Energy. you will learn

Transcription

1 3 Energy What you will learn What is energy? Heat on the move - 1 Heat on the move - 2 Heat on the move - 3 Energy resources and conservation - 1 Energy resources and conservation

2 Energy 3a What is energy? You probably use the word energy to describe how you feel. When you are tired you might say I have got no energy today. But when you feel lively you might say I m full of energy. What do you think energy is? Where do you think it comes from? In science, the word energy is used in a special way. Energy is all about things changing. In fact energy is needed to make anything change. Whether you run a marathon or just blink, energy is always involved. Some things have energy stored in them ready to make changes. Energy which is stored is called potential energy. Gravitational energy Spring energy Chemical energy Things that you lift up gain potential energy. You can get the energy out by letting them fall. A wound up spring has potential energy. Look what happens when you let it go! Do you know? Fireworks have potential energy too. The energy is released when you set light to them. Energy cannot be created or destroyed. 20

3 Energy and its forms Energy is measured in joules (J). Large amounts are sometimes measured in kilojoules (kj). 1kJ = 1000J Like money, energy doesn t vanish when you spend it. It just goes somewhere else! The total amount of energy always stays the same. Electrical energy Kinetic energy Thermal energy Light energy Wherever there is a current flowing, there is electrical energy. It is easily converted into other forms. Anything that moves has kinetic energy. Everything has thermal energy. The hotter the object is, the more thermal energy it has. Anything luminous gives off light energy. Elastic energy Anything stretched, has elastic energy. Potential energy Anything that has been lifted off the ground has potential energy. Chemical energy Anything that has stored energy can be released by a chemical reaction. Sound energy Anything that makes a noise, gives off sound energy.? 1 Give an example of something which has a. kinetic energy b. potential energy c. chemical energy d. sound energy e. light energy f. elastic energy g. electrical energy h. thermal energy 2 A fire gives out 15kJ of energy. What is this in joules? 3 Scientists say that energy can never be destroyed. Explain what they mean. K Ideas!Everything that is moving has energy.!when energy changes from one form to another, the total amount stays the same.!all changes involve some form of energy. 21

4 Energy 3b Heat on the move - 1 Everything is made of particles which are constantly on the move. If something hot cools down, its particles lose energy. The energy given out is called heat. It can travel in three ways. Conduction Heat can travel through solids. If you try stirring boiling curry using a metal spoon the heat energy quickly flows from the curry, through the spoon, to your hand. And your fingers begin to get hot! The movement of heat through the solid metal is called conduction. The metal is a good conductor because heat can flow easily through it. It is much more sensible to use a wooden spoon or one with a plastic handle to stir curry. Wood and plastic don t allow heat to flow easily through them. Substances like these are called insulators. Right material for the right job Conductors and insulators can be both useful, but for different jobs. The bottom of a saucepan may be made from a metal, like iron or aluminium, which lets heat flow quickly from the cooker to the food inside. But its handle is likely to be wooden or plastic. These materials would not heat flow to your fingers and burn them. Good Conductors Insulators (poor conductor) metals especially: silver copper glass water plastic wood wool fibrewool aluminium plastic materials with foam air trapped in fur them air feathers 22 Do you know?! When heat travels by conduction the heat is passed on from one atom to the next. The heat flows, the heated atoms don t!! Copper and silver are the best heat conductors. Copper conducts ten times better than iron.

5 Air a poor conductor The air is an excellent insulator. The clothes that you wear keeps you warm by trapping air between their fibres. Your body is warmer than the air around you and so it is always losing heat. The insulation supplied by clothes cuts down the amount of heat which escapes and so saves valuable energy. This is more important if you are living in colder climates. Feathers, fibrewool, plastic foam, wool and fur are all good insulators because they contain tiny pockets of trapped air. This is why birds are able to stay warm even in cold places. ptarmigan? 1 What is meant by saying that a. metal spoon conducts heat? b. wood is an insulator? 2 Pick out the conductors and insulators from this list: aluminium, wool, glass, tin, iron, cork, plastic, air 3 How could you protect yourself if you had to lift a hot plate? Why would this protect you? 4 What kind of natural insulation do birds and mammals have? K Ideas!Conduction is movement of heat through solid objects.!substances that allow heat to pass through them easily are called heat conductors. Those that do not allow heat to flow through them easily are called heat insulators. 23

