ENERGY. Unit 12: IPC

Transcription

1 ENERGY Unit 12: IPC

2 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

3 THERMAL internal motion of particles The ability to cause change. NUCLEAR changes in the nucleus CHEMICAL bonding of atoms ENERGY joules (J) MECHANICAL motion of objects ELECTRICAL motion of electric charges

4 ENERGY Kinetic Energy (KE) - energy in the form of motion depends on mass and velocity KE = 1/2mv 2 Which has the most KE? 80 km/h truck Which has the least KE? 50 km/h motorcycle 50 km/h 80 km/h

5 ENERGY Potential Energy (PE) - stored energy depends on position or configuration of an object PE = mgh g = 9.8 m/s 2 on Earth Which boulder has greater gravitational PE?

6 TYPES OF POTENTIAL ENERGY Gravitational due to position above Earth s surface Elastic- stored in something stretchy Chemical result of a chemical reaction

7 WORK W Work transfer of energy through motion F d force exerted through a distance W = Fd W: work (J) F: force (N) d: distance (m) 1 J = 1 N m J = Joules Distance must be in direction of force!

8 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

9 ENERGY Thermal Energy

10 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

11 UNITS OF TEMPERATURE Celsius scale defined by two readily determined temperatures: Freezing point of water = 0 o C Boiling point of water = 100 o C 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 = o C + 273

12 COMMON TEMPERATURE SCALES

13 TEMPERATURE CONVERSIONS

14 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

15 THERMAL ENERGY Which beaker of water has more thermal energy? B - same temperature, more mass 80ºC 80ºC A B 400 ml 200 ml

16 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

17 OTHER UNITS FOR THERMAL ENERGY calorie = 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) = 1.5x10 3 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.

18 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

19 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

20 STATES/PHASES OF MATTER

21 PHASE CHANGES

22 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.

23 HEAT TRANSFER Specific Heat (C p ) amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin units: J/(kg K) or J/(kg C) Specific Heat Values (J/(kg K)) Water Alcohol Aluminum Carbon (graphite) Sand Iron Copper Silver

24 HEAT TRANSFER Which sample will take longer to heat to 100 C? 50 g Al 50 g Cu Specific Heat Values (J/(kg K)) Water Alcohol Aluminum Carbon (graphite) Sand Iron Copper Silver Al - It has a higher specific heat. Al will also take longer to cool down.

25 HEAT TRANSFER-CALORIMETRY Q = m T C p Q: heat (J) m: mass (kg) T: change in temperature (K or C) C p : specific heat (J/kg K) T = T f - T i Q = heat loss + Q = heat gain

26 HEAT TRANSFER Calorimeter device used to measure changes in thermal energy in an insulated system Coffee cup Calorimeter heat gained = heat lost

27 A 32-g silver spoon cools from 60 C to 20 C. How much heat is lost by the spoon? GIVEN: m = 32 g T i = 60 C T f = 20 C Q =? C p = 235 J/kg K WORK: Q = m T C p m = 32 g = kg T = 20 C - 60 C = 40 C Q = (0.032kg)(-40 C)(235J/kg K) Q = 301 J

28 How much heat is required to warm 230 g of water from 12 C to 90 C? GIVEN: m = 230 g T i = 12 C T f = 90 C Q =? C p = 4184 J/kg K WORK: Q = m T C p m = 230 g = 0.23 kg T = 90 C - 12 C = 78 C Q = (0.23kg)(78 C)(4184 J/kg K) Q = 75,061 J