Heat, Temperature, Heat Transfer, Thermal Expansion & Thermodynamics

Heat vs. Temperature Temperature Heat Avg. Kinetic Energy of the particles Measured in C, F, K “hot” & “cold are relative terms Absolute zero is zero Kelvin 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

Heat Transfer Conduction - requires direct contact or particle to particle transfer of energy; usually occurs in solids Convection - heat moves in currents; hot air rises and cold air falls; only occurs in fluids (liquids and gases) 3.Radiation – electromagnetic (heat) waves travel through empty space, no matter is 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 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 (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 106 J/kg or 540 cal/g

Heat Calculations Q = mHf Q = mHv Q = mcΔT Phase Change Temperature Change Phase Change Q = mHf Q = mHv Q = mcΔT Q = heat absorbed or released m = mass of substance changing phase, kg Hf = heat of fusion, J/kg (liquid solid) Hv = heat of vaporization, J/kg J/kg (liquid gas) Q = heat absorbed or released, J m = mass of substance being heated, kg c = specific heat of substance, J/(kg°C) 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).

Thermodynamics The study of changes in thermal properties of matter

0th Law of Thermodynamics Heat will be transferred between objects until thermal equilibrium (same temperature) is reached.

1st 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.

2nd 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.

3rd 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.