EDEXCEL IGCSE / CERTIFICATE IN PHYSICS 7-4

ParticlesEdexcel IGCSE Physics pages 226 to 231

March 5th 2013

All content applies for Triple & Double Science

Edexcel SpecificationSection 7: Radioactivity and particlesc) Particlesdescribe the results of Geiger and Marsden’s experiments with gold foil and alpha particlesdescribe Rutherford’s nuclear model of the atom and how it accounts for the results of Geiger and Marsden’s experiment and understand the factors (charge and speed) which affect the deflection of alpha particles by a nucleusunderstand that a nucleus of U-235 can be split (the process of fission) by collision with a neutron, and that this process releases energy in the form of kinetic energy of the fission productsunderstand that the fission of U-235 produces two daughter nuclei and a small number of neutronsunderstand that a chain reaction can be set up if the neutrons produced by one fission strike other U-235 nucleiunderstand the role played by the control rods and moderator when the fission process is used as an energy source to generate electricity.

Models of the atomIn about 500 BC The Greek philosopher Democritus suggested that all matter was made up of tiny, indivisible particles. The word ‘atom’ meaning ‘cannot be cut or divided’

In 1803, Thomas Dalton proposed that atoms were tiny hard spheres. He further proposed that an element was made up of identical atoms all have the same mass but each different element was made up of its own type of atom.

Thomas Dalton 1766-1844

Electrons and sub-atomic particlesIn 1897 J.J. Thomson discovered the electron.

He found that this was a negatively charged particle that was much smaller than an atom.

This discovery showed that atoms were not indivisible but were made up of smaller sub-atomic particles.

J.J. Thomson 1856-1940

The Plum Pudding Atomic Model

Based on his discovery of the electron J.J. Thomson suggested that atoms consisted of a ball of:- positively charged matter - embedded with negatively charged electrons like the plums in a pudding

This became known as the plum pudding model of the atom.

The ‘Plum Pudding’ Model

Geiger & Marsden’s alpha particle scattering experiment

In 1909 Hans Geiger and Ernest Marsden performed an experiment that involved firing alpha particles onto a very thin foil of gold.

The results from this experiment would show that the plum pudding of the atom was incorrect.

Geiger and Marsden

The experiment

vacuum

θ

lead container containing an alpha source

inner zinc sulphide coating

evacuated container

very thin gold foil

The apparatus is contained in an evacuated container otherwise the alpha particles would not be able to reach the gold foil and the zinc sulphide coating.

The alpha particles cause the zinc sulphide detector to emit a flash of light.

Geiger and Marsden noted the distribution of the angles of deflection, θ of the alpha particles.

What was observedthin metal foil

1. Virtually all of the alpha particles went straight through the metal foil.

2. A few alpha particles were deflected through a small angle.

3. About 1 in 10 000 were deflected backwards.

alpha source

nucleus (highly enlarged)

atom

How the results can be explained

1. Deflections occur because there is a force between a charged nucleus and the positively charged alpha particles.

2. Most of the alpha particles do not go near enough to the nucleus to be deflected.

3. Backwards deflections occur when the alpha particles make near head on collisions with a positively charged nucleus.

Rutherford’s Atomic ModelBased on the results of Geiger and Marsden’s experiment Ernest Rutherford suggested that an atom consists of a a tiny positively charged nucleus surrounded by negatively charged electrons.

Lord Rutherford 1871 - 1937

How their results supported Rutherford’s atomic model

1. The relatively small number of deflections indicates that most of the atom is empty space with only a very small nucleus.

2. The backward deflections can only occur if the nucleus is positively charged and contains most of the atom’s mass.

3. The ‘plum pudding’ model would not produce backward deflections.

Choose appropriate words to fill in the gaps below:

According to __________ an atom consists of a tiny, ___________ charged __________ surrounded by a cloud of ________ electrons. The nucleus also contains most of the ______ of an atom.

This model was supported by the ______ particle scattering experiment in 1909. In this experiment most alpha particles passed ________ through a thin metal foil with only about 1 in 10000 being deflected _________.

positively nucleus

mass

alpha

Rutherford

straight

backwards

WORD SELECTION:

negative

positively nucleus

mass

alpha

Rutherford

straight

backwards

negative

Nuclear fissionNuclear fission is the splitting of an atomic nucleus.

The process of nuclear fission was discovered in 1938 by Lise Meitner, Otto Hahn and Fritz Strassmann.

Lise Meitner 1878-1968

Otto Hahn 1879-1968

Fissile materialsA fissile material easily undergoes fission.

The mostly commonly used fissile material is uranium-235.

When this undergoes fission two daughter nuclei are produced that are called fission fragments.

The process also produces neutrons, gamma radiation and a considerable amount of kinetic energy.

The fission of uranium-235The fission of a nucleus uranium-235 can be initiated by its absorption of a slow-moving neutron.

U236

92U

235

92n

1

0+

Ba144

56U

236

92

Uranium-236 is very unstable and splits apart.

