 J

Question 8

Describe the method of the LED experiment

Answer 8

1) set up a circuit with a variable resistor and a voltmeter and LED in parallel
2) place a black tube over the LED and gradually increase R (or V) until the LED just starts to emit light, record the voltage
3) plot a graph of P.D. against 1/lambda

Question 9

How to analyse the results of the LED experiment

Answer 9

Given 
W = VQ and E = hc/lambda 
This means 
VQ = hc/lambda
V=(1/lambda)(hc/e)

Thus our gradient = hc/e so we can calculate h

Question 10

Summarise the wave model of light

Answer 10

• the energy delivered by the wave depends on the intensity

- energy arrives continuously and can build up over time

Question 11

Summarise the photon model of light

Answer 11

• light is made of many individual quanta called photons
• each photon has an energy given by E = hf
• the intensity of the light is the number of photons per second
• Each photon can be absorbed by each electron

Question 12

Briefly explain the photoelectric effect experiment

Answer 12

• a gold leaf electroscope is negatively charged (by induction) both the stem and the leaf become negatively charged and the lead rises
• if electrons are emitted from the plate’s surface, the lead falls down

Question 13

What were the findings of the photoelectric effect experiment

Answer 13

• threshold frequency: no matter the intensity, if the incident radiation was below the threshold frequency no photoelectrons are emitted
• time : providing the correct frequency, the emission of electrons was instantaneous
• effect of light intensity : if the frequency is correct, increasing the intensity doesn’t affect KE but does affect number of electrons emitted
• MAX KE: increasing the frequency increases KE

Question 14

What is the photon explanation of the photoelectric effect

Answer 14

• light is a stream of photons
• each photon has a set energy
• each electron needs a certain energy to escape the material (work function)
• photons and electrons interact one-to-one
• this explains why there is no time delay, electrons cannot accumulate energy from different photons

Question 15

What is Einstein’s photoelectric effect equation

And a real terms explanation

Answer 15

E(Kmax) = Hf - (work function)

I.e. the energy of an incident photon will

• free an electron
• any leftover energy goes to the KE of the electron

Question 16

Why is it KE(max) and not just KE

Answer 16

Electrons at different points of the metal structure will have different work functions

Question 17

How to calculate threshold frequency

Answer 17

Occurs where E(k) = 0
This means
HF(0) = work function

Question 18

What are the important features of an Ek(max) against f graph

Answer 18

• the y-intercept = -(work function)
• gradient = h
• where y<0 no electrons are emitted

Question 19

What is the electron gun diffraction experiment

Answer 19

• a thin piece of wire is heated by a very high P.D. where work done by pd = E
• electrons are emitted and the electron ‘gun’ fires at a thin piece of crystalline graphite
• the gaps between the atoms is similar to the do Broglie wavelength of the electrons
• thus they diffract and interfere and and form interference patterns on a fluorescent screen

Question 20

What are some notable observations from the electron gun experiment

Answer 20

• when the electrons are accelerated by the pd they act as particles
• when they diffract, they act as waves
• they act as particles upon impact with the screen

Question 21

What is the de Broglie equation in terms of p, hmv, and Ek

Answer 21

Lambda = h/p
P = momentum 

Lambda = h/mv

Lambda is inversely proportional to sqrt(Ek)

Question 22

What is threshold frequency (definition)

Answer 22

The minimum frequency of light required to produce electrons from a metal

Question 23

What is the effect of increasing frequency on the photoelectrons produced

Answer 23

They will have a greater kinetic energy

Question 24

what are some key points to mention when describing the photoelectric effect

Answer 24

• Photoelectric effect is the removal of electrons (from metals) when exposed to EM radiation
• Surface electrons are involved
• energy is conserved

Question 25

what to always mention if explaining why different wavelengths of light have different effects in the photoelectric effect

Answer 25

- energy of one wavelength of photon < work function - energy of the other wavelength of photon > work function - intensity is independent of energy - one-to-one interaction - e = hf

Question 26

what improvement can you make to the LED practical to get a more accurate reading for when the LED first starts to emit light

Answer 26

- place a sensitive ammeter in series with the LED | - when it starts to register a reading you know you have reached the threshold P.D.