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Forms of Energy Freshman Seminar Energy • Energy The ability & capacity to do work – Energy can take many different forms – Energy can be quantified • Law of Conservation of energy “In any change from one form of energy to other forms, the total quantity of energy remains constant” Energy Energy =The ability & capacity to do work Measured by the capability of doing work: potential energy or the conversion of this capability to motion: kinetic energy Examples of Potential Energy: Stretching a rubber band.. -Stores energy Water at the top of a waterfall.. -Stores energy Yo–Yo in held in your hand.. -Stores energy because of position Drawing a Bow… -Stores energy because of position Calculating Potential Energy… • Energy due to position or stored energy. Example: PE= (mass) (gravity) (height) Due to gravity In this case potential energy is calculated by: The object’s mass, multiplied by the earth’s gravitational pull (9.8 m/sec sq), multiplied by the distance the object can fall. Potential Energy Converted to Kinetic Energy… When stored energy begins to move, the object now transfers from potential energy into kinetic energy. Standing still Running Examples of Kinetic Energy… • Shooting a rubber band. • Water falling over the fall. • A Yo-Yo in motion. • Releasing the arrow from the bow. Definition of Kinetic Energy… The energy of motion. Measured by: 2 KE= ½ (Mass) (Velocity) Kinetic energy is calculated by one half of the object’s mass, multiplied by the object’s speed- squared. Mechanical Energy Mechanical energy E is the sum of the potential and kinetic energies of an object. E=U+K The total mechanical energy in any isolated system of objects remains constant if the objects interact only through conservative forces: E = constant Ef = Ei Uf + Kf = Ui+ Ki Conservation of Mechanical Energy If friction and wind resistance are ignored, a bobsled run illustrates how kinetic and potential energy can be interconverted, while the total mechanical energy remains constant. Heat and temperature Kinetic molecular theory Measures of heat Metric units English system • calorie (cal) - energy needed to raise temperature of 1 g of water 1 degree Celsius • kilocalorie (kcal, Calorie, Cal) - energy needed to raise temperature of 1 kg of water 1 degree Celsius • British thermal unit (BTU) - energy needed to raise the temperature of 1 lb of water 1 degree Fahrenheit Mechanical equivalence • 4.184 J = 1 cal Heat • A form of energy transfer between two objects • External energy - total potential and kinetic energy of an every-day sized object • Internal energy - total kinetic energy of the molecules in that object • External can be transferred to internal, resulting in a temperature increase Heat versus temperature Temperature • A measure of hotness or coldness of an object • Based on average molecular kinetic energy – Recall difference in KE and temp. between solids, liquids & gases Heat • Based on total internal energy of molecules • Doubling amount at same temperature doubles heat Specific heat Variables involved in heating • Temperature change • Mass • Type of material – Different materials require different amounts of heat to produce the same temperature change – Measure = specific heat Summarized in one equation Energy, heat, and molecular theory Two responses of matter to heat 1. Temperature increase within a given phase – – Heat goes mostly into internal kinetic energy Specific heat 2. Phase change at constant temperature – – Related to changes in internal potential energy Latent heat Heat flow Three mechanisms for heat transfer due to a temperature difference 1. Conduction 2. Convection 3. Radiation Natural flow is always from higher temperature regions to cooler ones Conduction • Heat flowing through matter • Mechanism – Hotter atoms collide with cooler ones, transferring some of their energy – Direct physical contact required; cannot occur in a vacuum • Poor conductors = insulators (Styrofoam, wool, air…) Sample conductivities Material Relative conductivity Silver 0.97 Iron 0.11 Water 1.3x10-3 Styrofoam 1.0x10-4 Air 6.0x10-5 Vacuum 0 Convection • Energy transfer through the bulk motion of hot material • Examples – Space heater – Gas furnace (forced) • Natural convection mechanism - “hot air rises” Radiation • Radiant energy - energy associated with electromagnetic waves • Can operate through a vacuum • All objects emit and absorb radiation • Temperature determines – Emission rate – Intensity of emitted light – Type of radiation given off • Temperature determined by balance between rates of emission and absorption – Example: Global warming Thermodynamics • The study of heat and its relationship to mechanical and other forms of energy • Thermodynamic analysis includes – System – Surroundings (everything else) – Internal energy (the total internal potential and kinetic energy of the object in question) • Energy conversion – Friction - converts mechanical energy into heat – Heat engines - devices converting heat into mechanical energy – Other applications: heat pumps, refrigerators, organisms, hurricanes, stars, black holes, …, virtually any system with energy inputs and outputs The first law of thermodynamics • Conservation of energy • Components – Internal energy – Heat – Work • Stated in terms of changes in internal energy • Application: heat engines Electric current • A flow of charge is called an electric current I Q / t • It is measured in ampere (A=C/s) • Need free charge to have electric current. Use conductors. Note: net charge =0 + + + + + + + Skiing electric circuit High PE High PE Low PE Low PE Skiers Charges go from points with high PE to low PE To complete the circuit need a device that brings you back to high PE: Ski lift Battery Ohm’s law • Electric current is proportional to voltage. I V V IR • Coefficient in this dependence is called resistance R • Resistance is measured in Ohm (W = V/A) I R V Resistance and Temperature • When electrons move through the conductor they collide with atoms: – Temperature of the conductor increases because of the current (through collisions) – Electrical energy is transformed into thermal energy – Resistors dissipate energy – Power – energy per unit of time- (in W=J/s) dissipated by a resistor PI R 2 Electric power • Electric energy can be converted into other kinds of energy: – – – – Thermal ( toaster) Light (bulbs) Mechanical (washer) Chemical • Electric power (energy per unit of time): P IV What Produces Voltage? A Battery 9V Lab Power Supply Solar Cell 1.5 V Electric Power Plant 13,500 V A few Volts Nerve Cell A few millivolts when activated by a synapse Simple Electromagnetic Energy Moving electric charges. Examples: • Power lines carry electricity • Electric motors are driven by electromagnetic energy • Light is this form of energy (X-rays, radio waves, laser light etc.) Or Radiation Energy? • Electromagnetic radiation -- photons (visible light, infrared, ultraviolet, x-rays, and radio waves). • Light is an electromagnetic wave Chemical Energy • Energy that exists in the bonds that hold atoms together. • When bonds are broken, chemical energy is released. Examples: • Digesting food…bonds are broken to release energy for your body to store and use. • Sports… your body uses energy stored in your muscles obtained from food. • Fire–a chemical change. Sodium metal reacts with water. Nuclear Energy • When the nucleus of an atom splits, nuclear energy is released. • Nuclear energy is the most concentrated form of energy. • Fission/fusion