GGCCSSEE SScciieennccee RReevviieeww Volume 13Number 2November 2002

Fireworks!

EDITORIAL TEAM

Nigel CollinsKing Charles I School,

Kidderminster

David MooreSt Edward’s School, Oxford

David SangAuthor and editor

Jane TaylorSutton Coldfield Grammar

School for Girls

Editorial telephone01562 753964

ADVISORY PANEL

Eric AlboneClifton Scientific Trust

Tessa CarrickFounder Editor, CATALYST

David ChaundyFounder Editor, CATALYST

Peter FinegoldThe Wellcome Trust

Roland JacksonScience Museum

Chantelle JayBiotechnology and Biological

Sciences Research Council

Peter JonesScience Features, BBC

David KneeCREST Awards

Ted ListerRoyal Society of Chemistry

Founder Editor, CATALYST

Ken MannionSheffield Hallam University

Andrew MorrisonParticle Physics and

Astronomy Research Council

Jill NelsonBritish Association

Silvia NewtonCheshunt School

and Association for Science Education

Toru OkanoThe Rikkyo School in England

John RhymerBishop’s Wood Environmental

Education Centre

Julian SuttonTorquay Boys’

Grammar School

Nigel ThomasThe Royal Society

Fireworks: an explosive businessRonald Lancaster 1Improve your gradeI knew the answer so why did I only get 1 mark? 4Where on Earth?David Sang 6Energy transferJane Taylor 9Places to visitThe International Life Centre 12Effects of climate changeMax de Boo 14Try thisSpheres in space 17A life in scienceAlfred Nobel and the Nobel prizes 18Yes, it is rocket scienceDavid Sang 20Rocket man 22The front cover shows a firecracker exploding (ErichSchrempp/SPL).

© PHILIP ALLANUPDATES 2002ISSN 0958-3629

Publishing Editor: Jane Buekett.

Artwork: Gary Kilpatrick.

Reproduction by DeMontfort Repro, Leicester.

Printed by Raithby,Lawrence and Company,

Leicester.

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EditorialDon’t be a damp squib!

Exams are a bit like fireworks. You wait

all year for them to happen. Then sud-

denly there is a very busy spell, after which

they are all over and you can look forward

to the next time they come round. But why

are we thinking about exams now — surely

they are not until next June? Well, just as a

good firework display needs good prepara-

tion, so do exams. It’s no good just turning

up on the day and expecting things to go

well.

All exams — whether they are mocks or

GCSEs — need preparation. CATALYST can

help you with this. With this issue you

receive your useful GCSE Science Essential

Word Dictionary, written by editors of this

magazine. The entries are topical and up-

to-date. Use your dictionary as part of your

reference material and to help when

answering tests and questions.

In this issue of CATALYST there are useful

articles in all three sciences as well as a

section on examination technique. Read

the articles well — they will help you to

understand a topic better, or give you

another way of looking at a particular

subject.

Newton’s third law underpins many of

the articles in this issue — whether it be in

forces for propelling rockets and fireworks,

the movement of satellites, or the explo-

sive force of dynamite. Even the movement

of animals involves this law. Don’t become

narrow-minded and think that the three

sciences exist on their own — they are all

interconnected and what you read in one

article may help you understand what is

mentioned in another on a quite different

subject.

Don’t be like a damp squib — work at

things and go out in a blaze of glory! David Moore

Fireworks are an explosive combination ofcolour and noise. Whistles, bangs and hum-ming noises combined with many different

kinds of light effects give pleasure and delight tothousands of people all over the world. Althoughfireworks were invented hundreds of years ago theyhave developed as science has progressed, particu-larly over the last 200 years.

IN THE BEGINNING

Many people think that fireworks came from theFar East, but this is only part of the story. It was theWest which taught the East how to make modernfireworks.

Gunpowder was, undoubtedly, discovered inChina. The people who made it were not looking forgunpowder but for the ‘elixir of life’. They mixedtogether various salts, some of which must have beennitrates, with charcoal, honey and eventually sul-phur, and found that this mixture caught fire readily.

By AD 800 gunpowder had been produced in a

form similar to that we use today. It is pretty certainthat Arabs brought it to Europe in the thirteenthcentury. This mixture of potassium nitrate, charcoaland sulphur has a long history, but the Chinese didnot use it in weapons, instead they made it into fire-crackers. The powder was packed into bambootubes and thrown on to the fire so that it explodedand scared away evil spirits. Such crackers are stillmade today, and are traditionally covered in redpaper, which has a spiritual significance.

1November 2002

Nitrates containthe group (NO3

− ).

12 g of charcoal(about 6 cm3)when completelyburnt produces 24 000 cm3 of gas.If this expansionoccurs quicklyenough it can beused to propelshells or to causean explosion.

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RONALD LANCASTER

GCSE key wordsCombustion

Oxidation

Compounds

Flame tests

Fireworks have been known for many

hundreds of years. What gives them their

particular colour or effect? How are they made

and how are they set off? This article includes

some basic chemistry relevant to your GCSE

science course — with an explosive twist.

Left: TheGunpowder Plot in1605 was a famousbut unsuccessfulattempt to blow upthe king andparliament, stillcelebrated withfireworks madefrom gunpowder.

Above: KimboltonFireworks gainedfirst prize at aninternationalfireworks and musiccompetition inCannes in 2001.Displays on threebarges 300 m outto sea weresynchronised by acomputer on thepromenade.

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INTRODUCING COLOURS

Fireworks as we know them today are a compara-tively recent invention. Some were made with gun-powder before 1800, but they could only burn goldor white and consisted mainly of rockets, bangersand gold or white fountains. The charcoal and thesulphur burn in the oxygen provided by the potas-sium nitrate (an oxidising agent) to provide goldenflames. By-products include gases such as carbondioxide and sulphur dioxide and some solid potas-sium compounds. It is mainly the charcoal whichproduces the gold colour.

Nitrates of barium and strontium provided someother colours, but the discovery of potassium chlo-rate (another oxidising agent) by Berthollet in 1794changed everything. By 1820 European fireworkmakers were producing fine coloured fireworks, butthey were very dangerous because potassium chlo-rate is an unstable compound and, when mixed withsulphur, can spontaneously combust. It did this

many times, causing serious accidents, before themore stable potassium perchlorate came into use tosupply the oxygen.

It was not long before the isolation of magnesiumand aluminium (due to the development of electro-chemistry) allowed manufacturers to produce bril-liant silver fireworks. More recently titanium hasbecome available and has the same effect.

The range of colours used to be restricted to red,blue, green, yellow and white. High temperatureflames have now been developed with the addition ofeither magnesium or magnesium–aluminium alloy,known as magnalium alloy, which corrodes muchless easily than magnesium alone. These have madepossible intermediate colours such as citron (betweengreen and yellow), turquoise (between green andblue) and rich oranges and violets.

HOW IT WORKS

Colour occurs in the flame due to the formation ofhalides, but these are unstable and only exist in theflame itself. The best colour emitters are strontium(I)chloride (SrCl) which produces red, copper(I) chlo-ride (CuCl) for blue and barium(I) chloride (BaCl)for green. Sodium atoms produce yellow quitepowerfully, and traces of sodium can overwhelmany other colour present. Blue has always been themost difficult colour to produce because the copperchloride molecule is easily destroyed in high-tem-perature flames.

