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Modern Physics
26 January 2015

18:54

Cathode ray tube (CRT)

Electron beam

Vacuum

Grid

Light

Heater

Cathode

Anode

x-plates

-

+

High voltage supply approx
...

 Activity
○ λ
 Decay constant
 High value
□ Very radioactive material
○ Radioactivity
 Measured
□ Half-life of an element
Time taken for half the nuclei in any given sample to decay
Modern Physics Page 6

 Time taken for half the nuclei in any given sample to decay
 Time taken for the activity of any given sample to decrease to half its original
value
 Relegated
◊ Decay constant
◊ Formula

 Unit of activity
□ Becquerel
 One
◊ Disintegration
 One nucleus per second
• STS
○ Radioactivity
 Effects
□ Dangerous
□ Positive
 Ionising effect
□ Damage to human cells
□ Lead to cancer
 Radon gas exposure
□ Alpha emitter
□ Lung cancer causer
 Used
□ Treat cancer
 Small doses
◊ So as not to cause cancer while treating it

Nuclear fission
• Nuclear fission
○ Breaking up of a large nucleus into a smaller nuclei of similar size with release of energy
• Uranium
○ U-238
• Rare isotope
○ U-235
○ Most important atom
 Undergo fission
○ Bombarded with neutrons
 Splits in two
 Creates
□ Barium
□ Krypton
 Releases
□ 2/3 neutrons

Barium

92 protons

Neutrons

143 neutrons
Neutron
235

Krypton

• On average
○ One new neutron
 Fission in another atom
□ Reaction sustained
□ Gradual release of energy
• Chain reaction
○ All 3/2 new neutrons create fission
○ Large release of energy
Quickly
Modern Physics Page 7

+ energy

○ Quickly
• Difference
○ Atomic bomb
○ Nuclear power station
• U-235
○ Exceeds particular mass
 Chain reaction occurs
 Critical mass
□ 10kg
• STS
○ Fission reactor
 Fuel rods
□ Enriched uranium
□ Uranium
 Extra U-235
 Control rods
□ Absorb neutrons
□ Control speed of reaction
 Moderators
□ Slow down neutrons
□ Most efficient
 Slightly slower than speed emitted from atom
 Energy released
□ Converts water to steam
□ Drive turbines
Control rods

Steam

Water

Coolant
Moderators

Fuel rods

Nuclear fusion
• Nuclear fusion
○ Two small atomic nuclei join to form a larger nucleus accompanied by the release of large
amounts of energy
○ Sun
○ Stars
○ Forms
 Deuterium ( ) atoms
□ Isotope of hydrogen
□ Forms
Helium
Modern Physics Page 8

 Helium

• Source of energy
○ Advantages
 Huge resource
□ Ocean full of hydrogen
 No dangerous waste products
 Large energy release
• Only occurs at high temps
○ Hard to sustain safely

Modern Physics Page 9

Demonstration Experiments
26 January 2015

18:54

Rutherford's gold foil experiment
Apparatus

~2%

<1%

Alpha particles

~97%
Gold foil

Method
Rutherford fired a beam of alpha particles towards a thin sheet of gold
...

Observation
What Rutherford found is that most particles went through the foil, without causing damage to the foil
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Conclusion
This experiment gave rise to the idea of the Rutherford-Bohr model of the atom, which is now the standard
view of the atom
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Protons are the nucleus of a hydrogen atom
...

• The loss of energy and gain in mass fitted perfectly with Einstein's equation:
Remember
• The Cockcroft and Walton experiment demonstrated for the first time that the splitting of a nucleus was
possible by the use of artificially accelerated particles
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Modern Physics Page 11

Definitions
26 January 2015

18:54

Thermionic emission
Thermionic emission is the emission of electrons from the surface of a hot metal
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Threshold frequency
The threshold frequency of a metal is the minimum frequency of electromagnetic radiation
that will cause photoemission from that metal
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Pair annihilation
Pair annihilation is the transformation of a particle and anti-particle into 2 photons of
electromagnetic energy
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Radioactivity
Radioactivity is the emission of either particles or electromagnetic radiation from the
nucleus of an atom
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Law of radioactive decay
This law states that the number of disintegrations per second is proportional to the number
of nuclei present
...


One Becquerel
One Becquerel is defined as the disintegration of one nucleus per second
...


Modern Physics Page 12

Nuclear fusion
Nuclear fusion occurs when two small atomic nuclei join together to form a larger nucleus,
accompanied by the release of large amounts of energy

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