Heat, Temperature, Heat Transfer & Thermodynamics
Heat vs. Temperature Heat Temperature A form of energy Measured in calories or Joules There is no “coldness” energy Any object with temperature above zero Kelvin has heat energy Temperature Avg. Kinetic Energy of the particles Measured in C, F, K, R “hot” & “cold are relative terms Absolute zero is zero Kelvin
Heat Transfer (3 methods) 1. Conduction - requires direct contact or particle to particle transfer of energy; usually occurs in solids 2. Convection - heat moves in currents; only happens in fluid states of matter 3. Radiation - heat waves travel through empty space, no matter needed; IR
Thermal Equilibrium A system is in thermal equilibrium when all of its parts are at the same temperature. Heat transfers only from high to low temperatures and only until thermal equilibrium is reached.
Temperature Scales There are four temperature scales – Celsius (Centigrade), Kelvin, Fahrenheit, & Rankine Celsius, C – metric temp. scale Kelvin, K – metric absolute zero temp. scale Fahrenheit, F – customary (english) temp. scale Rankine, R – english absolute zero temp scale
Comparing Temperature Scales All temperatures listed are for water Celsius - Freezing = 0°C, Boiling = 100°C Kelvin - Freezing = 273K, Boiling = 373K Fahrenheit- Freezing = 32°F, Boiling = 212°F Conversions between Scales °F = 1.8 *°C+32 K = °C + 273
Change of State steam vaporization Heat of fusion 100 condensation water Temp ° C melting Heat of vaporization ice freezing -20 Increasing Heat Energy (Joules) As heat is added to a substance it will either be absorbed to raise the temperature OR to change the state of matter. It can NEVER do both at the same time. Temperature will NOT change during a phase change!
Specific Heat The amount of heat energy needed to raise the temperature of 1 gram of substance by 1°C. Substances with lower specific heats change temperature faster. Symbol : c units : cal/g°C or J/kg°C For water: c = 1 cal/g°C = 4.18 J/g°C = 4180 J/kg°C
Latent Heat The amount of heat energy required to change the state of 1 gram of substance. Heat of fusion - latent heat for changes between the solid and liquid phases. Lf =80 cal/g for water Heat of vaporization - latent heat for changes between the liquid and gas phases. Lv =540 cal/g for water
Heat Calculations Q = mL Q = mcΔT Phase Change Temperature Change Q = heat absorbed or released m = mass of substance changing phase L = latent heat of substance Lf = heat of fusion (liquid solid) Lv = heat of vaporization (liquid gas) Q = heat absorbed or released m = mass of substance being heated c = specific heat of substance ΔT = change in temperature
Thermodynamics The study of changes in thermal properties of matter Follows Law of Conservation of Energy 1st Law – the total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it 2nd Law – natural processes tend to increase the total entropy (disorder) of the universe.
1st Law of Thermodynamics The total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it. ΔU = W + Q ΔU = change in the thermal energy of the system W = work done on the system (W = Fd or W=ΔK) Q = heat added to the system (Q is + if absorbed, Q is – if released) *All measured in Joules*
Heat engines Convert thermal energy to mechanical energy Require high temp heat source and low temp heat sink. (Takes advantage of heat transfer process) Examples: Steam engine, Automobile engine
Refrigerators and Heat Pumps It is possible to remove heat from a cold environment and deposit it into a warmer environment. This requires an outside source of energy. Examples: Refrigerators, Air conditioning units Heat pumps are refrigeration units that work in either direction.
2nd Law of Thermodynamics All natural processes go in a direction that increases the total entropy of the universe. Entropy is a measure of the disorder of a system. If heat is added, entropy is increased. If heat is removed, entropy is decreased. Work with no ΔT, entropy is unchanged