Most coloured fireworks consist of four main com-ponents with a number of additives. The oxidisingagent potassium perchlorate provides the oxygen, thefuel is normally a gum resin such as shellac fromIndia or gum yacca (acaroid resin) from Australia.The colouring agent is the insoluble salt of the appro-priate metal. Insoluble salts are chosen to avoidchemical reactions which produce deliquescence. Inaddition there is the halogen donor and sometimesstarches to slow down the burning rate or to act as anadhesive for making pellets which burst as stars.

2 Catalyst

Potassium chlorateis KClO3, potassium

perchlorate isKClO4.

The best blue‘stars’ were

originally madewith an easily-

decomposedpigment called

Paris green. It wasmade by boiling

arsenic oxide withcopper sulphate

and thenprecipitating the

pigment withethanoic acid. It isimpossible to buy

this pigment todaybecause of its

toxicity.

Deliquescence iswhen a chemical

absorbs waterfrom the

atmosphere.

Many street lightscontain sodium —hence their colour.

BOX 1 FLAME TESTS

Flame tests are used to show whether certain metalions are present in a compound. A tiny sample ofcompound on a piece of platinum (or nichrome)wire is placed in a non-luminous bunsen flame. Theenergy of the flame causes electrons in the metal ionto rise to higher energy levels, and as they fall backto their original level they give out specific frequen-cies of light.Metal ion ColourLithium Deep redSodium Persistent yellow-orangePotassium LilacRubidium RedCaesium BlueCalcium Orange-redBarium Pale greenCopper(II) Blue-green

BOX 2 CONGREVE ROCKETS

Around the end of the eighteenth century rocketsexperienced a brief revival as a weapon of war.Indian rocket barrages were used against the Britishin 1792 and again in 1799. These caught the interestof an artillery expert, Colonel William Congreve,who set out to design rockets for use by the Britishmilitary.

Congreve rockets were highly successful inbattle. They were used by British ships to poundFort McHenry in the 1812 war against the USA, andthis inspired Francis Scott Key to write the line ‘therockets’ red glare,’ in his poem that later became‘The Star-Spangled Banner’ — the US nationalanthem.

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INTO THE AIR

There are four main kinds of fireworks: rockets,roman candles, fountains and shells, but there are lit-erally hundreds of variations of each type.

Rockets have been made for a few hundred years,not only for pleasure but also as weapons (Box 2).The gas produced in the rocket motor issues from theend with great force and the flight of the rocket isguided with a stick, fins or additional stabilising jets(see Figure 1). The payload of stars is ejected at theapex of their flight, but rockets cannot carry largepayloads.

Shells are packages of paper — either compressedpapier-mâché spheres or paper cylinders bound withcord (see Figure 2). They are propelled into the air by a lifting charge of gunpowder. The mortars

from which shells are fired can be made of paper,polyethene or fibreglass. In earlier times steel wasused, but this is dangerous if the mortar bursts. Inabout 1926, Aoki in Japan developed the modernchrysanthemum shell which has produced some ofthe most innovative shell patterns in recent years.

The roman candle is a very English kind of fire-work and consists of a long narrow tube. Stars orsmall bombettes are fired out of the tube at intervals,using some form of delay mechanism between eachshot.

SETTING OFF

Large firework displays require a lot of skilland time to set up. Many displays are nowfired with electric matches which areexpensive, but allow multiple firing along-side pyrotechnic delay systems. Computer-fired systems are also available at consider-able expense but they do not always workwell in poor weather conditions. A good,experienced operator can fire perfectly wellby hand without electricity.

Fireworks continue to give pleasure tomany people, whether in large public displays or your own back garden. As theyare partly based on good old blackpowderit is doubtful whether modern science willbe able to enhance them further — but onlytime will tell.

The Reverend Ronald Lancaster taught chemistryand religious studies for many years and is now man-aging director of Kimbolton Fireworks, the UK’sprincipal manufacturer of display fireworks.

3November 2002

Fuel

Before ignition

After ignition

Casing

Figure 1 How a rocket works.

Bursting charge

Black powder

Stars

Fuse

Figure 2 Thestructure of asimple shell.

BOX 3 SAFETY

All fireworks are potentially dangerous. They mustbe handled only as instructed on the packaging. Noattempt should be made to ‘investigate’ or disman-tle any firework. Do not attempt to make your ownfireworks — it is dangerous and illegal.

Above: KimboltonFireworks settingup a display on theThames.

Below: TheReverend RonaldLancaster, withsome of hisproducts.

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In the last Improve Your Grade we looked at revi-sion. Once you have learned your work thor-oughly the battle is half won. This time we deal

with how to get maximum marks for the answersyou write. Exam technique is all about understand-ing what is required in an answer. The examinationpaper itself gives you clues about what, and howmuch, you should write.

TIMING

Examiners don’t just sling together the first 20 ques-tions they come across. They construct a paper withthe right number and length of questions. Thisshould allow you a few minutes to read the papercarefully before beginning, time to write appropriateanswers to all the questions on the paper and a littletime left over to check your answers.

If you often run out of time in exams you need toask yourself if you are trying to put too much intoyour answers. You may need to be more selective.Instead of ‘carpet-bombing’ the examiner with everyfact you know, choose just the information that ispertinent to the question. You may be penalised ifyou include irrelevant material — and you haven’tthe time to write it. Don’t make the same point intwo different ways, and don’t rewrite the question.

If you finish your paper a good while before theend of the exam:• you are very knowledgeable and write concisely

— and there are a lot of very good studentsaround; or

• you haven’t written enough in your answers —read the advice in the next section; or

• you haven’t learnt enough to write a properanswer — in which case get on with it now!

HOW MUCH SHOULD YOU WRITE?

Questions carry between 1 and 4 marks. Thenumber of marks is a good guide to the number ofpoints you should make in your answer, and whetheryou should be writing a sentence or a paragraph.

WHY ISN’T THERE ENOUGH SPACEFOR MY ANSWER?

The space given for answers is enough for someonewith average handwriting to write a full answer —and have a little bit of space left. Bigger handwritingwill just about fit. If you have to write in the spaceunderneath or in the margin then you are goingwrong. Students often waste time and space byrepeating or rephrasing the question. You areencouraged to do this when answering homeworkquestions because it gives you more informationwhen you look back through your exercise book.However you don’t have the space or time to do thisin the exam — just go straight into the answer.Example 1 shows a better start to an answer.

SHORT ANSWERS

Some questions are very straightforward and requirejust one word or a phrase as an answer, for example:

4 Catalyst

IMPROVEyour grade I knew the answer

so why did I only get 1 mark?

EXAMPLE 1

Q Bile is a juice that helps to digest food in thesmall intestine. Explain how. (3 marks)

A1 Bile helps digest food in the small intestinebecause…

A better start would be:

A2 Bile breaks down the food to give it a largersurface area.

Q Name one organism in this food web which is botha primary consumer and a secondary consumer.

(1 mark)A Water fleas.

But if the question includes words such as ‘describe’or ‘explain’ you must answer in whole sentences.Example 2 shows you how.

YOUR ANSWER DIDN’T QUITE MAKE IT

Each exam paper has questions that require you togive fuller answers with reasons. To get the marksyou may need to make two points and give expla-nations for both, for example:

Q Explain how walls made of concrete blocks help tokeep a room warm. (2 marks)

Your answer should explain that concrete blockshave trapped air bubbles and that air is a good insu-lator that reduces heat transfer through the block, orthat the air is trapped and that this reduces heat lostby convection.

A lot of students write factually-correct informa-tion but fail to get full marks because their answersare too general, include irrelevant material or missout a crucial phrase.