6 Energy 3c Heat on the move - 2 Convection How heat energy travels through the water in the kettele Heat energy can travel through liquids. When you boil water in an electric kettle, the kettle s heating element only warms the water next to it, but the heat energy is carried all through the water until it boils. direction of heated water movement direction of cold water movement Heat energy can travel through gases. When you burn a camp fire the air above the fire is heated and moves around the fire carrying tiny particles of ash with it. As air becomes warmer, it becomes lighter and rises. Conversely, when air becomes cooler, it gets heavier and sink. heated air moving cold air moving How air moves round about a fire The heat can not be travelling through water and air by conduction. Water and air are both bad conductors. So the heat travels in a different way, by convection. Convection cannot take place in solids. It only takes place where particles can move once they are heated. 24 Do you know?! Birds make use of thermals (rising currents of hot air) to soar in the air.! Glider pilots often find thermals by watching birds in flight.

7 Convection and monsoons The monsoon is a convection current wind. It blows because of temperature differences between the land and the sea. In Maldives we have the south west monsoon (Hulhangu moosun) and the north east monsoon (Iruvai moosun). The south west monsoon has warm, wet winds carrying rain. The winds blow because hot air rises from the land and cold air from the sea flows into take its place. The north east monsoon has cooler, drier winds. The winds blow because warm air rises from the sea and cold air flows from the land to replace it. hot air rising monsoon wind? 1 Copy and complete the following: a. Heat travels through and by convection. b. Heat cannot travel through by convection. 2 How does convection occur in north east monsoon? K Ideas!Convection is the main way in which heat travels through all liquids and gases. It takes place whenever one part of a liquid or gas is heated more than the rest. 25

8 Energy 3d Heat on the move - 3 Radiation There is empty space between us and the Sun. But we still get heat from the Sun. The energy travels as tiny waves of radiation. If you absorb any of the Sun s radiation, it heats you up. That is why it is sometimes called heat radiation. Everything gives off radiation. The hotter it is, the more it radiates. Earth-warming radiation The Earth is warmed by heat radiation. This comes from the Sun. We get most of our energy from the Sun. We can see the light it gives us, and we can feel its heat on our skin. Only a small fraction of the Sun s heat radiation reaches the Earth. To get there, it travels through space. Since space has very few particles in it, this shows that heat radiation is not carried by moving particles. When the heat radiation reaches the Earth, some of it is taken in, or absorbed. Some is bounced back, or reflected. It is the absorbed heat which warms up the Earth. The more heat absorbed, the hotter it gets. Clouds can cut down the amount of heat radiation reaching the Earth. Cloudy days are usually cool days! That is because heat radiation travels in straight lines. The heat can not bend round the edges of the cloud to reach the Earth underneath The sun gives off lots of heat radiation Heat radiation travels in straight lines 1 2 Less than 100,000,000 of this heat radiates to Earth through space Do you know? Clouds absorb and reflect some of the heat from the sun 3 The Earth is warmed by the heat radiation it absorbs Dull and cool sunny and warm! Burglar alarms can be set off by heat radiating from a burglar s body.! The solar energy reaching the Earth in 1 year is more than times greater than the world s energy needs.! Solar panels can absorb useful amounts of heat even on cloudy days.

9 Black absorbs better Black surface are the best at giving off radiation. They are also the best at absorbing it. Silvery or white surfaces are good at reflecting radiation. That means they are poor at absorbing it. In hot, sunny countries, buildings are often painted white so that they absorb as little of the Sun s radiation as possible. Kettles are usually made silvery or white so that they lose heat slowly. The thermos flask- 3 in 1 1 stopper 2 part-vacum (space with some air removed) glass 3 silvery surfaces 1 stopper prevents convection 2 part-vacuum (space with some air removed) reduces conduction 3 silvery surfaces reduce radiation. A thermos flask can keep drinks hot or cold for hours.? 1 Why can the Sun s heat not reach us by conduction or convection? How does the Sun s heat reach us? 2 What is meant by: a. absorbing heat? b. reflecting heat? 3 Give an example of convection, conduction and radiation that you come across in your daily life situations. K Ideas!Radiation is the movement of energy across air from warm to cold surfaces.!everything gives off heat radiation. The hotter the object, the more radiation given off. 27

10 Energy 3e Energy resources and conservation - 1 Most of the energy around us originates from the sun. We can see the light it gives us, and we can feel its heat on our skin. Industrial societies need huge amounts of energy. Most comes from fuels which are burnt in power stations, factories, and vehicles. The energy in fuels originally came from the sun. So did the energy in the foods we eat. The sun gives out its nuclear energy in the form of LIGHT HEAT Solar energy Solar energy Energy in plants Green plants trap light energy for growth and in turn provide food for other living things. Earth s weather The heat from the Sun causes the various weather conditions like wind and rain on Earth. Hydroelectric energy Do you know? 28 Fossil fuels Wind energy The solar energy reaching the Earth in 1 year is more than times greater than the world s energy needs.

