Chapter 14 - Matter: Very Hot and Very Cold

Resources: Page numbers refer to your Advancing Physics textbook, codes to the CD. Access to Advanced Level Physics and Physics Foundation Unit (both thin red books) are on the shelves just inside the door of S4 - help yourself if needed (please sign them out).

Key: Demo = Demonstration, Pres = Presentation, SoftAct = Software Activity, Exp = Experiment, Home = Home Experiment, Dis = Display

SAQ = Short Answer Question, Est = Estimate, Comp = Comprehension, Data = Data Handling, ExExQ = Explanation-Exposition Question, Warm = Warm Up Exercise Question, MC = Multiple Choice Question

Work in class

Work out of class

Section 14.1 The magic ratio E/kT

14.1.1 Energy per particle
Pres 10P Temperatures everywhere. Add own temps to a copy.
Discussion using SAQ 20S Molecules and change .
Units of energy: J per particle, eV, kJ per mole. SAQ 25S Energy per particle.
Home Experiment 120H Energy for one molecule of water to escape, Home Experiment 20H Creme brulee or Home Experiment 30H Chilled confectionery.
Students choose one and present others with their results next lesson.

14.1.2
Present results from home experiments.Help with SAQ 25S if needed .
SAQ 130S energy per atom in some solids for extension.
Pres 40P Hot glows.
Pres 50P Slow motion syrup.
Discuss and note possible explanations for observations.
Refer to p110 diagram

14.1.3 Energy and processes
Diagram of processes on p 110.
SAQ 35S Value of energy E=kT.
Practice with estimation.
Processes start to happen when E is about 10-30 kT.
Do ExExQ 45X Matter comes apart.
Start to use the ratio E/kT.

1. Read p 105 -112.

2. Fill in your own temperatures on a copy of Pres 10P.

3. Learn the different units of energy: J per particle, eV, kJ per mole and complete SAQ 25S Energy per particle.

4. Choose one from : Home Experiment 120H Energy for one molecule of water to escape, Home Experiment 20H Creme brulee or Home Experiment 30H Chilled confectionery.

5. Complete SAQ 35S Value of energy E= kT

6. P 116 Qu 1 -6

7. Complete ExExQ 45X Matter comes apart. And write your own explanations for a process which occurs at a low, normal and high temperature in terms of particles and energies.

Extension
a) Reading 10T Einstein's solid
b) Reading 20T Equipartition of energy.

Quick check - do you know:
Particles have an energy of the order kT.
Values of kT can be in J per particle, eV, kJ per mole.
Bonds are characterised by the energy E needed to break them.
When E/kT is small, bond break and matter comes apart.
When E/kT is large, bond hold and matter condenses or solidifies.
Processes start to happen when E/kT is in the range 15-30

Section 14.2 The Boltzmann factor exp(-E/kT)

14.2.1 Getting extra energy by chance
Processes happen when E is 15 - 30 kT -we want to explain this.
Particle energy is quantised on levels.
SoftAct 170S Getting lucky… Particles are constantly exchanging energy so some have more than the average .
Exp 200E Multi level energy shuffling.
SoftAct 190S Following one particle: exponential behaviour
Note exponential form of the energy distribution. Dis 40O / p113 diagram indicates where the factor comes from.

14.2.2. The Boltzmann factor exp (-E/kT)
Discuss example of atmosphere p 114 - 115 and Disp 50O. N/N' = exp (-E/kT).
Rate of 'reaction' is proportional to this factor . Many processes require particles to get over a 'hill' or activation energy to happen.
Disp 60O / p117.
SAQ 90S The Boltzmann factor.
ExExQ 60X Thinking about the Boltzmann Factor.
Paragraph bottom p 115 has concise explanation of E/kT numbers.
Discuss change of Boltzmann factor with temp p 116.

14.2.3
Exp 240E Conduction in a semiconductor.
Exp 260E Chemical clock reactions - only if some keen chemists are available.
Exp 280E Flow ate of sticky liquids.
Exp 290E Thermionic emission.
The point is that rate of reaction rate depends on exp(-E/kT). Students should do one of these and be prepared to present their results.Students write a brief paragraph explaining the results for their experiment using the ideas of this chapter and present to the others.

14.2.4
Read and report back on one of these:
Reading 30T Flow in liquids.
Reading 40T Why you can't get to absolute zero.
Reading 50T The story of rubberReading 60T Liquid crystals

1. Read p 113 - 120

2. Describe for three processes: a) what is happening to the particles, b) why the speed of that process varies approximately exponentially with temperature. Use Disp 40O and 60O and p 113 - 117.

3. ExExQ 80X Exponential distributions

4. P 120 Qu 1-6.

5. Complete ExExQ 60X Thinking about the Boltzmann Factor.

Extension
a) SAQ 150S Electrons from hot metals.
b) Comp 140C contaminated surfaces.
c) Readings 30T, 40T, 50T, 60T.

Quick check - do you know:
Boltzmann factor = ratio of numbers of particles on two energy levels separated by energy E.
The origin of the Boltzmann factor is the small probability of repeatedly gaining extra energy at random from a large number of other particles.
Rate of a reaction is approximately proportional to the Boltzmann factor. It increases rapidly with temperature.
Reactions can involve changes in the number of particle arrangements, as well as of their energy.