For example, in a question about changes in therate of reaction and production of gas when a fixed

amount of acid reacts with plenty of limestone, youranswer must refer to changes in the frequency of collisions, because this is what the rate is. Example 3 illustrates this. Example 4 looks like agreat answer from a good student — but it only gets 2 out of 3 marks. Read the commentary on theanswer.

Don’t forget, when you are asked to do a calcula-tion always write down your workings, even if youare using a calculator. You can get a mark for correctprocedure even if you make an arithmetical error.

Finally, 3 and 4 mark questions require you towrite a paragraph making several points. With thesesorts of questions you may be able to make five orsix points, but it is better to cover four properly thanjust to list things. These questions cover broad topicssuch as the effects of acid rain on the environment,the nitrogen cycle, the effects of pesticide use on foodwebs or insulation in a house.

Though you may be tempted to write an essay,don’t. You have several lines so you need to makeeach point in one and a half lines.

Jane Taylor teaches biology, writes textbooks andedits CATALYST.

5November 2002

EXAMPLE 2

Q Describe one way in which the dolphin isadapted for life in the water. (1 mark)

A1 Its shape. (0 marks)

A2 A dolphin has a smooth streamlined shapethat allows it to move rapidly through thewater with least resistance. (1 mark)

Notice that in answers like these it’s not enough tosay ‘it has a streamlined shape’; you need to add thereason why this makes it better.

EXAMPLE 3

Q (refers to a graph of amount of gas producedas limestone reacts with acid) Explain why therate of reaction changes during theexperiment. (3 marks)

A1 It slows because there is less acid.(1 mark for ‘slows’)

A2 The rate slows down because the concentra-tion of acid decreases during the reaction andthere are fewer collisions per second.

(3 marks for ‘slows’, ‘acid concentration decreases’, ‘fewer collisions per second’)

EXAMPLE 4

Q Bile is a juice that helps to digest food in the small intestine. Explain how. (3 marks)

A Bile breaks down the food so it has a larger surface area. It is easilydigested then. It breaks down fat. It is made in the liver and is stored inthe gall bladder. Enzymes break down large molecules to smallermolecules.

Let’s look at this answer from the examiner’s view point.

Bile breaks down the food so it has a larger surface area (1 mark given for’increasing surface area‘). It is easily digested then (no explanation of why— it allows enzymes to work on the fat molecules — so no mark given). Itbreaks down fat (1 mark given for the idea of helping to break down fats).It is made in the liver and is stored in the gall bladder (true but not relevantto the question so no mark). Enzymes break down large molecules tosmaller molecules (true but bile is not an enzyme — no mark).

This candidate could easily have scored another mark by mentioning that bileemulsifies fats, or that it neutralises acids from the stomach.

EXAMPLE 5

Use what you have learned to decide which is the betteranswer.

Q Describe the type of orbit used by weather satellites,and explain why this type is used. (3 marks)

A1 A polar orbit is used, so that the satellite observes differ-ent areas as the Earth rotates. A low orbit is used, so thatthe satellite can see more detail.

A2 The satellite is in a low orbit that takes it over the poles,so it gets a better view of all of the Earth’s surface.

6 Catalyst

DAVID SANG Where on Earth?GCSE key wordsSatellites

Orbits

Electromagneticwaves

Digital signals

Spacecraft move around the Earth in many

different orbits, according to their purpose.

In this article, we look at one use of satellites

— navigation.

An artist’simpressionof a GPSsatellite.

In a flat, rectangular field near Bognor Regis inSussex, a tractor moves steadily up and down,spreading fertiliser. But this is no ordinary tractor.

For a start, there is no driver. The vehicle is con-trolled by an onboard computer which uses satellitetechnology to determine its position in the field.

The computer has a detailed record of the cropyield from every square metre of this field lastautumn; where the yield was poor, extra fertiliser isadded to the soil.

The tractor’s onboard computer relies on theglobal positioning system (GPS), a navigation systemoriginally set up by the United States Department ofDefense but now more widely available. If you gosailing, you may use a GPS receiver for navigating,an increasing number of mobile phones have GPSbuilt in, and so do many cars.

THE GPS SATELLITES

The GPS ‘constellation’ is made up of 24 satellites.They occupy six orbits around the Earth, with foursatellites spaced around each orbit (Figure 2).• Orbit radius: 20 190 km — about three times the

radius of the Earth.• Orbital period: about 12 hours, so each satellite

orbits the Earth twice a day.• Orbital tilt: 55° relative to the plane of the

equator.The satellites are spaced out in this way so that,wherever you are on the Earth’s surface, you will bein ‘line-of-sight’ of at least four satellites at anymoment; in other words, your GPS receiver will bereceiving at least four signals.

GPS SIGNALS

Each of the 24 satellites transmits a signal towardsthe Earth. This is a digital signal — it consists of asequence of on–off pulses of different lengths (Figure1). The signal encodes a range of information,including the identity of the satellite and the time.The receiver can decode this information to workout the user’s position on the Earth.

Signals are transmitted at two different frequen-cies, L1 = 1575.42 MHz, and L2 = 1227.6 MHz.G

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7November 2002

GPS satellites arenot positioned in a geostationaryorbit, alongsidetelecommunicationsand televisionsatellites, becausethis orbit cannot be seen from polarregions.

The EuropeanSpace Agency plans to establish asecond satellitenavigation system.

Today’s GPSsystems aretypically accurate to within a metreor so; for surveyingpurposes, they canbe accurate towithin 1 cm.

1 MHz= 1 megahertz= 106 Hz.

Kwajalein

Diego GarciaAscension Island

Hawaii

Falcon AFB Colorado Springs

Master control station Monitor station

Figure 3 Thecentral GPS controlstation is at FalconAir Force Base,Colorado Springs.The satellites aremonitored fromfour other stationsaround the world.

These frequencies are in the microwave region ofthe electromagnetic spectrum, with a wavelength of 20 or 30 cm. The wavelength must be as short asthis if a position is to be determined to within ametre or so.

Two frequencies are used because microwaves areslowed down as they pass through the ionosphere, acharged region high in the atmosphere, and thisintroduces an error into the measurements. Differentfrequencies are slowed down by different amounts,just as violet light is slowed down more than redlight as it passes through glass; this is an example ofdispersion. Expensive GPS systems can compare thetwo signals and correct for the error. Most civilianreceivers use only the L1 frequency.

CALCULATING POSITION

If you have a GPS receiver, how can it use the fouror more signals reaching it to work out your preciselocation? Here is an analogy, using sound.

Imagine that you are in the countryside, sur-rounded by mountains. Three of the mountains havea cannon on the summit. At midday, each cannonfires a blank. What do you hear?

If you are the same distance from all three moun-tains, all three bangs will reach your ears simultane-ously. But elsewhere, you are likely to hear threebangs in succession. If you look at your watch, youwill be able to determine how many seconds aftermidday each bang arrives. Knowing the speed ofsound, you can work out your distance from eachmountain, and find your position. (This technique isknown as triangulation; you have probably comeacross it in geography or maths — see Figure 4.)

The GPS system works in the same way, but usingradio waves. The signals are transmitted continu-ously, not just at midday. Of course, in the soundanalogy, you need to know which bang has comefrom which cannon. In the case of GPS, each signalencodes the details of the satellite which transmits it,together with the time.

The advantage of having signals from at least foursatellites is that the receiver’s computer can workout four things: latitude, longitude and altitude(equivalent to x, y and z coordinates), and the precisetime.

Figure 2Satellite orbits.

