Conservation of Energy PHYSICAL SCIENCE The Law of Conservation of Energy Energy cannot be created or destroyed; it may be transformed from one form into another, but the total amount of energy never changes. What is energy? “the ability to do work” The combination of energy and matter make up the universe: ◦ Matter is substance, and energy is the mover of substance. Energy is nature’s way of keeping score. We sense energy only when the score changes, either a transformation from one form of energy to another, or a transfer of energy from one point to another. Work is equal to the force that is exerted times the distance over which it is exerted. W=Fxd The unit of work combines the unit of force (N) with the unit of distance (m) Newton-meter (N-m) aka Joule. You carry a 20 kg suitcase upstairs, a distance of 4m. How much work did you do? W=Fxd F = ma = (20 kg) (10m/s2) = 200 N W=Fxd = (200 N) (4m) = 800 J Power omeasures the rate of work done. oor the rate at which energy is expended. oPower is the amount of work done, divided by the time it takes to do it. oPower (watts) = work (joules) / time (sec) oP = W/t Power Since work performed equals energy expended, Power (watts) = energy (joules) / time (sec) The watt is defined as the expenditure of 1 joule of energy in 1 second. (75 watt light bulb consumes 75 J/sec) Important formulas and units Quantity Definition Units Force mass x accel. newtons Work force x distance joules Energy power x time joules Power work / time watts Heat and Temperature THIS LESSON BROUGHT TO YOU BY: THE LET TER 3 AND THE NUMBER M Temperature and Heat Temperature and heat are NOT THE SAME. Temperature How hot or cold it is. Measured in degrees Celsius. Heat The amount of thermal energy, measured in joules or J. A cup of hot tea has heat energy in the form of kinetic energy from its particles. A swimming pool at 30°C is at a lower temperature than a cup of tea at 80°C. BUT the swimming pool contains more water, so it stores more thermal energy or heat. The small beaker of water boils first The large beaker contains more water and needs more thermal energy or heat to reach 100°C. Transfer of Heat Heat A form of energy associated with the motion of atoms or molecules. Transferred from higher temperature objects to objects at a lower temperature. How Heat Can Be Transferred Conduction Convection Radiation Conduction Transfer of heat through direct contact. Occurs anytime objects at different temperatures are touching each other. As long as the objects are in contact, transfer of heat will continue until the temperature of the objects is the same. Conduction (continued) Example: If you leave a metal spoon in a pan of soup that you are heating on the stove, it may burn your fingers. The spoon is in direct contact with the hot soup and heat is transferred to the spoon. Conductors and Insulators Some materials conduct heat better than others. Materials that transfer heat well are called conductors. Metals are usually good conductors. Wood, paper and plastic are not. Materials that stop the transfer of heat are called insulators (styrofoam, wool, fiberglass). Convection The transfer of energy in a liquid or gas. When part of a gas or liquid is heated, the particles it is made up of move faster and spread out more. The moving particles bump into other particles, causing them to move faster and spread out more. Convection Currents When particles in the air spread out, they become less dense and generally rise above the unheated, more dense particles around them. The denser masses of the gas or liquid move in to fill the space left by the heated particles. The particles that move away from the source of heat become cooler and more dense. Radiation Energy transferred in the form of rays or waves or particles. We will concentrate on the type of radiation that travels as electromagnetic waves. Heat From the Sun You can feel the sun warm your skin on a sunny day. This is because the energy causes the particles in your skin to move faster = more heat energy. Electromagnetic Waves Include visible light, microwaves and infrared light Can travel through space. The sun is our major source.