Heat, Temperature, Heat Transfer, Thermal Expansion & Thermodynamics

Heat vs. Temperature Heat Heat A form of energy A form of energy Measured in calories or Joules Measured in calories or Joules There is no “coldness” energy There is no “coldness” energy Any object with temperature above zero Kelvin has heat energy Any object with temperature above zero Kelvin has heat energy Temperature Temperature Avg. Kinetic Energy of the particles Avg. Kinetic Energy of the particles Measured in  C,  F, K, R Measured in  C,  F, K, R “hot” & “cold are relative terms “hot” & “cold are relative terms Absolute zero is zero Kelvin 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. 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. 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 There are four temperature scales – Celsius (Centigrade), Kelvin, Fahrenheit, & Rankine Celsius,  C – metric temp. scale Celsius,  C – metric temp. scale Kelvin, K – metric absolute zero temp. scale Kelvin, K – metric absolute zero temp. scale Fahrenheit,  F – customary (english) temp. scale Fahrenheit,  F – customary (english) temp. scale Rankine, R – english absolute zero temp scale Rankine, R – english absolute zero temp scale

Comparing Temperature Scales Celsius - Freezing = 0°C, Boiling = 100°C Celsius - Freezing = 0°C, Boiling = 100°C Kelvin - Freezing = 273K, Boiling = 373K Kelvin - Freezing = 273K, Boiling = 373K Fahrenheit- Freezing = 32°F, Boiling = 212°F Fahrenheit- Freezing = 32°F, Boiling = 212°F Conversions between Scales °F = 1.8 *°C+32K = °C All temperatures listed are for water

Change of State Temp ° C Increasing Heat Energy (Joules) ice water steam melting vaporization condensation freezing 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! Heat of vaporization Heat of fusion

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 : cunits : 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. L f =80 cal/g for water Heat of vaporization - latent heat for changesbetween the solid and liquid phases. L v =540 cal/g for water

Heat Calculations Q = mcΔT Temperature Change Q = heat absorbed or released m = mass of substance being heated c = specific heat of substance ΔT = change in temperature Phase Change Q = mL Q = heat absorbed or released m = mass of substance changing phase L = latent heat of substance L f = heat of fusion (liquid solid) L v = heat of vaporization (liquid gas)

Thermodynamics The study of changes in thermal properties of matter The study of changes in thermal properties of matter Follows Law of Conservation of Energy Follows Law of Conservation of Energy 1 st 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 1 st 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 2 nd Law – natural processes tend to increase the total entropy (disorder) of the universe. 2 nd Law – natural processes tend to increase the total entropy (disorder) of the universe.

1 st 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 Convert thermal energy to mechanical energy Require high temp heat source and low temp heat sink. (Takes advantage of heat transfer process) Require high temp heat source and low temp heat sink. (Takes advantage of heat transfer process) Examples: Steam engine, Automobile engine 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. It is possible to remove heat from a cold environment and deposit it into a warmer environment. This requires an outside source of energy. This requires an outside source of energy. Examples: Refrigerators, Air conditioning units Examples: Refrigerators, Air conditioning units Heat pumps are refrigeration units that work in either direction. Heat pumps are refrigeration units that work in either direction.

2 nd 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

Thermal Expansion Substances expand as they heat and contract as they cool. Substances expand as they heat and contract as they cool. The rate of expansion depends on the substance’s coefficient of expansion (α) The rate of expansion depends on the substance’s coefficient of expansion (α) 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. 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) Calculations LoLo ΔLΔL Linear expansion: objects expand along linear dimensions such as length, width, height, diameter, etc. ΔL = L o α ΔT ΔL = change in length measurement (same units as original length) Lo = original length (may be in any units); ΔT = change in temperature (°C); α = linear coefficient of expansion (/ °C)

Thermal Expansion (Volume) Calculations Volume expansion: since objects expand in all dimensions, volume also expands. ΔV = V o β ΔT ΔV = change in volume (same units as original volume) V o = original volume (may be in any units); ΔT = change in temperature (°C); β = linear coefficient of expansion (/ °C)