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AQA 2011 Physics Unit 2
This PowerPoint supports sections P2.5 and P2.6 of the 2011 AQA Physics Unit 2 module
W Richards

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P2.5.1 – Atomic Structure

3. The structure of the atom
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ELECTRON – negative, mass nearly nothing
The nucleus is around 10,000 times smaller then the atom!
PROTON – positive, same mass as neutron (“1”)
NEUTRON – neutral, same mass as proton (“1”)
Atoms always have the same number of protons and electrons so they are neutral overall. They can gain or lose electrons to form ions.

4. Structure of the atom
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A hundred years ago people thought that the atom looked like a “plum pudding” – a sphere of positive charge with negatively charged electrons spread through it…
Ernest Rutherford, British scientist:
I did an experiment (with my colleagues Geiger and Marsden) that proved this idea was wrong. I called it the “Scattering Experiment”

5. The Rutherford Scattering Experiment
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Alpha particles (positive charge, part of helium atom)
Thin gold foil
Most particles passed through, 1/8000 were deflected by more than 900
Conclusion – atom is made up of a small, positively charged nucleus surrounded by electrons orbiting in a “cloud”.

6. The structure of the atom
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Particle
Relative Mass
Relative Charge
Proton 1 +1
Neutron
Electron
1/2000 (i.e. 0) -1
MASS NUMBER = number of protons + number of neutrons He 2 4
SYMBOL
PROTON NUMBER = number of protons (obviously)

7. Mass and atomic number revision
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How many protons, neutrons and electrons? 1 11 16 H B O 1 5 8 23 35
238 Na Cl U 11 17 92

8. Isotopes
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An isotope is an atom with a different number of neutrons:
Notice that the mass number is different. How many neutrons does each isotope have? O 8 16 O 8 17 O 8 18
Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.
A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

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P2.5.2 – Atoms and Radiation

10. Introduction to Radioactivity
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Some substances are classed as “radioactive” – this means that they are unstable and continuously give out radiation at random intervals:
Radiation
The nucleus is more stable after emitting some radiation – this is called “radioactive decay”. This process is NOT affected by temperature or other physical conditions.

11. Ionisation
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Radiation is dangerous because it “ionises” atoms – in other words, it turns them into ions by “knocking off” electrons:
Alpha radiation is the most ionising (basically, because it’s the biggest). Ionisation causes cells in living tissue to mutate, usually causing cancer.

12. Background Radiation 13% are man-made Radon gas Food Cosmic rays
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13% are man-made
Radon gas
Food
Cosmic rays
Gamma rays
Medical
Nuclear power

13. Background Radiation by Location
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In 1986 an explosion occurred at the Chernobyl nuclear power plant. Here is a “radiation map” showing the background radiation immediately after the event:
Other “risky” areas could be mining underground, being in a plane, working in an x-ray department etc

14. Types of radiation
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1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)
Unstable nucleus
New nucleus
Alpha particle
2) Beta () – an atom decays into a new atom by changing a neutron into a _______ and electron. The fast moving, high energy electron is called a _____ particle.
Beta particle
New nucleus
Unstable nucleus
3) Gamma – after  or  decay surplus ______ is sometimes emitted. This is called gamma radiation and has a very high ______ with short wavelength. The atom is not changed.
Words – frequency, proton, energy, neutrons, helium, beta
Unstable nucleus
New nucleus
Gamma radiation

15. Changes in Mass and Proton Number
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Alpha decay: Am
241 95 Np α +
237 4 93 2
Beta decay: Sr 90 38 Y β + 90 39 -1

16. Sheet of paper (or 6cm of air will do)
Blocking Radiation
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Each type of radiation can be blocked by different materials:   
Sheet of paper (or 6cm of air will do)
Few mm of aluminium
Few cm of lead

17. Summary Property Alpha Beta Gamma Charge Mass Penetration ability
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Property
Alpha
Beta
Gamma
Charge
Mass
Penetration ability
Range in air
What is it?
Ionising ability

18. Deflection by Electric Fields
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2 protons, 2 neutrons, therefore charge = +2
Alpha and beta particles have a charge:
1 electron, therefore charge = -1 -
Because of this charge, they will be deflected by electric fields: + - + -
1) Why did they move in opposite directions? 2) Which particle had the more curved path and why?

19. Deflection by Magnetic Fields
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Recall: +
2 protons, 2 neutrons, therefore charge = +2
1 electron, therefore charge = -1 -
Because of this charge, they will also be deflected by magnetic fields: + -
1) Why did they move in opposite directions? 2) Which particle had the more curved path and why?
Region of magnetic field

20. Uses of radioactivity 1 Sterilising medical instruments
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Sterilising medical instruments
Gamma rays can be used to kill and sterilise germs without the need for heating. The same technique can be used to kill microbes in food so that it lasts longer.

21. Uses of radioactivity 2 - Tracers
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A tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe:
Gamma source
The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.
Tracers can also be used in medicine to detect tumours:
For medicinal tracers, you would probably use a beta source with a short half life – why?

22. Uses of radioactivity 3 – Smoke Detectors
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Smoke detectors
Alpha emitter
+ve electrode
-ve electrode
Alarm
Ionised air particles
If smoke enters here a current no longer flows

23. Uses of Radioactivity 4 - Treating Cancer
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High energy gamma radiation can be used to kill cancerous cells. However, care must be taken in order to enure that the gamma radiation does not affect normal tissue as well.
Radioactive iodine can be used to treat thyroid cancer. Iodine is needed by the thyroid so it naturally collects there. Radioactive iodine will then give out beta radiation and kill cancerous cells.