WAVELENGTH OF L1 WAVES

wavelength = speed of light/frequency = 3 x 108 m/s________________

1575.42 x 106 Hz = 0.19 m

Data signal (digital)

Carrier wave

Figure 1 The digital GPS signal transmits informationat a slow rate — about 50 bits per second.

8 Catalyst

Figure 4How triangulationworks.

In addition, your own speed can also be found,using the Doppler effect. This is the change in fre-quency which occurs when a source of waves and thereceiver move relative to one another.

USING GPS

Aircraft and shipping make great use of GPS for nav-igation. In the past, they used a system called dead-reckoning — by monitoring speed, direction andtime taken, they could plot their routes on a map.Usually, they had to travel in this way from oneradio beacon to the next. Now, an aircraft usingGPS can follow a great circle around the Earth, theshortest distance between two points.

The US government is insisting that all mobilephones should include GPS. This will mean that therescue services will be able to pinpoint the positionof anyone making an emergency phone call. Somepeople suspect that it will also make it possible totrack anyone carrying a phone — this is seen as aninvasion of personal privacy.

Cars may combine a GPS receiver with an on-board geographical information system (GIS). TheGIS is a computer with details of road networks,tourist information and so on, which can alertdrivers to useful information as they drive around. Aspecialised GPS device can warn drivers of high-sidedvehicles when they are approaching low bridges;another can warn drivers when they are nearing aspeed camera, so that they will slow down. Is this aform of cheating?

David Sang writes textbooks and is an editor of CATALYST.

Sailor Ellen MacArthur.GPS is an important aidto navigation.

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9November 2002

You may have wondered why the real-life

food chains you have studied aren’t very long.

In this article we show how the loss of energy

from food chains limits the number of animals

that can survive on the energy fixed by a patch

of vegetation.

The Earth’s outer atmosphere is irradiated byhuge amounts of energy from the Sun. Theatmosphere reflects and absorbs some of this

energy, but most reaches the Earth’s surface. Energyenters food chains only when sunlight is used inphotosynthesis.

Over 3 million kilojoules of visible light energyreaches each square metre of ground in the UK in ayear. Relatively little of this kick-starts food chainsthrough the process of photosynthesis. Some isreflected by bare soil and some is absorbed, heatingthe soil — think of walking barefoot on a dry sandybeach on a sunny day. Plants can benefit from thewarming of soil because it makes them grow faster.About half of the light energy is used evaporatingwater from moist soil; this is what dries the groundout after rain.

Plant leaves reflect and transmit green and yellowlight — which is why they look green — so thisenergy is lost too. They absorb red and blue wave-lengths of light and use them for photosynthesis.

Of each thousand kilojoules reaching the Earth’ssurface less than 25 kJ/m2 is used for photosynthesis.This small amount is what ecosystems are built on.

PLANTS AS FIXERS

Only a portion of the sugar made in photosynthe-sis contributes to the increase in biomass. Some ofit is used in respiration as a source of energy todrive the synthesis of other substances the plantneeds in its growth. The energy transformationsinvolved are not 100% efficient so energy is alsolost to the environment as heat (Figure 1). Only themolecules used in tissues, for example sugars usedto make cellulose for new cell walls, proteins forcytoplasm and lipids for waterproofing the leaves,contribute to the biomass of plants. This biomassamounts to 1–5% of the light energy received persquare metre and is the source of energy for primaryconsumers.

l Why do plantsgrow faster inwarmer ground?Hint: think aboutrates of chemicalreactions.

JANE TAYLOR

Energy transferGCSE key wordsEfficiency ofenergy transfer

Heat

Respiration

Egestion

Trophic level

sugar + oxygen carbon dioxide + water

Figure 1 Energy is mostly lost as a result ofrespiration.

Above: Food websare very complex. It is not only largeherbivores that eatplants.

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CONSUMING EXPERIENCES

A herbivore — a primary consumer — browses onvegetation. Of course the energy in the parts of theplant it does not eat is lost to that food chain, but itmay enter another. The roots or remnants of leavesdamaged by large herbivores may be eaten by othersmall herbivores such as slugs or millipedes.

Food webs are very complex. Ecologists often con-sider energy transfer between trophic levels ratherthan energy passing down a food chain. Theymeasure the energy per square metre fixed byprimary producers, how much energy passes to the

primary consumers grazing on that area of vegeta-tion, how much then passes on to secondary con-sumers, and so on. It is very difficult to accuratelymeasure and portray energy transfer becauseanimals’ feeding habits can be complex (see Box 2).

Quite a lot of the energy in the biomass an animalconsumes is not available for growth. For example,cellulose, the polymer of glucose making up plantcell walls, is by far the commonest substance in veg-etation, but few herbivores can digest it and use itsenergy. Even animals that have microbes in their gutto digest cellulose, such as cows, cannot make use ofall of it. Most cellulose is egested as faeces. However

10 Catalyst

Heat generated bythe respiration of

microorganismscan cause a fire ina haystack or kill

weed seeds insidea compost heap.

Detritus includesall fragments of

living things, suchas dead leaves, fur,

remnants ofcarcass and solid

waste — the foodof detritovores.

Primary producer

Primary consumer

Secondary consumer

LightTransmitted light

Some plants consumed

energy

Energy in leaf litter and other dead plant parts

Leaves heated

Detritus contains energy used by detritovores and

decomposers

Energy in faeces, urine and dead

primary consumers

See Figure 3

Respiration including heat loss

Energy in faeces, urine and dead

secondary consumers

Energy in parts of some rabbits

consumed

Reflected light

Figure 2 Energy flow in a food chain.

Detritus Detritovores

Energy in faeces and dead

detritovores etc.

Decomposers Bacteria and

fungi

Carnivores

Respiration including heat loss

From Figure 2

Figure 3 What happens to the energy in detritus.

BOX 1 DIFFICULTIES MEASURINGENERGY TRANSFER

It is difficult to measure how much of the energypresent in 1 m2 of grassland passes into each trophiclevel above it. We can quantify the energy transferin only the simplest systems with a few organismsand few levels. Such simple systems are rare, and sothere are few fully-investigated examples.

Food chains are simplified versions of the real-life situation in which predators such as starlings eatseveral prey organisms and so link several foodchains — which change as they migrate acrossEurope each year. Field mice fall victim to many dif-ferent predators, herbivores browse on differentfood plants in different places. It does not get easierif scientists attempt to quantify energy transfer in afood web, as many predators and scavengers such asfoxes feed on primary and secondary consumers aswell as taking the occasional fruit meal.

microbes in the soil — fungi and bacteria — candegrade cellulose in dead plant material and herbi-vore faeces to release organic compounds. They canabsorb and use these for their own respiration andgrowth. In addition, animals do not use all the nutri-ents absorbed from the gut, and some energy is lostas urea in urine.

RESPIRATION LOSSES

Animals use sugars in respiration to provide energyfor movement and metabolism; again, the energytransformations are not 100% efficient so energy islost to the environment as heat (Figure 1). Mammalsand birds use most of their energy intake to maintaina steady body temperature.

Less than 10% of the energy taken in ends up innew biomass. This biomass is the source of energyfor organisms at the next trophic level in the chain.

As Figures 2 and 3 show, the diminishing proportionof the energy originally stored in plants that is trans-ferred on through the chain severely reduces thequantity of energy available for the animals in thenext level.

