Heat and Temperature

ISNS 3371 - Phenomena of Nature

Heat and Temperature

Heat: The amount of energy transferred from one body to another by virtue of a temperature difference between them.

Heat is not the energy content of a body although it is frequently used that way. The proper term is thermal energy.

Unit of heat is: Joule

Temperature: A measure of the average kinetic energy of the molecules of a substance.

That property of a body of matter that gives rise to sensations of hot and cold.


Temperature and Thermal Energy

Why does water burn your skin so much quicker than air?Why is falling into a 32º F lake more dangerous than standing outside naked on a 32º F?

Temperature - measure of the average kinetic energy of the particles in a substance - particles in box on right have higher temperature - higher velocity = more KE = higher temperature

Both boxes have same temperature - particles have same average velocity/KE - box on right has more thermal energy - energy contained in a substance - more particles


Temperature is a measure of the average kinetic energy of the particles in a substance. This diagram compares three common temperature scales. The Fahrenheit scale is used in the United States, but nearly all other countries use the Celsius scale. Scientists prefer the Kelvin scale because O K represents absolute zero, the coldest possible temperature.


TEMPERATURE SCALES

WATER __________________________

ABSOLUTE ZERO FREEZING POINT BOILING POINT_________________________________________________FAHRENHEIT -459° 32° 212°CELSIUS -273° 0° 100°KELVIN (ABSOLUTE) 0° 273° 373°

CONVERSIONS:CELSIUS TO FAHRENHEITF = 9/5C + 32FAHRENHEIT TO CELSIUSC = 5/9 x (F - 32)CELSIUS TO KELVINK = C + 273.


Thermal Energy

Thermal Energy: Total of all energies, kinetic plus potential, internal to a substance.

Quantity of Heat: Calorie or Joule

1 calorie = Amount of thermal energy required to change the temperature of 1 gram of water 1°C.

1 joule = 0.239 calories (1 Calorie = 4.187 joules)

1 kilocalorie = 1,000 calories (usually spelled with a capital C)


Specific Heat Capacity, c: Thermal inertia

Specific Heat Capacity is the quantity of heat required to change the temperature of 1 gram of a substance by 1° C.

If Q units of of thermal energy added to 1 gram a substance produce a temperature change of ∆T,

Q = c x ∆T

Specific heat , c, of a substance is the heat capacity per unit mass.

For m grams of a substance,

Q = cm ∆T

Water has high specific heat capacity - used as a cooling fluid.

Specific heat capacity of water is 1calorie per gram-deg. C.


Heat Transfer Processes

Conduction – transfer of heat from a region of higher temperature to a region of lower temperature by increased kinetic energy moving from molecule to molecule through collisions between molecules. Occurs in solids.

Convection – transfer of heat from a region of higher temperature to a region of lower temperature by the flow of higher energy molecules. Occurs in gases and liquids.

Radiation – transfer of heat by emission and absorption of radiant energy (energy that can travel through space as electromagnetic radiation, like visible light).


The Laws of Thermodynamics

First Law:

Whenever heat flows in or out of a system, the gain or lass of thermal energy equals the amount of heat transferred.

Second Law:

Heat never spontaneously flows from a cold substance to a hot substance.

Third law:

No system can reach absolute zero.


The state or phase of matter is determined by its temperature.

Consider water:

Below 32º F - ice - relatively low KE - each molecule tightly bound to it neighbors - solid

At 32º F molecules have enough energy to break solid bonds of ice - remain together but move relatively freely - liquid

At 212º F water boils and turns to gas - molecules break free of all bonds with neighbors - move independently of other molecules - gas


Phase/State Changes

Melting - when a solid changes to a liquid

Evaporation - when a liquid changes to a gas

Sublimation - when a solid changes directly to a gas

Condensation - when a gas changes to a liquid

Freezing - when a liquid changes to a solid

Energy is absorbed

Energy is released

Heat transfer always occurs whenever a substance changes phase

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Heat and Temperature