+ Kr89

36γ

0

0+n

1

0+ 3

fission fragments

barium-144 and krypton-89

Other fission examplesOther fission products can be produced. Their masses usually have a roughly 3:2 ratio.

Xe134

54U

236

92+ Sr

90

38γ

0

0+n

1

0+ 2

Cs137

55Pu

239

94+ Y

89

39γ

0

0+n

1

0+ 3

Plutonium-239 is also fissile.

fission products = xenon-134 and strontium-90

fission products = caesium-137 and yttrium-89

Fission equation questions

Cs143

55U

236

92+ Rb

91

37γ

0

0+n

1

0+ 2

Ba145

56Pu

239

94+ Sr

92

38γ

0

0+n

1

0+ 2

I131

53U

235

92+ Y

91

39γ

0

0+n

1

0+ 3

This last example combines the uranium-235 neutron absorption with the fission of uranium-236

A

B

D

C

E

F

Complete the equations below:

Rubidium

Iodine

Chain reactionThe fission of a nucleus of Uranium-235 can be initiated by a slow-moving neutron.

When this nucleus splits further neutrons are produced.

These neutrons in turn can cause more nuclei to split.

An avalanche effect, called a ‘chain reaction’ can then occur.

Nuclear fission reactorsA nuclear reactor is used to produce substantial and controllable energy from nuclear fission.

The thermal energy produced is used to convert water to high pressure steam.

This steam is used to drive turbines which rotate generators to produce electricity.

top view of a nuclear reactor

Nuclear power plant

Structure of a

reactor

What each part does1. Uranium fuel rodsThese contain enriched uranium. This consists of uranium-238 with a higher proportion of uranium-235 than is found in natural reserves of uranium. The fuel rods become very hot and dangerously radioactive due to nuclear fission.

2. Removable control rodsMade of boron or cadmium, when placed in-between the fuel rods these absorb neutrons and so reduce the rate of fission. Their depth is adjusted to maintain a constant rate of fission.

3. Moderator (graphite core)Made of graphite. This surrounds the fuel rods and slows the neutrons down to make absorption of neutrons by uranium-235 more likely.

uranium fuel rods

4. CoolantThis can be water, carbon dioxide gas or liquid sodium. This transfers the thermal energy of the fuel rods to the heat exchanger.

5. Heat exchangerHere water is converted into high pressure steam using the heat energy of the coolant. 6. Concrete shieldThis absorbs the radiation coming from the nuclear reactor.

reactor underneath water coolant

Choose appropriate words to fill in the gaps below:

Nuclear fission is the _________ up of the nucleus of an atom into two smaller nuclei. Energy, neutrons and _________ radiation are also emitted.

Nuclear ________ use Uranium _____ or Plutonium _____to produce energy by nuclear ________. A controlled chain reaction is maintained by the use of _______ rods which absorb some of the _________ produced.

An _______ bomb is the consequence of an uncontrolled chain reaction.

239

neutrons

reactors

control

splitting

fission

gammaWORD SELECTION:

235

atomic

239

neutrons

reactors

control

splitting

fission

gamma

235

atomic

Nuclear fusion

Nuclear fusion is the joining of two atomic nuclei to form a larger one.

Hydrogen nuclei undergo fusion in stars to make helium nuclei

Energy from fusionNuclear fusion is the process by which energy is released in the Sun and other stars.

It is also the energy source of the hydrogen bomb.

Nuclear fusion reactors

Scientists are currently working to make nuclear fusion reactors.

The fuel for fusion reactors is the isotope hydrogen 2 (deuterium) which is found in sea water.

An experimental fusion reactor in Seatle USA

Online SimulationsNuclear Fission - PhET - Start a chain reaction, or introduce non-radioactive isotopes to prevent one. Control energy production in a nuclear reactor! Nuclear Fission - Powerpoint presentation by Richard Miller of 5SJW (2005) Nuclear Fission - Powerpoint presentation that includes a link to the 'mousetrap' demonstration Power Station Animation - eChalk Managing a Nuclear Power Plant Simulation - by Henrik Eriksson BBC AQA GCSE Bitesize Revision: Nuclear fission Nuclear fusion

ParticlesNotes questions from pages 226 to 231

1. Describe Geiger and Marsden’s alpha particle experiment (see pages 227 and 228).

2. Explain how the alpha particle experiment supports Rutherford’s nuclear model of the atom (see pages 227 and 228).

3. What is meant by (a) nuclear fission, (b) fission product and (c) chain reaction. (see pages 228 and 229).

4. Give an example of an equation of the fission of uranium-235 (see page 229).

5. Draw a diagram showing the structure of a nuclear reactor (see page 230).

6. Explain the function in a reactor of (a) control rods, (b) moderator and (c) heat exchanger (see pages 229 and 230).

7. Answer the questions on page 231.8. Verify that you can do all of the items listed in the end of chapter

checklist on page 231.