As the animals at the next level use energy insimilar ways, there is a similar reduction in theenergy fixed as biomass. Predators take the mosteasily accessible energy-rich parts of their prey, suchas the liver, and leave less digestible parts such as fur,bone and feathers. However these are a food sourcefor scavengers or detritovores (Figure 3).

By the time energy has been passed from plants tosecondary consumers there is very little left for topcarnivores. A family of carnivores needs a huge ter-ritory of vegetation to support it (see Box 3).

Jane Taylor teaches biology, writes textbooks andedits CATALYST.

11November 2002

It has beenestimated that ahuman needs eighttimes as muchenergy as acrocodile of thesame size. This isbecause humansneed to maintain aconstant bodytemperature,normally abovethat of theirsurroundings.

l Try to work outthe size of territorya domestic catwould need if ithad to fend foritself.

BOX 3 CARRYING CAPACITY

The productivity of an area of vegetation (that isenergy fixed per square metre per year) limits howmany animals can live there. The total energy fixedeach year in plant biomass in an area can feed manydifferent herbivores, but there is a limit to the totalanimal mass it can support. If there are too manyherbivores the vegetation will be grazed faster thanit grows and will not be able to flower, set seed andreplace itself.

The number, or biomass, of herbivores that canbe maintained in an area is called its carrying capacity. There are also limits to the number ofpredators that can live on the herbivores.

This information is important to people manag-ing areas of land. Farmers need to know, forexample, how many sheep can be satisfactorilygrazed on a given size of farm. Similarly, game parkmanagers have to know how many elephants cangraze in the park. A growing population risks dam-aging the vegetation so badly that the whole systembecomes unbalanced.How does a howler monkey

choose which leaves to eat?

How many elephants can a game park support?

Ferr

ero

/Lab

at/A

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M. W

atso

n/A

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BOX 2 FORAGING STRATEGY

Animals make life complicated for ecologists tryingto estimate who eats how much. Errors creep inbecause animals do not forage evenly within a habi-tat, they are choosy about what and where they eat.They have to make important decisions when forag-ing, and choosing the wrong strategy may impairtheir chances of survival.

Howler monkeys eat leaves but may completelyignore those of nearby trees and search for the raretrees whose leaves contain less of a toxic chemical.They eat just the youngest leaves with the leasttoxin and fibre. They will select individual matureleaves but only if they have a high protein content.A ‘snapshot’ visit by an ecologist estimating leavesconsumed may not accurately reflect the situation.

Animals forage away from their usual territoryfrom time to time if it makes sense in terms ofenergy lost and gained. A starling will ignore smallsoil invertebrates where it usually feeds and useslightly more energy to travel further for largerenergy-rich leatherjackets. Again this upsets theenergy figures.

12 Catalyst

Visit the LIFEcentre’s website

http://www.centre-for-life.co.uk

PCR stands forpolymerase chain

reaction, a processused to make lotsof extra copies of

pieces of DNA. It isused in research on

DNA and byforensic scientists.

Currentexhibitions:

weirdscience@life(November 2002)and skating@life

(December–January).

Teachers’ contactfor education visits

0191 243 8211.

P L A C E St o v i s i t

The International

Centre

The LIFE visitor attraction gives you a chanceto find out more about evolution, cells, genetics and neuroscience. These are the

sciences which are likely to have the same impact in the early twenty-first century as computers had at the end of the last century.

LIFE builds on our fascination as human beingswith ourselves — our origins, our makeup, our rela-tionships and our capabilities. It has three themes:• What is life and how does it work?• What makes life for you and me?• Celebrating life.DNA is the thread that holds this all together. Itconnects us with our ancestors and with each other.At the same time, it makes each one of us different— unless you are identical twins!

Visits start with a journey along the River of Life,back through 4 billion years of evolutionary history.Then there are theatre shows including a motionsimulator, and interactive exhibits devoted to sport,diet, creativity and the workings of the brain.

HANDS-ON IN THE LAB

Ever looked at some DNA — your own, even? If youvisit the LIFE centre with a school party, you willhave the chance to work in the Senior LIFElab along-side students, researchers and professional scientists.You might try such things as using DNA analysis toidentify species collected from the rainforests andfind out if they are new to science. You might alsotry DNA photocopying, in which you can collect,extract, ‘amplify’ and visualise your own DNA usingthe latest techniques of PCR and electrophoresis.

FAST FORWARD — TOO FAST?

Perhaps you think that the science and technology ofgenetics are getting out of hand. Well, the LIFEcentre is also home to a research institute looking atthe ethics of life sciences. Workers look at the ethicalissues raised by developments in genetics and humanbiology, and they find ways to involve members of

From time to time, CATALYST brings you news about scientific places to visit. Here’s news of an

attraction in the north of England, at Newcastle-upon-Tyne.

the public in debating these issues. At the same time,they make sure that scientific researchers are awareof public concerns.

Through the LIFE visitor attraction, you can addyour voice to this work.

REGENERATION

The International Centre for Life is a dramatic devel-opment in the heart of Newcastle-upon-Tyne. A run-down site which was previously a bowling green,army barracks, hospital, air raid bunker, livestockmarket, abattoir and timber mills has been trans-formed. As well as the visitor centre, hands-on labsand ethics centre, it houses the Institute of HumanGenetics (part of Newcastle University) and the Bio-science Centre, a specialist space for small biotech-nology companies. Come and take a look.

13November 2002

GETTING THERE

The International Centre for Life is in Newcastle CityCentre, next to the city’s mainline railway stationand 2 minutes from Newcastle Arena. See Figure 1.

P

The Close

River Tyne

Grey

St.

Dean

St.

A167

A167

Ble

nh

eim

St.

Mar

lbor

ough

Cr.

Westgate Road

Neville St.

Railway St.

Forth St.

✝

High

Level BridgeTyne

Bridge

Railway station

Life

Figure 1 How to get there.Li

fe

Do your grandparents ever complain, ‘You

don’t have to suffer the really cold winters we

had when we were young’? If so, they are

right. Records began 350 years ago, and nine

of the warmest years recorded have been since

1990. You might think warmer temperatures

are a good thing, but climate change has far-

reaching implications. You need to know

about the impacts we have on our global

environment for your GCSE course. This article

explores climate change in detail.

Climate is defined as the prevailing conditionsof factors such as temperature, humidity,rainfall and wind in a region. The UK is in the

temperate zone, one that does not exhibit extremesof heat or cold.

Animals and plants have annual cycles of growthand reproduction. Phenology is the study of thetimes each year that certain life-cycle events occur. It

involves recording things like when you heard thefirst cuckoo or saw the first frogspawn, hawthorntrees in blossom, blackberries fruiting or leaveschanging colour. If the climate is changing, thetiming of these events might change as well.

CLIMATE IS CHANGING

The debate among scientists now is not whetherclimate change will happen but how fast it willoccur. The United Nations’ IPCC (Intergovern-mental Panel for Climate Change) predicts warmingof 2.4–5.8°C over the next century. Nine of the tenwarmest years since temperature records began inBritain, in 1659, occurred in the 1990s. The warmestyear on record was 1998, the second warmest was2001 and the Meteorological Office predicts a 75%probability that 2002 will be warmer than 2001.January 2002 was the warmest January for 9 years(the average temperature of 5.8°C was even warmerthan the average of 5.2°C in 1998), February 2002was 3.2°C warmer than the 30-year average, andMarch was 1.8°C above the average.

