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Thermal Energy & Conservation

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Thermal Energy & Conservation ENERGY Unit 12: IPC WHAT IS ENERGY? • Energy- is the ability to do work. • Energy is the ability to cause a change. • Energy can change an object’s: motion shape temperature color THERMAL The ability to cause change. internal motion of particles MECHANICAL NUCLEAR ENERGY motion of objects changes in the nucleus CHEMICAL ELECTRICAL motion of electric bonding of atoms joules (J) charges ENERGY • Kinetic Energy (KE) - energy in the form of motion • depends on mass and velocity • KE = 1/2mv2 • Which has the most KE? 80 km/h truck 80 km/h • Which has the least KE? 50 km/h 50 km/h motorcycle ENERGY • Potential Energy (PE) - stored energy • depends on position or configuration of an object • PE = mgh • “g” = 9.8 m/s2 on Earth • Which boulder has greater gravitational PE? TYPES OF POTENTIAL ENERGY • Gravitational – due to position above Earth’s surface • Elastic- stored in something stretchy • Chemical – result of a chemical reaction WORK W • Work F d • transfer of energy through motion • force exerted through a distance W: work (J) F: force (N) d: distance (m) W = Fd 1 J = 1 N·m J = Joules Distance must be in direction of force! CONSERVATION OF ENERGY • Law of Conservation of Energy- Energy may change forms, but it cannot be created or destroyed under ordinary conditions. • EX.’s of Changing Forms: • PE KE • mechanical thermal • chemical thermal ENERGY Thermal Energy TEMPERATURE • Temperature - measure of the average kinetic energy (KE) of the particles in a sample of matter • (hotness or coldness) • Adding or removing energy usually changes temperature UNITS OF TEMPERATURE • Celsius scale defined by two readily determined temperatures: • Freezing point of water = 0 oC • Boiling point of water = 100 oC • Kelvin scale does not use the degree sign, but is just represented by K • absolute zero = 0 K = all motion stops (thus no negative values) • formula to convert: K = oC + 273 COMMON TEMPERATURE SCALES TEMPERATURE CONVERSIONS THERMAL ENERGY • Thermal Energy - the total energy of the particles in a material • KE - movement of particles • PE - forces within or between particles due to position • depends on temperature, mass, and type of substance THERMAL ENERGY • Which beaker of water has more thermal energy? • B - same temperature, more mass 80ºC 80ºC A B 400 mL 200 mL HEAT TRANSFER • Heat - thermal energy that flows from a warmer material to a cooler material • Like work, heat is... • measured in joules (J), named after James Prescott Joule • is a transfer of energy OTHER UNITS FOR THERMAL ENERGY • calorie = 4.186 Joules • calorie is the amount of thermal energy that must be added to water to change the temperature of one gram of water by one degree Celsius • kilocalorie (kcal, or Calorie) = 4,186 J. • Food energy is measured in Calories • Btu (British Thermal Unit) = 1.5x103 J. • the amount of thermal energy that must be added to water to change the temperature of one pound of water by one degree Fahrenheit. HEAT TRANSFER Why does A feel hot and B feel cold? . Heat flows from A to your hand = hot. Heat flows from your hand to B = cold. 80ºC 10ºC A B HEAT & PHASE CHANGES • There are situations in which the addition or removal of heat does not cause a temperature change • Heat must be added or removed to make a material change its phase • Amount of heat added or removed depends on the type of material and nature of the phase STATES/PHASES OF MATTER PHASE CHANGES HEAT = TRANSFER OF ENERGY • The 3 methods that heat can be transferred: • Conduction – heat transfers by direct contact • Convection - is the process in which heat is carried from one place to another by the bulk movement of a fluid. • Radiation- heat transfers by electromagnetic waves. HEAT TRANSFER • Specific Heat (C ) Specific Heat Values p (J/(kg·K)) • amount of energy Water 4184 required to raise the Alcohol 2450 temp. of 1 kg of Aluminum 920 material by 1 degree Carbon (graphite) 710 Kelvin Sand 664 Iron 450 • units: J/(kg·K) Copper 380 or J/(kg·°C) Silver 235 HEAT TRANSFER • Which sample will take Specific Heat Values longer to heat to 100°C? (J/(kg·K)) Water 4184 Alcohol 2450 Aluminum 920 Carbon (graphite) 710 50 g Al 50 g Cu Sand 664 Iron 450 Copper 380 Silver 235 • Al - It has a higher specific heat. • Al will also take longer to cool down. HEAT TRANSFER-CALORIMETRY Q = m T Cp Q: heat (J) m: mass (kg) T: change in temperature (K or °C) Cp: specific heat (J/kg·K) – Q = heat loss + Q = heat gain T = Tf - Ti HEAT TRANSFER • Calorimeter • device used to measure changes in thermal energy in an insulated system Coffee cup Calorimeter heat gained = heat lost A 32-g silver spoon cools from 60°C to 20°C. How much heat is lost by the spoon? GIVEN: WORK: m = 32 g Q = m·T·Cp Ti = 60°C m = 32 g = 0.032 kg Tf = 20°C T = 20°C - 60°C = – 40°C Q = ? Q = (0.032kg)(-40°C)(235J/kg·K) Cp = 235 J/kg·K Q = – 301 J How much heat is required to warm 230 g of water from 12°C to 90°C? GIVEN: WORK: m = 230 g Q = m·T·Cp Ti = 12°C m = 230 g = 0.23 kg Tf = 90°C T = 90°C - 12°C = 78°C Q = ? Q = (0.23kg)(78°C)(4184 J/kg·K) Cp= 4184 J/kg·K Q = 75,061 J .
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