Chapter 12 Thermal Energy Glencoe 2005 Honors Physics Bloom High School

12.1 Temperature & Thermal Energy Thermal Energy- total energy of the molecules in a substance – Translational, rotational, vibrational, bending energies of molecules Average energy- related to temperature Solids- only exhibit vibrational energies in bonds

Thermal Energy & Temperature Which would you rather? – 1 drop of 100°C water on your arm – Fall into a pool of 100°C water Temperature- independent of amount of substance Thermal energy- dependant on amount of substance

Equilibrium & Thermometry Conduction- transfer of kinetic energy when particles collide Thermal equilibrium- rate of energy flow between the two objects is equal

Temperature Scales: Celsius & Kelvin Celsius scale- 100 degrees difference between freezing and boiling of pure water – 0°C corresponds to the freezing point of pure water – 100°C corresponds to the boiling point of pure water Kelvin- 100 degrees difference between freezing and boiling of pure water – 273K corresponds to the freezing point of pure water – 373K corresponds to the boiling point of pure water – 0K (absolute zero) represents zero kinetic energy of a substance

Temperature Scales Kelvin = °C °C = (°F-32) x °F = (1.8 x °C) +32

Heat and the Flow of Thermal Energy Heat (Q, Joules)- energy that is transferred between objects – Always flows from high energy area to low energy area – Nothing ever feels cold Conduction- heat flow due to physical contact Convection- the rising of a higher temperature fluid – Hot air or water rising; colder fluid sinking Radiation- transfer of energy through electromagnetic waves – Infrared increases average kinetic energy

Specific Heat Specific heat capacity (C, J/g°C)- the quantity of heat required to raise the temperature of 1g by 1°C – Ability to store internal energy A measured value for each substance (Table 12-1) Q=mC  T – Q=heat (J or kJ) – m=mass(kg or g) – C=specific heat (J/g°C or J/kgK) –  T=change in temperature

Calorimetry: Measuring Specific Heat Calorimeter- measures the  T and/or C of unknown substances with a reference substance Physics Physlet E.19.3

Conservation of E & Calorimetry E A +E B =constant – Any energy lost by the hotter object goes to colder object  E=Q=mC  T – In a closed, isolated system, the change in energy is equal to the heat transfer

Thermal Equilibrium m A C A  T A +m B C B  T B =0 – Substituting the second equation into the first – Equals zero because all energy is conserved – Can be rearranged to solve for any variable! – For  T, T f will be equal for both substances

12.2 Changes of State & the Laws of Thermodynamics Figure – Energy continues to be added throughout – Energy used to increase average kinetic energy or change state Melting/Freezing point- exists in solid and liquid state Boiling/Condensation point- exists in liquid and gas state

Phase Diagram of Water

Heat of Fusion Heat of fusion (H f, J/kg)- amount of energy needed to melt a kg of substance without  T – Q=mH f – Q is negative when solidifying (heat removed) Heat of Vaporization (H v, J/kg)- amount of energy needed to vaporize a kg of substance without  T – Q=mH v – Q is negative when condensing (heat removed) See Table 12-2 for values

Transition of Water 0°C ice -100°C steam How much energy does 1kg of ice (0°C) need to change to 1kg of steam (0°C)? Convert solid to liquid- Q=mH f – Q 1 =(1kg)(3.34x10 5 J/kg) Increase temperature of water- Q=mC  T – Q 2 =(1kg)(4180J/kgK)(100K) Convert liquid to gas- Q=mH v – Q 3 =(1kg)(2.26x10 5 J/kg) Q 1 +Q 2 +Q 3 =(3.34x10 5 J)+(4.18x10 5 J)+(2.26x10 5 J)

1 st Law of Thermodynamics First Law- changes in internal thermal energy (  U) of an object are equal to the heat (Q) that is added to the system minus the work (W) done by the object –  U=Q-W Heat Engine- converts thermal energy to mechanical energy

Combustion Engine Physics Physlet E.21.2

Efficiency, Refrigerators & Heat Pumps Efficiency- ratio of heat in to useful work out Refrigerator- heat engine in reverse – Heat energy flows from high to low – Refrigerator reverses this energy flow Heat Pump- a refrigerator that can be run in reverse – Remove heat or add heat to system

2 nd Law of Thermodynamics 2 nd Law- all processes naturally go in the direction of increasing disorder and increases the entropy of the universe Entropy- a measure of the disorder in a system – Dependant on thermal energy of system –  S=Q/T – If heat flows into system, disorder is increased – Hg tube

Entropy Can be reversed if work is added to the system