14 Catalyst

GCSE key skillsand conceptsConsidering ideasand evidence

The power andlimitations ofscience,uncertainties inscientificknowledge

Patterns andrelationships indata

The impact ofhumans on theenvironment

Interrelationshipsof organisms

MAX DE BOO

Above: The firsthorse chestnutleaves, bluebells,and woodanemone flowersare all signs ofspring — but arethey appearingearlier?

Effects of climate change

Mar

gar

etB

arto

n/T

he

Wo

od

lan

dTr

ust

RECORDING CHANGE

Over the centuries both scientists and non-scientistshave kept records of natural events. In 1736, RobertMarsham began recording 27 indications of springin the UK and we have recently found a few evenolder records dating back to 1703. When correlatedwith temperature these phenological records showhow nature is responding to a changing climate. TheMarsham family continued recording until 1958. Anationally coordinated scheme ran from 1875 to1947 and the UK Phenology Network (UKPN)began in 1998.

In autumn 2000 UKPN had 350 recorders. Twoyears on there are more than 14,000 recorders spreadacross the UK, mostly working over the internet.Although recordings of natural events by the publicare sometimes dismissed as unscientific, the UKPNrecords are producing some important results whichare statistically significant. When a large number ofpeople are recording, the data as a whole becomecredible because anomalous records can be spotted.

Phenology is taken very seriously. The IPCC seesit as legitimate research into climate change and phe-nological events are now part of the UK govern-ment’s Climate Change Indicators.

INDICATORS OF CHANGE

Chemical reactions, including those of respirationand growth, speed up with increased temperature.The UK Phenology Network has found that for each1°C rise in temperature, events happen on average6–8 days earlier in spring!

Earlier this year we predicted, based on tempera-tures and phenological data, that spring 2002 wouldbe 2–2.5 weeks earlier than the norm and that sea-sonal changes in autumn 2002 would be delayed orprolonged.

IMPLICATIONS FOR WILDLIFE

Let’s consider a woodland. Organisms in woodlandcommunities are interdependent, not only in the wayrepresented in food webs, but also in the timings oftheir life cycles. For example, herbivorous insectsneed to hatch from eggs when their food plant is

available. If this breaks down as climate patternschange, feeding relationships will be disrupted.

At present, insects seem to be responding to tem-perature changes at the same rate as their foodplants. Orange tip butterflies are active and egg-laying earlier; but garlic mustard, their food plant, isgrowing sooner. The same is happening with wintermoths and oak trees. However, there is evidence thatthe hatching of blue tits may no longer coincide withthe peak of caterpillar numbers in a wood in Oxford-shire, and the arrival dates of some summer migrantbirds are already lagging behind the rest of thewoodland community.

Climate change may also affect woodland compo-sition. Trees such as sycamore and oak are respond-ing more quickly to climate change than, for example,beech and ash. Bluebells and snowdrops may lose the competitive advantage gained by starting growth

15November 2002

At the end of 2001,observers notedvery intenseautumn colours ontrees. Warmerweather and alonger growingseason increasethe concentrationof sugars in theleaves. At the endof the season, thechlorophyll breaksdown, revealingthe otherpigments,particularlycarotene andanthocyanin, bothof which are moreintense withincreased sugarconcentrations.

Mea

nan

nu

alte

mp

erat

ure

(°C

)

Year

8

1900 1950 2000

9

10

11

Figure 1 Graph showing temperature change overthe last 100 years. Predict the rise in temperatureover the next 20 years if the same trend continues.

BOX 2 CAUSES OF CLIMATE CHANGE

Some variation in climate is natural, but the primarycause of climate change is the increase in carbondioxide concentration in the atmosphere. Levels ofthis gas were relatively stable for thousands of yearsbut have increased by 31% since 1750. Burning offossil fuels (industry, domestic use, vehicles on theroad) has contributed 75% of this increase, whiledeforestation accounts for most of the remainder.

Carbon dioxide is quantitatively the most signifi-cant contributor to global warming but some othergases play a part. These include methane (CH4),nitrous oxide (N2O), hydrofluorocarbons, perfluoro-carbons and sulphur hexafluoride (SF6).

The USA releases most of the global carbon diox-ide (85% of all emissions).

BOX 1 REGISTERING AS A RECORDER

Wherever you live in the UK you can join the Wood-land Trust’s network of recorders. Connect toNature’s Calendar at http://www.phenology.org.uk.You will get help online and offline to identifyspecies for observation. It is easy to record some nat-ural events — for example the first snowdrops, thefirst leaves on oak trees, and autumn events such asblackberry fruits and leaf fall.

in the previous autumn, as leaf growth of many otherplants now begins earlier in the spring.

Ancient woodland is by far the richest habitat forwildlife in the UK. Such woods are often small andisolated — surrounded by intensively-managed land.A rise in annual temperatures will mean that thosespecies that can will spread north to a more suitableclimate. Many of the species found in ancient wood-land, such as lichens, fungi and invertebrates, arevery immobile, rare and threatened. Effectively theywill be locked into these small wildlife ‘islands’.

IMPLICATIONS FOR PEOPLE

Winters will be warmer, wetter and increasinglyfrost-free. A longer growing season can be useful forfarmers, although other factors, such as flooding,the survival of pests through the winter and thechance for them to produce more generations in oneyear, might counteract these benefits. Temperate

plants often rely on winter chills to break dormancy.Research is being done on this at the moment, forexample investigating the potential impact on yieldsof fruits such as blackcurrants. Longer growingseasons might even mean longer pollen seasons andmore hay fever.

WHAT CAN WE DO?

All the major carbon dioxide emitting countries (EUstates, Japan and the US), signed the Kyoto Protocolof 1997. This committed them to reducing overallemissions of greenhouse gases by at least 5% of the1990 levels by 2005. When the protocol was due tobe ratified in 2001, one country withdrew its agree-ment to cut back emissions (see Box 3).

In Britain, from April 2002, businesses will beliable to the Climate Change Levy (adding 15% totypical energy bills) but discounts will be awarded tothose who cut their energy use.

There are ways that we as individuals can reducecarbon dioxide emissions: • use the car less — walk, cycle or use public trans-

port instead;• make sure our homes are well insulated;• install energy-saving devices;• avoid using energy-consuming equipment like air

conditioning unnecessarily — open a windowinstead.

And why not contribute to research on the localeffects of global warming by joining the UK Phenol-ogy Network (Box 1)?

WEBSITES

The UK Phenology Network http://www.phenology.

org.uk

Eco Schools http://www.eco-schools.org.uk promotesschool and community involvement in improvingthe environment, citizenship and healthy lifestyles.

http://www.greencode.org.uk for schools wishing toinclude education for sustainable development inthe curriculum.

The Met Office http://www.meto.gov.uk/research/

hadleycentre/index.html

Friends of the Earth http://www.foe.co.uk

Green Energy Options http://www.greenenergy.org.uk

Government energy projections for the UKhttp://www.carboncalculator.org/faq.html

Max de Boo has taught science for many years, inprimary and secondary schools and in teacher edu-cation. She is grateful to Nick Collinson (Conserva-tion Policy Advisor, The Woodland Trust/UKPhenology Network), Tim Sparks (Centre forEcology and Hydrology/UK Phenology Network)and Jill Attenborough (Phenology Project Manager,The Woodland Trust UK).

16 Catalyst

BOX 3 ACTIVITIES

(1) Try this at home. Can you find out how muchelectricity is used per week by:• a computer used for 6 hours per day?• all the LEDs (cooker, VCR, televison) left onpermanently in your home?

(2) Predict the increases/decreases in the animal andplant populations if blue tits hatch later thancaterpillars:• to the caterpillars. • to the birds.• to green plants. • to other wildlife.

