HEAT

Thermal Energy Defined Thermal energy – total energy of the particles in a material

Heat defined Heat - is the thermal energy that flows from something of a higher temperature to something of a lower temperature.

Temperature Defined Temperature – measure of the average kinetic energy of the particles in a sample of matter.

Temperature units depend of scales The temperature scales most widely used today are the Fahrenheit, Celsius and Kelvin Scales.

Daniel Gabriel Fahrenheit He discovered, among other things, that water can remain liquid below its freezing point and that the boiling point of liquids varies with atmospheric pressure. Fahrenheit temperature scale, scale based on 32 degree for the freezing point of water and 212 degree for the boiling point of water, the interval between the two being divided into 180 parts.   Fahrenheit originally took as the zero of his scale the temperature of an equal ice-salt mixture and selected the values of 30 degree for the freezing point of water and 90 degree for normal body temperature.

 After Fahrenheit died in 1736, scientists calibrated his model of thermometer using 212 degrees, the temperature at which water boils, as the upper fixed point, and 32 degree for water freezing. When the Fahrenheit thermometer was recalibrated, normal human body temperature registered 98.6 rather than 96.

The Celsius temperature scale was invented by Anders Celsius (1701-1744) a Swedish Astronomer For his meteorological observations he constructed his world famous Celsius thermometer, with 0 for the freezing point of water and 100 for the boiling point.  After his death in 1744 the scale was reversed to its present form with zero as water’s freezing point.

Celsius Temperature Scale The Celsius scale often used by countries using the metric system. It is based on water’s freezing point – set at 0o and its boiling point – set at 100o.

Kelvin temperature scale The temperature scale is named after the British mathematician and physicist William Thomson Kelvin (1824 – 1907) who proposed it in 1848.

Kelvin temperature scale, has an absolute zero below which temperatures do not exist. Absolute zero or 0 K, is the temperature at which molecular energy is a minimum, and it is equal to a temperature of -273.15° on the Celsius scale.  The Kelvin degree is the same size as the Celsius degree. (When writing temperatures in the Kelvin scale, convention the degree symbol is left out and we merely use the letter K.)

Temperature Conversion Formulas Fahrenheit to Celsius Celsius to Fahrenheit Kelvin to Celsius

Thermal equilibrium The state in which two bodies in physical contact with each other have identical temperatures. (Not net transfer of thermal energy or heat. Thermal equilibrium is the basis for measuring temperature with thermometers in that the thermometer must be in thermal equilibrium with an object before a temperature reading can be taken. Physics -Serway/Faughn (Holt)

State of matter is related to temperature Solids, liquids, gases- states of matter determined by the rate at which atoms or molecules move. Solids- tightly packed with fixed volume & shape Liquids – not as tightly linked with fixed volume, but not shape. Gases- particle with little or no attraction, no fixed volume or shape.

For gases with one type of atom (monoatomic) temperature is understood in terms of kinetic energy. For other kinds of substances, molecules can rotate or vibrate, so other types of energy is present. Serway/Faughn Physics

Thermal Expansion Increasing the temperature of a gas at constant pressure causes the volume of the gas to increase. (Also occurs in solids and liquids) In general, as the temperature of a substance increases, the volume also increases. This is known as thermal expansion.

Plasma – Fourth State of matter It comprises the major component of the Sun and other stars. Heating a gas may ionize its molecules or atoms (reducing or increasing the number of electrons in them), thus turning it into plasma, which contains charged particles: positive ions and negative electrons or ions.

Units of Heat Physics -Serway/Faughn (Holt)

Thermal conduction Thermal conduction is the flow of heat by transfer of energy between particles. It is due to the collision or diffusion of particles along a gradient (higher kinetic energy to lower kinetic energy). We usually think heating by conduction as a direct heat transfer between materials. Example: Heating food in a pot or pan on the stove.

Metals are very good thermal conductors Thermal conductors are substances that rapidly transfer energy as heat. Metals are very good thermal conductors

Thermal insulators Substances that slowly transfer heat.

R values of Insulation The R-value indicates resistance to heat flow. The R-value of a material is a measure of the resistance of that a 1 m x 1 m square slab of material has , to heat flow per centimeter of the material’s thickness.

To have a well-insulated home, materials with a total R-value of at least 19 should be used in the outer walls and those with total R-values of 30 – 44 should be used in the roof and ceiling because the warmest air inside the house is carried upward by convection currents. – Glencoe Physical Science

A puffy winter coat insulates with pockets of air trapped within the fill sewn into the coat. Bird feathers can insulate the bird by trapping air (dead air spaces).

Dead-Air Spaces as Insulation Even though glass is a good insulator, windows transfer thermal energy. A single pane of glass has an R-value of 1. A double-pane window traps air between the panes to produce greater insulating properties.

Transfer of heat through Convection Convection heat transfer involves the movement of molecules within fluids, such as gases and liquids. Convection heat and mass transfer happens as the result of diffusion.

Convection Currents Movement of cold or hot matter, such as heated air rising upward from a flame. Colder air is denser than warmer air. As it moves downward it caused convection currents which heat the air faster than by conduction alone. (Similar with water or other liquids)

Transfer of Heat Through Radiation Transfer of heat in the form of waves. It does not require matter to be present for transfer. Heat from the sun that warms the Earth is an example of radiant energy transfer.

Home Heating Systems Forced air (gas or oil furnace) convection–(uses fan in air handler) Hot Water Radiators – radiation Fireplace – radiation & convection (with fan) Electric heater – radiation & convection (with fan – air handler) Coal or wood stoves – radiant

Specific Heat Capacity The specific heat of a substance is defined as the energy required to change the temperature of 1 kg of a substance by 1oC. Every substance has a unique specific heat capacity. This value tells you how much the temperature of a given mass of that substance will increase or decrease, based on how much heat energy is added or removed.

Specific Heat of Water A relatively high specific heat capacity of water means that it resists changes in temperature. This makes it great for cell cytoplasm since it provides more stability in temperature.

Calculation of Specific Heat C = specific heat capacity Units [J/(kg.K)] m = mass T = change in temperature Tfinal – Tinitial Q = heat energy transferred

Sample Calculation A 32.0 g silver spoon cools from 60o C to 20oC. What is the change in thermal energy? m = 32.0g = 0.0320 kg C = 235 J/(kg.K) –from table of values for silver Tinitial =60oC Tfinal =20oC Q = (m)( T)(C) = (0.0320 kg)(-40.0oC)(235J/kg . K) = -301 J The spoon loses 301 J of thermal energy as it cools.