Transcript Ch 16 Thermal Energy and Heat

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