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7.1. Discrete Energy and Radioactivity

At GCSE we talked a lot about light behaving as a wave, doing wavey-type things, such as reflecting and refracting, as well as listening to the Electromagnetic Spectrum Song to death (at least, you would have done if you were in my class!).

Now light - or more generally all electromagnetic waves - have some very odd behaviour. There exists something called 'Wave-Particle duality' - basically saying that sometimes light can do wavey things (diffracting, interfering, refracting - basically all the stuff from Chapter 4), and sometimes it can behave more... particle-y (if that is a word) - and it's this particle-y behaviour that we will look at now.

This is the start of your introduction into the wonderful world of Quantum Mechanics. Crash Course Physics takes a whistle-stop tour through the first half of this section (N.B. beyond 4:15 is HL stuff only!)

I've split this section up as follows:

 

The Photon

Background : The UV Catastrophe

Here is my very oversimplified history of physics up until the 20th century:

So up until about the end of the 19th century (...also until the end of GCSE physics), most people thought light was a wave. In a nutshell, there was plenty of evidence (see this video for some of this evidence) to suggest light behaved as waves, and it mostly obeyed the mathematical rules of what we now call 'classical physics' (Newton, Galileo, Huygens and the like). However, when exploring mathematical models of something called 'black bodies' (stuff that radiates energy, explored in Section 8.2) , the rules of classical physics started to fall apart. This was termed the 'UV Catastrophe' - a rather melodramatic term for when the maths goes wrong - like when you end up dividing by zero. A fudge was found (... well, not quite, it was a bit more involved) by a guy called Max Planck to make the maths work - treating the emitted light as 'discrete' particles.

These discrete particles of light were termed 'photons'  - 'photo' meaning relating to light, an -ons being a standard suffix that Physicists add when talking about particle'y things (neutrons, electrons and so on).

A much better explanation is given by Physics Girl below:

 

(N.B. the history of UV Catastrophe is not part of the IB syllabus, but it's quite nice for a bit of context for where our equation came from)

The Photon

A photon is a particle of light, which has a discrete 'packet' of energy. This amount of energy carried by each photon is given by the following equation:

E = hf

(or E = hc/λ)

 

 

This tells us that the energy of a photon only depends on the frequency of that light.

For example, if we have violet light with a frequency of 7.5 x 10¹⁴ Hz, each photon will have an energy of :

7.5 x 10¹⁴ x 6.63 x 10ˉ³⁴ = 4.97 x 10ˉ¹⁹ J

where:

E = Energy of photon (in Joules)

h = Planck's constant (6.63 x 10ˉ³⁴ m²kg / s)

f = frequency of light (Hz)

λ = wavelength of light (m)

c = speed of light (3 x 10⁸m/s)

The Electronvolt

A Joule is very useful for measuring energy on macroscopic scales (e.g. the GPE gained when I lift a book onto a table). The Electronvolt is a unit of energy which we use to measure stuff on atomic energy scales (i.e. much much smaller than a Joule. THe conversion between Joules and electronvolvts is:

 

1 eV = 1.6 x 10ˉ¹⁹ J

(N.B. This is not explicitly given in your formula book, instead it is the same value as that of the electronic charge (on an electron) 1.6 x 10ˉ¹⁹ C. For the reason why, see PhysicsOnline's video below):

Video Lessons

Resources

IB Physics
Topic 7 Notes
IB-Physics.net
Chapter 7 Summary
IB Revision Notes
Isaac Physics
Electromagnetic Waves
Mr. G
7.1 Teaching Notes
7.1 Student Notes
Physics and Maths Tutor
Particles Definitions
Particles Key Points
Particles Detailed Notes
EM Radiation & Quanta Flashcards
A Level Resources - content slightly different

Questions

Cambridge University Press
Topic 7: Add Qs
Topic 7: Add Qs MS
Topic 7: MCQs
CUP Website Link
Freely available online
Grade Gorilla
7.1 (Radioactivity) MCQ
Topic 7 (Nuclear) End Quiz
Quick IB Specific Mixed MCQs
Isaac Physics
Quantum Calculations
Particle wave/ momentum HL only
Mr. G
7.1 Formative Assessment
Topic 7 Summary Qs
IB Specific Questions
 

The Bohr Model and Energy Levels

Bohr Model

Photons are discrete packets of energy - otherwise known as 'quanta'. The whole field of quantum physics is based around the idea that energy exists in discrete quantised packets. 

