Much of our day-to-day routines depend on energy transfers. This is the process in which energy changes from one form to another. For example, when you switch on a television, electrical energy in the wires are changed into light and sound energy. Heat energy is also produced as a by-product.
Now suppose you are walking up a hill. Since you are moving, you have kinetic energy, but you also have GPE, as you are getting higher. The higher you go, the more GPE you have, because more of the KE is changed into GPE. When you get to the top, the KE you started with would have all become GPE. However, you still have the same amount of energy. Energy can never disappear or be lost. It just get changed from form to form.
Youíve already got tired of walking, so you decide to ride a bike to complete the journey. As it is a foggy day, you switch your lights on. The energy for the lights come from your pedaling. So kinetic energy is changed into electrical energy, and transported on a wire to the lights, where it is transferred into light and thermal energy. The device used to change your movement into electricity (KE into electrical energy) is called a dynamo. It is the opposite of a motor, which, when connected to an electrical supply, make things move (electrical energy to KE). Dynamos are used in power stations to convert the turning of the wind turbine or water wheel into electrical energy, but they also come in smaller types which you can turn a handle or drop a weight to produce electricity. However, the amount obtained is only enough to light a lamp.
Here, when the load is let go, its potential energy becomes KE in the moving parts, which drive the dynamo into producing electrical energy. This is converted into light and heat energy by the bulb.
Youíre nearly at your destination, but you havenít had any breakfast and youíre really hungry. So you stop to have an apple. Food is where your energy comes from. It contains chemical energy which is stored in your body until you need to use it, when it is changed into another form.
If energy is never lost, then why do we get hungry? We constantly need more energy because some of our energy has been dissipated as thermal energy when we use it. This means it is wasted in heating up the atmosphere, and cannot be retrieved or re-used. However, the energy is not lost, because itís still in the air somewhere. For example, when we walk, some of our energy became wasted in friction with the ground, which is thermal energy. Whenever energy is used or transported, some of it gets dissipated inevitably. Thatís why by the time the electricity from the power station arrives at your house, 70% of it is already wasted.
When youíve finished the apple and returned to your bike, your mobile phone is ringing. This is sound energy, which also originates as Ďfoodí Ė the battery. A battery stores energy, too, in chemical form. Potential and chemical are the only forms in which energy can be stored.
Youíve finally arrived at the park where youíre meeting your friends. You decide to play tennis. As you hit the ball, what energy transfers occur? The chemical energy from your body (which you got from the apple) becomes KE, transported from your arm to the racket to the ball. The ball gains GPE as it travels higher, but this is transferred back to KE as it drops. When the ball hits the ground, it carries sound energy and EPE, which makes the ball bounce up again.
At each stage, some energy is wasted as sound energy and heat energy (friction with the air), so less EPE is left in the ball, which makes it bounce lower every time.
Energy plays a big part in our lives. Next time you travel, play or eat, think about the energy transfers involved. Look around the room you are in. See all the electrical appliances in our world today. All the objects use electricity, which is then converted into other types of energy. For example electrical energy from a stereo is converted into sound energy for the music, light energy for the display, and kinetic energy for the vibration of the speakers. To explain how much energy of each type is used we can draw "Sankey" diagrams.
Still puzzled?? Hereís an example to help you.
This hi-fi converts electrical energy into sound energy, light energy and kinetic energy. It produces more sound energy than light and kinetic energy.
On the diagram half the electrical energy given in, is produced into sound energy. The rest of the energy is then split into light and kinetic energy. Simple isnít it!
The conduction of heat occurs mainly in solids. One end of a solid containing the heat energy will cause its particles to vibrate, the vibration is then passed on from particle to particle until it reaches the other end of the solid.
All metals are good conductors because the electrons that they contain are free to move inside them. They carry the heat energy and diffuse through the metal particles until they get to the cold end. The movement of the electrons is much faster than the vibrating particles.
Convection is a much more effective process than conduction and only occurs in liquids and gases. This is because the particle in liquids and solids are free to move about, whereas solid particles are only vibrating. Convection takes place when the more energetic particles move from the hotter area to the cooler region, taking their thermal energy with them.
Evaporation is a procedure in which heat can be lost. In a liquid, the hottest particles will be moving the fastest. The fast moving particles near the surface of the liquid can easily evaporate off. When the hot and fast particles are gone, there are cold and slow moving particles left behind. As all the hot particles containing lots of energy are gone, the liquid soon has less energy causing the liquid as a whole to cool down.
Heat radiation is also known as infra-red radiation and it consists of only electromagnetic waves of a certain frequency. Heat radiation can travel in straight lines at the speed of light; it can also travel trough a vacuum. This is how light can reach us from the sun, the light travels through space. It can also be reflected back again by a silver shiny surface, heat radiation only travels trough transparent matter, e.g. air, water or glass. The behaviour of infra-red radiation depends totally on the surface colour and texture of the material. No particles are involved; it is a transfer of heat energy done just by waves.
Dark and matt surfaces absorb the light, whereas shiny white or glossy silver surfaces reflect almost all the heat radiation hitting them.
There is a distinct difference between "temperature" and "heat".
In terms of physics, heat is a form of energy coming from the random motion of the molecules of bodies, which may be transferred or changed by conduction, convection or radiation.
Temperature is a degree of heat of a matter in relation to other materials.
The differences in temperature can lead to heat transfer. There are three main methods of heat transfer, they are CONDUCTION, CONVECTION and RADIATION. Heat or thermal energy causes molecules in liquids and gases to move around more actively and solid particles to vibrate more rapidly. When particles are moving faster, they cause friction, which means a rise in temperature. The extra kinetic energy gets sent out to their surroundings. Moving out into the surroundings mean flowing away from the heated object. The hotter the object, the faster and more vigorously the particles will vibrate.
Heat flows FROM HOTTER objects TO COOLER surroundings.
We were asked to do a small project on how electrical appliances transfer energy. We had to draw Sankey diagrams to show the transfer of energy. We were also asked to write down the power of the appliances we used at home and work out how much energy was transferred in 1 hour.