Heat, Temperature, Heat Transfer, Thermal Expansion & Thermodynamics

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 Temperatur e 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 te r a w melting 0 -20 ic e Temp C 100 freezing ts ea m 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)

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 mHf Q mHv Q heat absorbed or released 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) ΔT change in temp., C or K J/kg (liquid gas)

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 L0 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, mL, cm3 ) 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 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.