11 Non-renewable resources Non-renewable resources will run out. Some examples of these are fossil fuels and crude oil. Fossil fuels include coal, gas, wood and peat. If we continue to use them at the present rate, scientists have predicted that there is only enough coal to last a few years and enough oil and gas to last about 50 years. Crude oil is important for making plastics and medicines. Thus it is important to conserve it. Black rock Coal is a useful fuel. Coal is formed from plants which lived hundreds of millions of years ago. When the plants died, they were gradually squashed by rocks which formed on top of them. The plants became fossils, so coal is known as a fossil fuel. Eventually all the fossil fuels in the Earth may be used up. So fossil fuels are non-renewable. Oil and gas Like coal, oil and gas are fossil fuels. They formed from the remains of sea creatures which lived hundreds of millions of years ago. Sometimes oil and gas are found under land and sometimes under the sea. We can try and make fossil fuels last longer by saving energy.? 1 What is our principal source of energy? Explain your answer. 2 What is a non-renewable source of energy? 3 Copy and complete the following using the words given below. wood gas sunlight oil coal a. fossil fuels b. not a fossil fuel c. where trees get their energy from K Ideas!The sun is our principal source of energy.!fossil fuels stored energy which came from the Sun millions of years ago.!fossil fuels will eventually run out, They are non-renewable. 29

12 Energy 3f Energy resources and conservation - 2 If you make a wood fire, you burn the wood. You get heat energy and light energy from the wood. This energy came from the sunlight that fell on the tree when it was growing. Wood from trees which are growing recently is called biomass fuel. Renewable resources The following energy resources are not re-usable, they are renewable. These sources are free and do not cause pollution. They renew themselves, e.g. trees do grow again, but cannot be used again once burned. Energy in plants Green plants trap light energy for growth and in turn provide food for other living things. Tidal energy Solar energy Solar panels absorb energy radiated from the Sun and use it to heat water. Solar cells use the energy in sunlight to produce small amounts of electricity. Energy from waste Rotting animal and plant waste can give off methane gas. This can be used as fuel. It will burn in air to give heat and light. Some waste can also be used directly as fuel by burning it. Wave energy Waves are caused by wind and partly by tides. Waves cause a rapid up-and-down movement on the surface of the sea. This movement can be used to drive generators. 30 Tides are caused by gravitational pull of the Moon and Sun on the water in oceans. The changing tides drive water towards or away from the land. The difference in height of the water at high or low tides is made use of in tidal power station to generate electricity. Do you know? There is enough heat in the top 10km of the Earth s crust to supply all our energy needs for hundreds of years.

13 Wind energy Traditional windmills convert the kinetic energy of wind into mechanical energy for pumping water. Modern windmills or wind turbines use wind energy to drive generators which produce electricity. Hydroelectric energy Rain that falls on high areas such as mountains have large amounts of gravitational potential energy. As if flows, its kinetic energy increases. This energy is used to turn turbines fixed at the base of dams. The turning turbines will then drive generators to produce electricity. Saving energy The energy sources on Earth are limited. Although we can use renewable energy sources such as solar energy, wind and tides, they too have their limitations. So there is a need for us to save energy and reduce energy wastage. Look at the diagram. How is energy being wasted? K Ideas? 1 Can you think of problems which can be caused by gases from burning fuels? 2 Give two ways of generating electricity in which no fuel is burnt and the energy is renewable. 3 List all the ways you can think of how you conserve energy. Then discuss them with your classmates.!energy which can be replaced are renewable. These include solar energy, tidal energy, wave energy, energy from waste, wind energy and hydroelectric energy.!energy need to be saved. 31

14 Energy JAMES PRESCO RESCOTT JOULE ( ) Joule was born into a wealthy Manchester brewing family, and was encouraged in his work by W Thomson. He was taught by John Dalton, who instilled in him a deep belief in the importance of mathematics and detailed measurement. He discovered that heat is a type of energy and established the principle of conservation of energy, and the equivalence of heat and other forms of energy. By 1840 he established Joule s law, which states that the heat generated by a steady electric current in a wire is related to the resistance of the wire, the square of the current, and time. One of Joule s main interests was to try to produce an electric motor driven by a zinc-acid battery. He hoped that this would replace the coal-driven steam engines of that time. 32