energy

• Different foods are stores of different amounts of energy. Energy measured in joules (J). One joule is a very small amount of energy so we often use kilojoules (KJ). 1kj = 1000j.
• Food labels tell you how much energy is in the store associated with food. The amount of energy stored in different foods varies greatly.
• The amount of energy in coal is 3000kj which is the same as 2 chocolate bars.
• Coal and chocolate are both stores of energy. Oil, wood and other further fuels are stores too. You need oxygen to burn both food and fuels. People use the energy fuels make to heat their house or cook their food. Electrical appliances need an electric current to work. When you burn fuel in a power station it produces a current that makes your microwave or hair straighteners work.
• You need different amounts of energy depending on what you do each day e.g. sleeping = 300 kj per hour
• There is an energy cost to everything that you do. You need energy to keep your body warm, to breathe, move and talk. While you are growing you need energy for your bones, muscles and brain to grow.
• Sports people need a lot more energy than the average person. People who walk to the north or south pole need even more energy because they need extra energy to keep warm.
• An adult should just take in energy they need for the activities that they do. If you take in more energy than you need to your body stores it as fat to use for the future.
• Energy cannot just disappear, an you cannot end up with more than you had at the start. Energy cannot be created or destroyed, only transferred. This is the law of conservation of energy.
• There is an energy associated with food and fuels (and oxygen). You can think of that energy as being in a chemical store. Energy is transferred from the store when you burn the fuel or respire. There are other types of energy store.
• food, fuels and batteries = chemical energy
• hot objects = thermal energy
• moving objects = kinetic energy
• position in a gravitational field = gravitational/potential energy
• changing shape, stretching or squashing = elastic energy
• total energy before = total energy after
• before = unburnt fuel, more oxygen = cold soup
• after = less fuel, more carbon dioxide and water = hot soup
• electric current, light and sound are ways of transferring energy between stores. After you use your phone, there is less energy in the chemical store and more energy in the thermal store of the surroundings.
• In a car you want the burning fuel to transfer energy to the store that you want (the kinetic store of the car), not to other stores that you don't want (such as thermal energy). You want the car to move, not heat up.
• In many situations energy is transferred to the thermal store of the surroundings. Scientists say that the energy is dissipated (to break up scatter of vanish).
• When we talk about 'saving energy', what we really mean is saving fuel. Energy is always conserved, but if we can transfer more of it to the useful store, we save fuel.
• It is tempting to say that things happen because they have energy. Energy tells you what changes are possible, but it does not explain why things happen. Forces, not energy, explain why things move. For example, you can put fuel in a car but that does not make the car move. The force provided by the engine makes the car move.
• Something that is hotter than your skin will feel hot, and something that is colder than your skin will feel cold. You cannot measure temperature with your skin.
• You use a thermometer to measure temperature. Some thermometers have a liquid inside a very thin glass tube that expands when it's heated. Other thermometers are digital. We measure temperature in degrees Celsius
• There is a difference between energy and temperature. You can have a swimming pool and a beaker of water at exactly the same temperature. Even though they are the same temperature the swimming pool represents a much bigger thermal store of energy than the beaker of water.
• Heating changes the movement of particles. If you heat a solid the particles vibrate more. If you heat a liquid or a gas the particles move faster and vibrate more.
• Individual particles in a solid, liquid or gas don't get hotter. They move or vibrate faster. The energy that you need to increase the temperature of a material depends on: the mass material, what the material is made of and the temperature rise that you want.
• Hot objects cool down. Energy is never transferred form a cold object to a hot object, only from a hot object to a cooler object. The temperature difference is reduced and eventually both objects will end up at the same temperature. They will be in equilibrium or thermal equilibrium. No more energy is transferred between their thermal stores.
• When you put the saucepan of soup on the stove, the soup heats up. The bottom of the saucepan is made of metal. A metal is a good conductor of energy. Energy transfers through it very quickly. This is conduction. Energy can be transferred by conduction, convection or radiation.
• In conduction, particles transfer energy by colliding with other particles when they vibrate. Energy transfer happens until the two surfaces are at the same temperature. If you keep one surface warm by heating it then you maintain the temperature difference. The solid will continue to conduct.
• Energy is not transferred very easily through materials like wood. Wood and many non-metals are poor conductors. They are insulators. This does not mean that they do not conduct at all but that energy is transferred very slowly through them.
• Liquids are poor conductors. Divers wear wetsuits, which use a thin layer of water against the skin as an insulator to keep them warm.
• Gases do not conduct well at all because their particles are much further apart than the particles in a solid. Duvets and warm clothing are designed to trap small pockets of air, which is a good insulator.
• When you heat up soup in a pan it all heats up, not just the layer in contact with the bottom of the saucepan. This is what happens: The soup that is in contact with the bottom of the pan gets hotter so the particles there move faster. The particles in the hotter soup mover further apart and so the soup becomes less dense. The hotter soup rises and cooler denser soup takes it's place. This is called a convection current. Convection also happens in gases.
• Convection does not happen in solids because the particles do not move freely.
• When you play music, energy is transferred to the surroundings. The air and wall get a bit warmer. The particles in the air move a bit faster, and the particles in the walls vibrate more.
• Very hot things such as burning coal give out light as well as infrared radiation. Some people call infrared ' thermal radiation' or 'heat'. The sun emits lots of different types of radiation, including light and infrared. Both light and infrared radiation travel as waves.
• You need particles to transfer energy by conduction and convection. You don't need particles to transfer energy by radiation. Light and infrared reach earth from the sun by travelling through space. Space is a vacuum. There are no particles in a vacuum.
• All objects including you give out, emit radiation: The type of radiation that they emit depends on their temperature. How much radiation they emit per second depends on the type of surface. Infrared can be transferred, absorbed or reflected just like light.