Survey
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
* Your assessment is very important for improving the workof artificial intelligence, which forms the content of this project
05/04/2017 Energy Transformations • law of conservation of energy • friction • As the ball moves upward, gravity pulls it and its speed and KE decrease, but the PE (gravitational PE) is increasing because the ball’s height is increasing. • At the ball’s highest point, the GPE is greatest, and the ball’s kinetic energy is the lowest. Energy Can Change Forms The Law of Conservation of Energy: energy cannot be created or destroyed, it can only be transformed from one form to another. • As the ball moves downward, PE decreases. At the same time, the ball’s kinetic energy increases. • When the ball reaches the player’s hand again, its KE is at the maximum value again. • When it stops in the player’s hand, KE is zero, while PE is high. Example 1: Changes between KE and PE: Throwing a basketball • When the player is holding the ball, the KE of the ball is zero. It does have GPE because if its height above the ground. • As the ball is thrown upward, it has a lot of KE. Example 2: Changes between KE and PE: A pendulum • When the ball is at rest, it has some PE because it is suspended off the ground. 1 05/04/2017 • When the ball is pulled back and held at the starting position, what happens to the energy of the ball? • PE increases, KE is zero. • When the ball is released, what happens to the energy of the ball? • KE increases, PE decreases. • When the ball is swinging and has reached the mid point, what happens to the energy of the ball? • KE is at its maximum, PE is at its lowest. It still has some PE because it is suspended off the ground. • When the ball is swinging and has reached the highest point on the other side, what is the balls energy? • KE is zero (the ball stops moving as it begins to swing back the other way). PE is at its maximum. Describe the energy at point A. Describe the energy at point B. Describe the energy at point C. Describe the energy at point D. A: the coaster is stopped at the top. Its PE is highest, KE is zero. B: As the coaster rolls downward, the PE is converted into KE. C: KE is highest, PE is lowest. D: The car slowed down as it climbed up. KE decreased, PE increased. 2 05/04/2017 Could the cars ever have enough energy to reach another point as high as point a? No, the car slows down and cannot reach that height again. If energy cannot be destroyed, what causes the cars to slow down? The cars slowed down because of friction. As the car slid along the track, friction caused some of the energy to be converted into thermal energy. Energy can be lost when it is converted into thermal energy. Friction is a force that resists the sliding of two surfaces that are touching. Friction and the Law of Conservation of Energy • What type of energy does a moving bicycle have? • What happens to mechanical energy when you apply the bicycle brakes and the bicycle stops? • A moving bicycle has mechanical energy. When you apply the brakes, the bicycle’s mechanical energy is not destroyed. Friction and the Law of Conservation of Energy (cont.) • The mechanical energy is transformed into thermal energy. Friction between the brake pad of a car and the moving wheel transforms mechanical energy into thermal energy (heat). The car slows down. • There is always friction between any two surfaces that are rubbing against each other. • EX: Rubbing hands. Using Energy • When you use energy, you usually change it from one form into another. • All forms of energy can be transformed into thermal energy. friction from Latin fricare, means “to rub” 3 05/04/2017 Using Energy (cont.) What happens to energy when it is used? Using Energy (cont.) • When energy changes form, some thermal energy is always released. • Scientists often refer to thermal energy that cannot be used as waste energy. • Whenever energy is used, some energy is transformed into useful energy and some is transformed into waste energy. Using Energy (cont.) • Using Thermal Energy: – All forms of energy can be transformed into thermal energy. • a gas stove transforms the chemical energy in gas to thermal energy. • An electric heater transforms electricity into heat • A jet engine burns fuel, releasing thermal energy from chemical energy, and then transfers the thermal energy into mechanical energy. Using Energy (cont.) • Using Chemical Energy: – Plants convert radiant energy (light) into chemical energy that is stored in the chemical bonds of glucose molecules. – When animals eat glucose, they release the chemical energy from the food and transform it into kinetic energy and thermal energy (body heat). Using Energy (cont.) • Using Radiant Energy: – Solar panels transform radiant energy into electrical energy. – Talking on your cell phone: Cell phones receive radiant energy (microwaves that are sent from the cell phone tower to your phone) and transforms it into electrical energy to power the speaker. The speaker transforms the electric energy into sound energy. Waste Energy • Energy transformation is not efficient. • Whenever energy is transformed, some is lost to the surroundings as heat energy. – Your cell phone gets hot – Light bulbs get hot – Car engines get hot • When energy is lost during transformation, it is called waste energy. It does NOT disappear, but transforms into a type of energy not being used. 4