Ch 16 Thermal Energy and Heat

16.1 Thermal Energy and Matter

• In the 1700’s scientists thought heat was a fluid called a caloric that flowed between objects.

• In 1798, the scientist Count Rumford concluded, from his observations, that heat could not be a kind of matter but instead was related to the motion of objects

Work and Heat

• Recall… some of the work done by machines is lost due to friction, which causes moving parts to heat up

• Heat = the transfer of thermal energy from one object to another because of a temperature difference

• Heat flows spontaneously from hot objects to cold objects

Temperature

• Temperature = a measure of how hot or cold an object is compared to a reference point. Such as the freezing and boiling pt of water.

Temperature is related to the average kinetic energy of the particles in an object due their random motions

Thermal Energy

• Recall…• Thermal Energy = the total potential

and kinetic energy of all the particles in an object

• Thermal energy depends on the mass, temp, and phase of an object

• Thermal energy, unlike temp, depends on mass

• Ex: a cup of tea vs a teapot full of tea at 98º– Both have the same temp but the

teapot has more particles = more thermal energy

Thermal Expansion & Contraction

• Thermal expansion = occurs when particles of matter move farther apart as temp increases

• Gases expand more than liquids and liquids more than solids

Ex: this is how thermometers work

Specific Heat

• Specific Heat = the amount of heat needed to raise the temp of one gram of material by one degree Celsius

• Units = J/g•Cº

• The lower a material’s specific heat, the more its temp rises when a given amount of energy is absorbed by a given mass

• Ex: water = 4.18, air = 1.01• Ex: water and alcohol

16.2 Heat and Thermodynamics

• When you take cookies out of a hot oven, why aren’t you burned by the hot air but would be burned by touching the metal rack?

• Air is not a good conductor of heat while metal is

Conduction

• Conduction = the transfer of thermal energy with no overall transfer of matter

• Conduction occurs within a material or between materials that are touching

• Collisions between particles transfer thermal energy

• Conduction in gases is slower than in liquids and solids because the particles are farther apart

• In solids, conduction occurs as particles vibrate in place and in metals conduction is faster because of the free electrons that collide and transfer heat

• Thermal conductor = a material that conducts heat well

• Ex: metals

• Thermal insulators = a material that conducts heat poorly

• Ex: air, plastics, foam, wool, wood

Convection

• Convection = the transfer of thermal energy when particles of a fluid move from place to place

• Ex: air circulating in an oven creates convection currents

• Convection currents are important in many natural cycles: ocean currents, weather systems, movement of hot rock in Earth’s interior

Radiation

• Radiation = the transfer of thermal energy by waves moving through space

• All objects radiate heat energy.

Thermodynamics

• Thermodynamics = the study of conversions between heat and other forms of energy

• First Law of Thermodynamics = energy is conserved

• Second Law of Thermodynamics = thermal energy can flow from colder to hotter objects only if work is done on the system

• Ex: refrigerators must do work to transfer thermal energy from the cold food compartment to the warm room

• Third Law of Thermodynamics = absolute zero cannot be reached

• Scientists have gotten close…within 3 billionths of a kelvin above absolute zero