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Energy notes Chapter 15 What is energy? The ability to do work Work is done when an object has a change in motion When a force causes something to move through a distance, work is done. Energy can be present when there is no motion Energy is observed only when it changes forms Units: Joules (J) 1 joule = 1 Newton meter 1 calorie = 4186 joules What is Kinetic Energy (KE)? The energy due to motion KE = ½ m v 2 m = mass of runner v = speed of runner Anything in motion has kinetic energy KE depends more on speed than on mass b/c of the squared term. Kinds of Potential Energy What is Potential Energy (PE)? Stored energy or Energy due to position Units: Joules Spring Potential Energy – the energy stored in anything stretched, bent or compressed This gives the object the ability to cause something else to move. SPE = ½ k x 2 k = the spring constant of the stretchy object, (more stretchy, lower k) x = the amount stretchy object is displaced from start Kinds of PE Gravitational Potential Energy – the energy stored in an object because it has height. GPE = mgh m = mass of object g = 10 m/s/s, the acceleration due to gravity h = the height of the object as measured from a reference point. Other forms of energy Mechanical energy– deals with motion and position of objects Thermal energy – the total potential and kinetic energy of all the particles in an object Chemical energy – the energy stored in the chemical bonds Electrical energy – the energy associated with electrical charges Electromagnetic energy – a form of energy that travels through space in the form of waves Nuclear energy – the energy stored in atomic nuclei Law of Conservation of Energy Energy can not be created nor destroyed. It only changes forms Energy doesn’t appear out of nowhere. Energy doesn’t disappear. At any given time, the total energy remains the same. Ex. A flashlight – chemical energy in the battery is converted into electrical energy when the switched on, which is converted into heat and light energy. Roller coaster energy What energy transformations occur in a roller coaster? The conveyor belt does work on the car to give it gravitational potential energy at the top of the hill. Then the car goes down the hill and the potential energy changes to kinetic energy. Some of the potential energy is changed to heat and sound energies. At the bottom of the hill, there is no more potential energy b/c the car can go no lower. Then as the car goes up the next hill, most of the kinetic energy changes back to potential energy. Heat transfer Heat – energy that flows as a result of a difference in temp. heat energy flow from hot objects to cold objects when molecules have thermal energy added to them, they move faster and bounce into each other, increasing their kinetic energy, which causes the particles to be pushed further apart 3 Methods of Heat Transfer Conduction – Heat transfer from particle to particle of 2 materials touching each other. Usually occurs between 2 solids. “Hot” particles bounce off each other then transfer their energy to the cold particles. Pot on stove Spoon in hot chocolate Convection – Heat transfer by movement of a heated substance by currents in a fluid (liquid or gas). The energy gets transferred by a warmer fluid moving through a colder fluid. Hot air popcorn poppers, convection ovens radiators where hot water flows through Radiation – Transfer of electromagnetic energy given off by “hot” materials that can be transmitted through objects or a vacuum at the speed of light. (No matter needed) solar energy Light ultraviolet rays Microwave ovens