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Types of Energy Chapter 11 Review Vocabulary System – The object or objects of interest that can interact with each other and with the outside world. Work-energy theorem – Doing work on a system, the result is a change in the system’s energy. How is energy used in every day speech? What are some examples you use? Food/drinks, athletes in sports, the companies that supply your home with electricity, gas and heat. How do scientist and engineers use the term energy? Work is the process that transfers energy between a system and the external world. When an agent performs work on a system, the system’s energy increases, when the system does work on the surroundings, the system’s energy decreases. This chapter looks at mechanical energy. Can energy change forms? Yes. It is like ice cream – there are a lot of varieties, but they are all types of energy What diagram is used to show the transformation of energy? Energy Bar diagram – keeps track of the energy stored or used We will use these diagrams when we look at the conservation of energy Types of Mechanical Energy What is kinetic energy? The energy that is due to the motion of a system’s center of mass. Think of it as your moving energy. What is potential energy? Energy that is stored due to interactions between objects in a system. There is more than one type of potential energy. Brain POP Energy in Roller Coasters Describe the work done by gravity as the orange rises from the juggler’s hand. Let h represent the orange’s height measured from the jugglers hand. On the way up, its displacement is upward, but the gravitational force on the orange (Fg) is downward, so the work done is negative. On the way back down, the force and displacement are in the same direction so work is positive. Describe the work done by gravity as the orange rises from the juggler’s hand. While the orange moves upward, gravity does negative work which slows the orange to a stop. When it goes back down, gravity does positive work, increasing the orange’s speed and kinetic energy. At the top, the orange has all GPE. At the person’s hand, the orange has all KE. Halfway between these points there is equal GPE and KE. Gravitational Potential Energy (GPE). Type of Potential Energy The stored energy due to gravity. What is a reference level? The position defined to be zero. If you look at the picture of the girl juggling oranges, you could use her hands as the reference level or the top height of the orange as a reference level. Elastic Potential Energy (EPE) The stored energy due to the object’s change in shape. What types of objects have elastic potential energy? Springs, rubber bands, trampolines, string on a bow and arrow. Albert Einstein Recognized that mass is a type of energy called rest energy (E0). Other forms of energy Chemical and nuclear – the chemical bonds (which store energy) are broken down. The energy is released when the structure of the atom’s nucleus changes. Thermal Energy– the sum of kinetic and potential energy of the particles in the system. Ex: rubbing your hands together. Electrical energy – associated with charged particles. Ex. Power plants Radiant energy – energy transferred by electromagnetic waves. Mechanical Energy Equations 1 2 EPE = 𝑘𝑥 2 k = spring constant x = displacement of object (how far it is stretched) 1 2 KE = 𝑚𝑣 2 m = mass v = velocity Mechanical Energy Equations GPE = 𝑚𝑔ℎ m = mass g = gravity (we are now using down as positive) h = height E0 = 𝑚𝑐 2 m = mass c = speed of light (3.0 x 108 m/s) Unit of Energy Energy is measured in Joules (J) This make sense because the change in energy is equal to work. Conservation of Energy Vocabulary Closed System – A system that does not gain or lose mass. Law of Conservation of Energy – states that in a closed, isolated system, energy can neither be created nor destroyed; energy is conserved. Mechanical Energy – the sum of the kinetic and potential energy of the system’s object. What is the equation that represents mechanical energy? ME = KE + PE Mechanical energy is the sum of kinetic and potential energy. Can you use both types of potential energy to find the mechanical energy of a system? Yes – gravitational and elastic Match the scenario with the correct energy bar diagram. 1 2 B C 3 A Conservation of Mechanical Energy Can mechanical energy be conserved? Yes What is the conservation of mechanical energy (KEi + PEi = KEf + PEf)? The sum of the system’s kinetic energy and potential energy before an event is equal to the sum of the system’s kinetic energy and potential energy after that event. If friction is present in a system, is energy is still conserved? Why? Yes, the energy is just transformed into a different type (non-mechanical energy). Energy cannot just disappear. EX: Friction and air resistance How do objects in everyday life (roller coasters, bouncy ball, etc.) stop without violating the law of conservation of energy? Some of the original mechanical energy in the system transforms into another form of energy within the system or transmits energy outside the system. Ex: warmer = thermal energy, sound = sound energy What are the problem solving strategies for conservation of energy? 1. Carefully identify the system. 2. Identify the forms of energy in the system. Identify which forms are part of the mechanical energy of the system. 3. Identify the initial and final states of the system. 4. Is the system isolated? There are no external forces acting on the system. Einitial = Efinal If there are external forces, then final energy is the sum of the inital energy and the work done on the system. Remember, work can be negative. Einital + W = Efinal What are the problem solving strategies for conservation of energy? 5. For an isolated system identify the types of energy in the system before and after. If the only forms of energy are potential and kinetic, mechanical energy is conserved. KEi + PEi = KEf + PEf 6. Decide on the reference level for gravitational potential energy. 7. Drawing an energy bar diagram showing the initial and final energies may help.