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AQA 2011 AQA 2011 Physics Unit 2Physics Unit 2

W Richards

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

18/04/2318/04/23P2.5.1 – Atomic StructureP2.5.1 – Atomic Structure

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The structure of the atomThe structure of the atomELECTRON –

negative, mass nearly

nothing

PROTON – positive,

same mass as neutron

(“1”)

NEUTRON – neutral,

same mass as proton

(“1”)

The nucleus is around 10,000 times smaller then the atom!

Atoms always have the same number of protons and electrons so they are neutral overall. They can gain or lose electrons to form ions.

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Structure of the atomStructure of the atom

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…

I did an experiment (with my colleagues Geiger and Marsden)

that proved this idea was wrong. I called it the “Scattering

Experiment”

Ernest Rutherford, British scientist:

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The Rutherford Scattering ExperimentThe Rutherford Scattering Experiment

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”.

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The structure of the atomThe structure of the atom

Particle Relative Mass Relative Charge

Proton 1 +1

Neutron 1 0

Electron 1/2000 (i.e. 0) -1

MASS NUMBER = number of protons + number of neutrons

SYMBOL

PROTON NUMBER = number of protons (obviously)

18/04/2318/04/23Mass and atomic number Mass and atomic number revisionrevision

How many protons, neutrons and electrons?

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IsotopesIsotopesAn isotope is an atom with a different number of neutrons:

Each isotope has 8 protons – if it didn’t then it just wouldn’t be oxygen any more.

Notice that the mass number is different. How many neutrons does each isotope have?

A “radioisotope” is simply an isotope that is radioactive – e.g. carbon 14, which is used in carbon dating.

18/04/2318/04/23P2.5.2 – Atoms and RadiationP2.5.2 – Atoms and Radiation

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Introduction to RadioactivitySome 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.

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IonisationRadiation 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.

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Background RadiationBackground Radiation

Radon gas

Food

Cosmic rays

Gamma rays

Medical

Nuclear power

13% are man-made

18/04/23Background Radiation by Location

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

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Types of radiationTypes of radiation

1) Alpha () – an atom decays into a new atom and emits an alpha particle (2 protons and 2 ______ – the nucleus of a ______ atom)

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.

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.

Unstable nucleus

Unstable nucleus

Unstable nucleus

New nucleus

New nucleus

New nucleus

Alpha particle

Beta particle

Gamma radiation

Words – frequency, proton, energy, neutrons, helium, beta

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Changes in Mass and Proton Number

Alpha decay:

237

93

4

2Am

241

95Np α+

90

39

0

-1Sr

90

38Y β+

Beta decay:

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Blocking RadiationEach 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

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SummaryProperty Alpha Beta Gamma

Charge

Mass

Penetration ability

Range in air

What is it?

Ionising ability

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Deflection by Electric Fields

Alpha and beta particles have a charge:

++

-

2 protons, 2 neutrons, therefore charge =

+21 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?

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Deflection by Magnetic FieldsRecall:

++ -2 protons, 2 neutrons,

therefore charge = +2

1 electron, therefore charge = -1

Because of this charge, they will also be deflected by magnetic fields:

+

Region of magnetic field

1) Why did they move in opposite directions? 2) Which particle had the more curved path and why?

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Uses of radioactivity 1

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.

18/04/23Uses of radioactivity 2 - Tracers

A tracer is a small amount of radioactive material used to detect things, e.g. a leak in a pipe:

Gamma source

Tracers can also be used in medicine to detect tumours:

The radiation from the radioactive source is picked up above the ground, enabling the leak in the pipe to be detected.

For medicinal tracers, you would probably use a beta source with a short half life – why?

18/04/23Uses of radioactivity 3 – Smoke Detectors

Smoke detectors

Alarm

+ve electrode -ve

electrode

Alpha emitter

Ionised air particlesIf smoke enters here a current no longer flows

18/04/23Uses of Radioactivity 4 - Treating Cancer

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.

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Dangers of radioactivity

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.

Alpha

Beta

Radiation will ionise atoms in living cells – this can damage them and cause cancer or leukaemia.

Gamma

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Half life

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…

At start there are 16 radioisotope

s

After 1 half life half have

decayed (that’s 8)

After 3 half lives another

2 have decayed (14 altogether)

After 2 half lives another

half have decayed (12 altogether)

= radioisotope = new atom formed

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A radioactive decay graph

Time

Count

1 half life

1 half life

1 half life

18/04/23Dating materials using half-livesQuestion: 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.

8

8

Answer: The sample was originally completely uranium…

…of the sample

was uranium

4

8

2

8

1

8Now 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

leadSo 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.

1 half life later…

1 half life later…

1 half life later…

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An exam question…

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

18/04/2318/04/23P2.6.1 – Nuclear FissionP2.6.1 – Nuclear Fission

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Nuclear fissionNuclear fission

Uranium or

plutonium nucleus

Unstable

nucleus New nuclei (e.g. barium

and krypton)

More neutron

s

Neutron

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Chain reactionsChain reactions

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

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

18/04/2318/04/23P2.6.2 – Nuclear FusionP2.6.2 – Nuclear Fusion

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Nuclear Fusion in starsNuclear Fusion in starsProton 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

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Stage 1: Nebulae

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

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

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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

Stage 2: Protostar

18/04/23Stage 3: Main Sequence

Our sun is an example of a main sequence star – it’s in the middle of a 10 billion year life

span

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.

Words – heavier, balanced, hydrogen, nuclear, temperatures

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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

Stage 4: Red Giant

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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:

Stage 5: The Death

White dwarf

Black dwarf

Red giant

18/04/232) If the star was a RED SUPERGIANT it will shrink and then EXPLODE, releasing massive amounts of energy, dust and gas.

AfterBefore

This explosion is called a

SUPERNOVA

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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.

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The dust and gas thrown out by a supernova can be used to form a new star…

Stage 6: Second generation stars

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

18/04/23The Life Cycle of a Star summary

Protostar

Main sequence

Red super giant

Supernova

Red giant

White dwarf

Black dwarf Neutron star Black hole

Basically, it all depends on the size of the star!

SMALLstars

BIGstars

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