24. Dangers of radioactivity
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Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia.
Alpha
Beta
Gamma
OUTSIDE the body  and  are more dangerous as  radiation is blocked by the skin.
INSIDE the body an  source causes the most damage because it is the most ionising.

25. Half life
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The decay of radioisotopes can be used to measure the material’s age. The HALF-LIFE of an atom is the time taken for HALF of the radioisotopes in a sample to decay…
= radioisotope
= new atom formed
After 2 half lives another half have decayed (12 altogether)
After 3 half lives another 2 have decayed (14 altogether)
After 1 half life half have decayed (that’s 8)
At start there are 16 radioisotopes

26. A radioactive decay graph
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Count
1 half life
1 half life
1 half life
Time

27. Dating materials using half-lives
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Question: Uranium decays into lead. The half life of uranium is 4,000,000,000 years. A sample of radioactive rock contains 7 times as much lead as it does uranium. Calculate the age of the sample.
Answer: The sample was originally completely uranium…
1 half life later…
1 half life later…
1 half life later… 8 4 8 2 8 1 8
…of the sample was uranium
Now only 4/8 of the uranium remains – the other 4/8 is lead
Now only 2/8 of uranium remains – the other 6/8 is lead
Now only 1/8 of uranium remains – the other 7/8 is lead
So it must have taken 3 half lives for the sample to decay until only 1/8 remained (which means that there is 7 times as much lead). Each half life is 4,000,000,000 years so the sample is 12,000,000,000 years old.

28. An exam question…
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Potassium decays into argon. The half life of potassium is 1.3 billion years. A sample of rock from Mars is found to contain three argon atoms for every atom of potassium. How old is the rock?
(3 marks)
The rock must be 2 half lives old – 2.6 billion years

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P2.6.1 – Nuclear Fission

30. Nuclear fission More neutrons Neutron Uranium or plutonium nucleus
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More neutrons
Neutron
Uranium or plutonium nucleus
Unstable nucleus
New nuclei (e.g. barium and krypton)

31. Chain reactions
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Each fission reaction releases neutrons that are used in further reactions.

32. Nuclear power stations
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Nuclear power stations use the energy from each reaction to heat water and use the steam to drive turbines:

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P2.6.2 – Nuclear Fusion

34. Nuclear Fusion in stars
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Proton
Neutron
Nuclear fusion happens in stars but it’s not possible to use it in power stations yet as it needs temperatures of around 10,000,000OC

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The Life Cycle of a Star

36. Stage 1: Nebulae A nebulae is a collection of dust, gas and rock.
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A nebulae is a collection of dust, gas and rock.
Some examples of nebulae…

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Dark nebula

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Emission nebula

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Reflection nebula

40. (This nebula is smaller and will only form a planet)
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Planetary nebula
(This nebula is smaller and will only form a planet)

41. Stage 2: Protostar Gravity will slowly pull these particles together…
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Gravity will slowly pull these particles together…
As they move inwards their gravitational potential energy is converted into heat and a PROTOSTAR is formed

42. Words – heavier, balanced, hydrogen, nuclear, temperatures
Stage 3: Main Sequence
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In a main sequence star the forces of attraction pulling the particles inwards are _________ by forces acting outwards due to the huge __________ inside the star.
Stars are basically ________ reactors that use _______ as a fuel. During its main sequence a star will release energy by converting hydrogen and helium (light elements) into _________ elements and this is why the universe now contains a number of heavier elements.
Our sun is an example of a main sequence star – it’s in the middle of a 10 billion year life span
Words – heavier, balanced, hydrogen, nuclear, temperatures

43. Stage 4: Red Giant
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Eventually the hydrogen and helium will run out. When this happens the star will become colder and redder and start to swell…
If the star is relatively small (like our sun) the star will become a RED GIANT
If the star is big (at least 4 times the size of our sun) it will become a RED SUPERGIANT

44. Stage 5: The Death
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What happens at this point depends on the size of the star…
1) For SMALL stars the red giant will collapse under its own gravity and form a very dense white dwarf:
Red giant
White dwarf
Black dwarf

45. This explosion is called a SUPERNOVA
2) If the star was a RED SUPERGIANT it will shrink and then EXPLODE, releasing massive amounts of energy, dust and gas.
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Before
After
This explosion is called a SUPERNOVA

46. If the star is big enough it could become a BLACK HOLE instead.
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The dust and gas on the outside of the supernova are thrown away by the explosion and the remaining core turns into a NEUTRON STAR.
If the star is big enough it could become a BLACK HOLE instead.

47. Stage 6: Second generation stars
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The dust and gas thrown out by a supernova can be used to form a new star…
Our sun is believed to be a “______ ______ star” – this is because it contains some __________ elements along with hydrogen and ________. These heavier elements would have been the products of a previous star that have been thrown out by a ________. These heavier elements are also found on planets, indicating that they might have been made from remains of previous _______ as well.
Words – helium, heavier, second generation, stars, supernova

48. The Life Cycle of a Star summary
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SMALL
stars
Protostar
BIG
stars
Main sequence
Red giant
Red super giant
White dwarf
Supernova
Black dwarf
Neutron star
Black hole
Basically, it all depends on the size of the star!

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