Cambridge Lower Secondary Science Digital Learner's Book 7 > 3.5 Energy With your partner, make a list of: some things that you need eneray to do some of the types of fuel that you know. 105/345Cambridge Lower Secondary Science Digital Learner's Book 7 3.5 Energy What is energy? Energy is something that must be changed or transferred in order to do something, ‘There are many different ways that energy can be stored or transferred. For example, kinetic energy is the energy in movement. The unit for measuring energy is called the joule (J) ‘You need about + 2000 F1o walk up the stairs between two floors in a building. + 200J for every metre you run + 400000 J to bring 1 litre of cold water to boiling point, Energy stores and transfers re many different ways in which energy is being stored or ferred around you all the time. These runners have kinetic energy because they are moving: 106/345< a Wa coo Cambridge Lower Secondary Science Digital Learner's Book 7 3.5 Energy 3.5 Energy ‘The table describes some of these stores and transfers. kinetic energy stored due to movement of an | store object chemical | energy stored in food, batteries, chemical | store fuels such as wood, oil and coal thermal heat energy stored in hot objects and store or transfer transferred to colder objects elastic energy stored when things are stretched | store potential __| or squeezed to change their shape gravitational | energy stored when an objectislifted | store potential | away from a source of gravity electrical | the flow of current in a circuit transfers _| transfer electrical energy sound energy transferred from vibrating objects | transfer Tight Visible energy from luminous objects _| transfer {objects that give out their own light) that you can see Look at the descriptions of energy in the picture. chemical the bus carries fue! kinetic the buss moving electrical for lights ‘thermal ~the engine ishot sound the engine. makes noise ‘gravitational potential ~ the bus is going uphill clastic the tyres get squeezed 7 e 107/345< a Wa Cambridge Lower Secondary Science Digital Learner's Book 7 3. Forces and energy > Storing energy Energy can be stored more easily in some ways than in others. For example, you can keep uncooked rice for a long time. That isa store of chemical energy. Coal and erude oil are stores of chemical energy that formed millions of y This shows that some energy stores can last for a very long time. A battery is another example of how chemical energy can be stored. It is quite easy to store By) Gravitational potential energy is also easy to store. The picture shows a tank containing water A pump has been used to lift up the water. The water stores gravitational potential energy chemical ener Some energy stores only last for a short time, Thermal energy (heat) is one example. Hot objects will eventually cool down (they will lose their store of thermal energy) Kinetic energy is another example. Kinetic energy is more difficult than chemical or gravitational potential energy to store. The tennis ball in the picture has a store of kinetic energy while the ball is moving, but the ball will eventually stop moving 108 / 3453.6 Changes in energy * eam about energy changing * discover that energy changes when something happens * eam how to give examples of changes in energy. Cee caer) 1 Make a list of all the energy stores and transfers that you can remember. 2 Give an example of each store or transfer on your list3. Forces and energy > Ei How does energy change? In Section 3.5, you learned that energy is something that must be changed or transferred in order to do something. Before energy can be changed or transferred, itis stored. When energy is stored, the energy is not doing anything. ‘The picture shows a cooking pot being heated ona fire. Walking up sais needs ray tobe changed ‘The fuel for the fire is wood. Wood is a store of chemical energy. Burning the wood changes the chemical energy to thermal energy (heat). The thermal energy is then transferred to the pot and the food inside. The people in the picture are walking up stairs. ‘They are changing chemical energy from their food into kinetic energy for movement. The movement is taking the people higher, so kinetic energy is being changed to gravitational This picture shows a power station3.6 Changes in energy This power station is using the chemical energy stored in natural gas. The gas is burned, which changes chemical energy to thermal energy. The thermal energy is then changed to in large generators that spin around, etic energy ‘The kinetic energy is then changed to electrical energy. ‘The electrical energy is then transferred through wires into homes and buildings. Energy changes are not always helpful. Typhoons, hurricanes, earthquakes and tsunamis are some examples of how energy changes can be very dangerous. Inall these examples, there is a process or event that changes or transfers the energy. For example, burning isa process. Strong wind can transfer energy in a damaging wey Burning changes chemical energy stored in a fuel to thermal energy. ‘You can represent the processes as arrows and draw diagrams to show changes in energy. Here are some other examples. AA fire that burns wood changes chemical energy to thermal energy. chemical. -}————>} thermal A television converts electrical energy to sound and light. sound electrical light is changed to kinetic energy as the book falls. ‘You can also represent events such as this in a diagram. gravitational potential |———>) kinetic When a book falls from a shelf, that is an event. When the book is on the shelf, the book has stored gravitational potential energy. This energy> 3.7 Where does energy go? Re ss Work with a partner. Discuss the energy that is changed or transferred in each of these processes. Burning wood for cooking. ] + Walking up stairs Cycling on a level road In each case, state where the energy comes from and the useful energy that is changed.3. Forces and energy > Ei Useful and wasted energy Every time you use energy to make something happen, energy is transferred or changed, Some of the energy transferred or changed is useful, but some of it is wasted. Look at the picture of fuel being added to a motoreyele. The motorcycle engine uses the chemical energy stored in the fuel. This chemical energy is changed to useful kinetic energy to move the motorcycle and rider. But chemical energy from the fuel is also changed into thermal energy and sound energy. In fact, only about } or 25% of the chemical energy in the fuel is used for movement. ‘The other } or 75% of the energy is wasted energy. This wasted energy is dissipated and cannot be recovered. Dissipated energy is energy that spreads out where there is no use for it. Gasoline ipeteo) is a store of eremica energy. “The motoroycle engine changes ony some of You cannot gather thermal energy or sound and bring them thins kinetic enotay. The rst cl the enoroy back into one place to be stored, changed or transferred, wasted Look at the two types of lamp in the picture. Both lamps A and B in the picture change electrical energy to light energy Lamp A only changes about 15% of the € electrical energy into light, 85% of the $ é clectrical energy is wasted from this lamp. This is dissipated as thermal energy. Lamp B changes about 50% of the electrical } \w energy into light. 50% of the electrical energy is dissipated as thermal energy { from this lamp. \: jit | i A e These two lamps emit the same brightness of light but they waste very different quantities of eneray.3.7 Where does energy go? Every time energy is changed or transferred, there is some thermal energy wasted. This ‘wasted thermal energy is dissipated. Even when you want to produce thermal energy, some of it is wasted. Look at the water being heated in this picture, In the picture, chemical energy from wood is being changed to thermal energy by the process of burning, being used to heat Thermal energy the water, ‘Thermal energy is also being used to heat the rocks, the metal container and the air around it ‘Some of the thermal energy is escaping in the steam. ‘The fire is also changing energy into light. Alll these represent wasted energy that is dissipated and cannot be recovered. For everything that uses energy change or transfer, some of that energy will always be dissipated. Questions 1 Which of these terms describes energy that is dissipated? energy that spreads out and energy that becomes more becomes less useful useful energy that can be energy that is not useful but used later can be stored 2 Which of these can be dissipated? Choose all that are correct. chemical thermal light sound _ elastic 3. List all the energy changes in these processes. List the energy as either useful or wasted. a Using electricity in a lamp. b Using petrol (gasoline) in a car engine. © Using electricity in a motor.