 # Heat Temperature Heat Transfer Thermal Expansion Thermodynamics Heat

• Slides: 22 Heat, Temperature, Heat Transfer, Thermal Expansion & Thermodynamics Heat vs. Temperature • Heat • 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 • “hot” & “cold are relative terms • Absolute zero is zero Kelvin Heat Transfer 1. Conduction - requires direct contact or particle to particle transfer of energy; usually occurs in solids 2. Convection - heat moves in currents; hot air rises and cold air falls; only occurs in fluids 3. Radiation - heat waves travel through empty space, no matter needed; sun 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 • Celsius, C – metric temp. scale • Fahrenheit, F – customary (english) temp. scale • Kelvin, K – metric absolute zero temp. scale • Rankine, R – english absolute temp. scale Comparing Temperature Scales (All temperatures listed are for water) • Freezing = 0°C = 273 K = 32°F • Boiling = 100°C = 373 K = 212 °F Conversions between Scales °F = 1. 8 x°C+32 = 9/5 °C + 32 °C = (°F – 32) / 1. 8 = 5/9 (°F – 32) K = °C + 273 or °C = K - 273 Change of State vaporization Heat of fusion w melting 0 -20 ice Temp ° C 100 freezing r ate m ts ea condensation Heat of vaporization 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 (or kg) of a substance by 1°C (or 1 K). Substances with higher specific heats, such as water, change temperature more slowly. Symbol : c units : cal/(g°C) or J/(kg°C) For water: c = 4. 186 J/(g°C) = 4186 J/(kg°C) or c = 1 cal/ (g°C) Latent Heat (Latent) Heat of fusion – the heat energy needed to melt (solid→liquid) or freeze (liquid → solid) one gram (or kg) of a substance. For water: Hf =334, 000 J/kg or 80 cal/g (Latent) Heat of vaporization – the heat energy needed to vaporize (liquid→gas) or condense (gas→liquid) one gram (or kg) of a substance. For water: Hv = 2. 26 x 105 J/kg or 540 cal/g Heat Calculations Temperature Change Q = mcΔT Phase Change Q = m. Hf Q = m. Hv Q = heat absorbed or released, J m = mass of substance changing phase, kg m = mass of substance being heated, kg Hf = heat of fusion, J/kg (liquid solid) c = specific heat of substance, H = heat of vaporization, J/kg v J/(kg°C) J/kg (liquid gas) ΔT = change in temp. , °C or K Melting & Boiling Point • Melting or Freezing Point – the temperature at which a substance melts or freezes. Water: 0°C • Boiling or Condensation Point – the temperature at which a substance vaporizes or condenses. Water: 100°C • For other substances, refer to your chart. Thermal Expansion • Substances expand as they heat and contract as they cool. • The rate of expansion depends on the substance’s coefficient of expansion ( α for linear, β for volume) • The exception to this rule is water. As water is cooled from 4°C to 0°C, it expands which explains why ice floats (it is less dense than water). Thermal Expansion - Linear Lo ΔL Linear expansion: objects expand along linear dimensions such as length, width, height, diameter, etc. ΔL = Loα ΔT LF = L 0 + ΔL ΔL = change in length measurement, (same units as original length) Lo = original length, (any length unit – m, cm, in) ΔT = change in temperature (°C) = Tf – Ti α = coefficient of linear expansion (1 / °C or °C -1 ) LF = final length, (same units as original length) Thermal Expansion (Volume) Volume expansion: since objects expand in all dimensions, volume also expands. ΔV = Voβ ΔT VF = VO + ΔV ΔV = change in volume (same units as original volume) Vo = original volume, (any volume units – L, m. L, cm 3 ) ΔT = change in temperature (°C) = Tf – Ti β = coefficient of volume expansion (1 / °C or °C -1 ) VF = final volume, (same units as original) Thermodynamics The study of changes in thermal properties of matter 0 th Law of Thermodynamics • Heat will be transferred between objects until thermal equilibrium (same temperature) is reached. 1 st Law of Thermodynamics • Conservation of energy • Energy can not created or destroyed but can change forms. • Thermal energy can be transferred from one substance to another. 1 st Law of Thermodynamics The total increase in 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 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* 2 nd Law of Thermodynamics • Natural (spontaneous) processes tend to increase the total entropy (disorder) of the universe. • Entropy increases when heat is added to a body and decreases when heat is removed. • Heat flows naturally from a hot body to a cold body. rd 3 law of Thermodynamics • As the temperature of an object approaches absolute zero (0 Kelvin), its entropy and kinetic energy approach zero. 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.