(3) Can you find out which country abandoned itscommitment to the Kyoto Protocol? Clue: It isthe country producing the most carbon dioxideemissions.

Above: Firstsightings of

frogspawn aroundthe British Isles are

recorded by the UKPhenology Network

each year.Ia

nB

eam

er/A

rdea

17November 2002

For this spectacular trick, you will need bouncyballs of three different sizes. Hold them at arm’slength, with the medium-sized ball on top of

the biggest one, and the smallest on top of that. (Likeall good tricks, this one is tricky to perform.)

Now, release the balls so that they fall downwards.When they hit the ground, you should find that thesmallest ball ricochets upwards and reaches a sur-prising height.

HOW IT WORKS

All three balls fall together at the same rate — that’ssomething which Galileo explained.

What happens when the big ball strikes theground? It bounces back upwards, imparting a shockto the medium sized ball, which in turn exerts a forceon the small ball.

Think about what happens when a larger objectcollides with a smaller one — a tennis racket with atennis ball, for example. Each feels the same force(that’s Newton’s third law), but the force will havea bigger effect on the object with the smaller mass.So the medium ball is accelerated by the force from

the large ball, andthe small ball is acceler-ated even more by the forcefrom the medium ball.

AN IMPROVED MODEL

Try fixing a wire or rod so itstands up vertically. Makeholes through the balls so thatthey will slide freely up anddown the wire or rod. Nowhold the balls at the topof the rod and let go as before. They will beguided by the rod — youcan even try the trick with four balls. The smallestball should fly at least 10 metres into the air.

This may seem like a trivial experiment, but ascaled-up version was once suggested as a way ofsending a simple spacecraft into orbit.

David Sang writes textbooks and is an editor ofCATALYST.

T R Yt h i s

Figure 1 Hold thethree balls on topof each otherbefore letting go.

Unscramble the following anagrams of substances used in the manufacture of fireworks.All the words can be found in the ‘Fireworks’ article on pages 1–3.

1 grow up, Den (9)2 resist amputation (9, 7)3 arch cola (8)4 thor spots primulaceae (9, 11)5 amusing me (9)6 CRT rush demolition (9, 8)

7 ah, cells (7)8 miniature Bart (6, 7)9 precooled chirp (6, 8)10 go mail manually (9, 5)

Answers on page 21

Fireworks quiz

Spheresin space

Co

rel

18 Catalyst

The website of theNobel Foundation

is athttp://www.nobel.se

l Find out aboutthe thalidomide

disaster of the1960s. Try entering

the word in asearch engine.

What link is therewith the Nobelchemistry prize

for 2001?

A L I F Ein science

Alfred Nobel was born in Stockholm on 21 October 1833. His father was aninventor and engineer, his mother came

from a wealthy family. In 1842 the family moved toSt Petersburg in Russia, where Alfred was educatedby private tutors. By the age of 17 Alfred was fluentin Swedish, Russian, French, English and Germanas well as being interested in literature and poetry,and chemistry and physics.

EXPLOSIVES

He travelled widely and it was on his travels thathe became interested in nitroglycerine. This highlyexplosive liquid was thought to be too dangerousto be of any practical use. Alfred returned toStockholm and started experimenting. His attemptsto make nitroglycerine safer to use resulted inmany accidents — in one explosion his brotherEmil and several other people were killed. Alfredwas soon banned by the authorities from experi-menting within the city limits and had to performall his experiments on a barge anchored in themiddle of a lake.

Alfred eventually found that mixing the nitro-glycerine with silica made it more stable. Hepatented the new material under the name ofdynamite. Together with detonators, which he alsoinvented, this explosive was soon used worldwidein the mining and construction industries. Within afew years there were 90 factories and laboratoriesin 20 different countries making and experimentingwith dynamite. Nobel liked to travel round hislaboratories. As well as studying explosivestechnology he also worked on making syntheticrubber and leather and artificial silk. He soonbecame immensely wealthy. Many of the companieshe founded have prospered and still play a part inthe world economy today.

Below: Sticks ofdynamite, which is

made fromnitroglycerine

(C3H5(NO3 )3 ) andan inert binding

substance.

Alfred Nobel.

Mar

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Pict

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The Nobel Foundation was 100 years old last year. Every October it awards prizes

for the best workers in the fields of physics, chemistry, medicine, literature,

economics and peace. Who was Alfred Nobel — and who won the prizes in 2001?

Alfred Nobel andthe Nobel prizes

NOBEL PRIZES

Alfred Nobel died in San Remo, Italy on 10December 1896, aged 63. His will decreed that hisfortune should be used to award prizes in physics,chemistry, medicine (or physiology), literature,economics and peace. The Nobel Foundation was set

up to take care of the finances and to award prizeseach year. This work still continues and a Nobelprize is one of the most prestigious awards anyonecan win.

David Moore teaches chemistry at St Edward’sSchool in Oxford and is an editor of CATALYST.

19November 2002

An adult humanbeing hasapproximately 100 000 billioncells, all originatingfrom a singlefertilised egg cell.

l What do wenormally think ofas the three statesof matter?

BOX 1 NOBEL PRIZE-WINNERS OF 2001

The chemistry prize last year was awarded to WilliamKnowles (USA), Ryoji Noyori (Japan) and BarrySharpless (USA) for their work on chiral molecules. Achiral molecule is one that exists in two differentforms — one being a mirror image of the other(think of a pair of gloves). One shape of the moleculemay undergo a particular reaction which its mirrorimage might not. The prize-winning chemists madecatalysts which could create molecules of one mirrorimage and not the other. This is important for someforms of drug synthesis.

The physics prize went to Eric Cornell, WolfgangKetterle and Carl Weiman (USA). They provedexperimentally a new state of matter — one whichhad been thought of as long ago as 1924 but hadnever been seen. It is relatively easy to make theparticles of light all have the same energy and tovibrate the same way (i.e. a laser), but this had notbeen shown to be true for matter. The researchers

managed to cool 2000 rubidium atoms down to0.000 000 02 degrees above absolute zero. At thispoint all the atoms had the lowest energy possibleand condensed together to vibrate in unison. Thetheory had been proved. This may be of use inprecision measurement and nano-technology in thefuture.

The medicine prize was won by Leland Hartwell(USA), Paul Nurse and Tim Hunt (UK) for their workon the cell cycle — the stages through which a cellgoes from one division to the next. Before the cellcan divide it has to duplicate and separate itschromosomes — this is regulated as part of the cellcycle. The researchers were able to identify some ofthe key molecules which control the cycle. Faults inthe cell cycle may result in the abnormalities ofchromosomes seen in cancer cells. It is hoped thatknowledge of how the cycle works may lead to newways of treating cancer.

Above: Sir PaulNurse and Dr TimHunt (right), Britishwinners (withAmerican LelandHartwell) of the2001 Nobel prizefor medicine (see Box 1).

Pop

per

foto

/Reu

ters

Rocket propulsion works by controlling anexplosion. In conventional rockets, the explo-sion is the result of burning fuel. As the fuel is

oxidised, energy is released. The exhaust gases arevery hot, and they push out of the back of the rocket.The effect is a forward thrust on the rocket.

One of the problems with space rockets is thatthey have to take with them their own supply ofoxygen — there’s no oxygen in space. This adds tothe overall load they must carry. One way to over-come this is with the new generation of solar-powered ion-propulsion rockets.