This led to the development of the Bohr Model of the atom. Click the link below to take a look at a nice simulation by Walter Fendt. 

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Geogebra have a lovely simulation on energy levels. First try changing the waveglength of the photon and seeing how the energy in eV changes (bottom right). Then try looking at di

Emission and Absorption Spectra

Neon lighting works by having neon (or others) gas enclosed in a glass tube, with a high potential difference across each end. The gas becomes excited, and emits light of a certain colour. We can see below a selection of gases - Neon is deep red, Argon is purple. However, what is really interesting is if we look at the different wavelengths of light emitted in each case. 

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1280px-Atomic_emission_spectrum_of_heliu

Video Lessons

Resources

IB Physics
Topic 7 Notes
IB-Physics.net
Chapter 7 Summary
IB Revision Notes
Mr. G
7.1 Teaching Notes
7.1 Student Notes
Physics and Maths Tutor
Particles Definitions
Particles Key Points
Particles Detailed Notes
EM Radiation & Quanta Flashcards
A Level Resources - content slightly different

Questions

 

Types of Decay - Alpha, Beta-, Beta+ and Gamma

Video Lessons

Resources

IB Physics
Topic 7 Notes
IB-Physics.net
Chapter 7 Summary
IB Revision Notes
Isaac Physics
Radioactive Decay Equations
Mr. G
7.1 Teaching Notes
7.1 Student Notes
Physics and Maths Tutor
Particles Definitions
Particles Key Points
Particles Detailed Notes
Particles Flashcards
A Level Resources - content slightly different

Questions

Cambridge University Press
Topic 7: Add Qs
Topic 7: Add Qs MS
Topic 7: MCQs
CUP Website Link
Freely available online
Grade Gorilla
7.1 (Radioactivity) MCQ
Topic 7 (Nuclear) End Quiz
Quick IB Specific Mixed MCQs
Isaac Physics
Nuclear Equations
Mr. G
7.1 Formative Assessment
Topic 7 Summary Qs
IB Specific Questions
Physics and Maths Tutor
Nuclear Physics (AQA 1)
Nuclear Physics MS (AQA 1)
Stable & Unstable Nuclei (AQA 2)
Stable & Unstable Nuclei MS (AQA 2)
A-Level Qs: overlapping content
Physics and Maths Tutor
Constituents of the Atom (AQA 2)
Constituents of the Atom MS (AQA 2)
A-Level Qs: overlapping content
 

Half Life Graphs

In this section, you should be able to link the above ideas of mass defect to calculate the energy produced through fission and fusion.

However, it is worth refreshing your memory on the key aspects of fission and fusion, so flick through the following resources.

Video Lessons

Resources

IB Physics
Topic 7 Notes
IB-Physics.net
Chapter 7 Summary
IB Revision Notes
Isaac Physics
Electromagnetic Waves
Mr. G
7.1 Teaching Notes
7.1 Student Notes
Physics and Maths Tutor
Particles Definitions
Particles Key Points
Particles Detailed Notes
EM Radiation & Quanta Flashcards
A Level Resources - content slightly different

Questions

Cambridge University Press
Topic 7: Add Qs
Topic 7: Add Qs MS
Topic 7: MCQs
CUP Website Link
Freely available online
Grade Gorilla
7.1 (Radioactivity) MCQ
Topic 7 (Nuclear) End Quiz
Quick IB Specific Mixed MCQs
Isaac Physics
Quantum Calculations
Particle wave/ momentum HL only
Mr. G
7.1 Formative Assessment
Topic 7 Summary Qs
IB Specific Questions
 

Additional Resources

IB Questions

A question by question breakdown of the IB papers by year is shown below to allow you to filter questions by topic. Hopefully you have access to many of these papers through your school system. If available, there may be some links to online sources of questions, though please be patient if the links are broken! (DrR: If you do find some broken links, please contact me through the site)

 

Questions on this topic (Section 7) are shown in dark purple.

Use this grid to practice past IB questions topic by topic. You can see from the colours how similar the question topic breakdown is year by year. The more you can familiarise yourself with the IB question style the better - eventually you will come to spot those tricks and types of questions that reappear each year.