SOLID ROCKET BOOSTERS

Solid rocket boosters (SRBs) are often used as thefirst propulsion system of a rocket. ‘Yes, we haveignition!’ — that’s the SRBs starting to burn. Therocket is bound to lift off, because there’s no way toturn off the SRBs. Within a few minutes, they areburnt out and jettisoned. They are the parts you see

falling to Earth shortly after launch.SRB fuel is often a mixture of nitrocellulose

and nitroglycerine, with an added plasticiser togive the fuel a solid, rubbery consistency.

Space shuttle SRB fuel is a mixture of alu-minium perchlorate (a source of oxygen) andaluminium powder, which burns.

LIQUID FUEL ROCKETS

For a more controlled thrust, rocket engines useliquid hydrogen and liquid oxygen as their fuelmixture. The two liquids are pumped into acombustion chamber, where they are ignited.

20 Catalyst

Take care with thepairs of forces in

Newton’s thirdlaw. They areequal in size

and opposite indirection and, most

importantly, theyact on different

objects.

l A catherinewheel uses the

action-and-reaction principle.

Explain how.

DAVID SANG

BOX 1 ACTION AND REACTION

Blow up a balloon and let go. It flies around the room as the air rushes out. This is an example of Newton’sthird law of motion. As the air is pushed in one direction, the balloon is pushed by an equal force in theopposite direction.

A rocket works in the same way. Hot exhaust gases (or high-velocity xenon ions) are pushed outthrough the nozzles at the rear and the rocket feels an equal and opposite force in the opposite direc-tion. (That’s why both types of rocket have to take something with them to push out backwards — fueland oxygen, or xenon gas.) These forces are often described as an ‘action and reaction pair’.

Here’s another way to think of it. Inside the rocket’s combustion chamber, fuel and oxygen burn.Roughly speaking, half of the resulting molecules fly backwards, and half fly forwards. The moleculesthat fly forwards bounce off the inside of the combustion chamber, exerting a force on it. That’s thephysical origin of the force which pushes the rocket forwards.

Yes, it is rocket scienceThe launch of a space rocket has become a familiar image.

But how do rockets work, and what is the future for rocket science?

Liquid fuel boosters

Solid fuel boosters

Figure 1 Arocket with

solid andliquid fuelboosters.

Artist’s impression of the launch of anAriane 5 rocket. Two large solidrocket boostersflank the mainliquid fuel rocketin the middle.

Dav

idD

ucr

os/

SPL

The temperature may reach 3000ºC. The hotexhaust gases emerge from the back of the rocket atspeeds of up to 5 km/s.

Other liquid fuels include kerosene in place ofhydrogen, and nitrogen peroxide (N2O4) in place ofoxygen.

Most rockets have a sequence of engines to liftthem into space. The first stages are the most pow-erful, with engines providing up to 500 000 newtonsof thrust. The later stages may provide only 20 000newtons.

Liquid hydrogen in

Combustion chamber

Liquid hydrogen cools walls

Exhaust gases (700°C)

Liquid oxygen in

21November 2002

Xenon is a noblegas, like heliumand argon.

Xenon ions areatoms which havelost one electron,leaving thempositively charged.

Ion-propelledmotors may beused for the firstmannedspaceflights toMars.

Above: Whenwalking in space,astronauts usecylinders ofcompressednitrogen to pushthemselves about.The gas is firedthrough 24 nozzles,and is controlled bya joystick.

Figure 2 How aliquid fuelbooster works.

science

SOLAR POWER

Most spacecraft have solar cells to provide electricalpower during their mission in space. This normallyonly powers the on-board instrumentation.

However, the first solar-powered craft are now beingproduced.

The European Space Agency’s SMART-1 missionto the Moon will be launched early in 2003, andwill make use of the latest ion-propelled motor. Thisworks as follows. Solar panels generate electricitywhich has two functions. First, it is used to ionisexenon gas. Second, the xenon ions are accelerated byhigh-voltage electricity, so that they emerge from theback of the motor at a speed of 30 km/s, much fasterthan the exhaust gases of a conventional rocket.

These motors have the great advantage that theydo not require an oxygen supply. They produce onlya tiny thrust — perhaps 70 millinewtons, less than amillionth of the thrust of a giant Ariane rocket.However, they can operate for months on end, grad-ually accelerating a spacecraft to speeds approaching5 km/s (11 000 miles per hour).

David Sang writes textbooks and is an editor ofCATALYST.

ANSWERS TO FIREWORKS QUIZ,PAGE 17

1 gunpowder2 potassium nitrate3 charcoal4 potassium perchlorate5 magnesium

6 strontium chloride7 shellac8 barium nitrate9 copper chloride10 magnalium alloy

Figure 3 An ion-propulsion booster. The solar panel is the ultimate source of energy for an ion-propulsion system. Liquid xenon gas is fed to theioniser, where the atoms are ionised before beingaccelerated by a high voltage. Electrons are also fedinto the beam of ions so that it is neutral and doesnot spread out.

Ioniser

Fluid ‘fuel’

Positive ion

Electron

Neutraliser

Solar panel

Electrons

High voltage generator

Accelerator

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BOX 2 IMPULSE OF A FORCE

A fuel-and-oxygen rocket provides a large force. Anion-propulsion motor provides a much smaller force,but it acts for a long time. A small force acting for along time can have a greater effect than a largeforce acting for a short time.

The quantity force × time is known as the impulseof the force, and tells us how much the rocket’smomentum is changed by the force.

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Robert Goddard was born in 1882, and grew up in Worcester, Massachusetts, USA. As a child, he was inspired by books such as

H. G. Wells’ The War of the Worlds and Jules Verne’s From the Earthto the Moon. He wrote, ‘They gripped my imagination tremendously.Wells’ wonderful true psychology made the thing very vivid, and possible ways and means of accomplishing the physical marvels setforth kept me busy thinking.’

What he was thinking about was how to leave the Earth and navigate space. In his twenties he came to the conclusion that a rocket was the only way to achieve this; the principle of action andreaction would permit people to explore the solar system. He madecalculations which showed that, in theory, a liquid-fuelled rocketcould carry enough energy to reach the Moon.

Goddard started work as a college physics teacher and soon foundfunding to work independently on his ideas. He devised systems formultistage rockets, gyroscopic stabilisers and propellant feedsystems, and these basic ideas are still in use today. His first liquid-fuelled rocket used liquid hydrogen and liquid oxygen.

For a scientist, Goddard led an unusual life. He worked almostalone, with help from a few assistants and funding from the US armyand navy. He set himself up at a ranch in the New Mexico desert, andeventually produced rockets which were capable of reaching aheight of 3 km, travelling at speeds of up to 900 km/h.

Gunpowder ignition

Valves

Asbestos cone to protect fuel tanks from exhaust gases

Liquid oxygen tank

Alcohol burner

Petrol tank

Flexible tube

Oxygen to keep the tanks pressurised

Rocket engine

Petrol supply tube

Cork pistons

Tubes for fuel pressurisation system

Control valve

Liquid oxygen supply tube

Goddard (left) and his team check over one of his mostsophisticated rockets in 1940.

Goddard’s first successful rocket.

Robert H. Goddard invented the liquid-fuelled rocket. His first

rocket flew in 1926, and the technology he developed became

the basis for all modern space rockets. He died in 1945, and

NASA’s Goddard Space Flight centre is named in his honour.

Rocket man

Goddard withhis first rocket,which is alsoshown in thediagram. It waslaunched in 1926and reached aheight of 41 feetin a flight of 21⁄2 seconds.

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