Chapter 6: Thermal Energy

Section 1: Temperature and Heat

• What is Temperature and how is it related to heat?

• In all materials: solids, liquids and gases particles are in constant motion

Temperature

• -of an object is related to the average kinetic energy of the atoms or molecule

• the faster the particles move, the more kinetic energy they have, the higher temp of the object

• Ex: cup of hot tea and glass of ice tea

Thermal Energy

• The sum of all kinetic and potential energy of all the molecules in an object is its thermal energy

• When the TEMP of an object increases, avg. kinetic energy increases

• SO: thermal energy also increases (sum of kinetic and potential energy)

• Similarly, if the mass of an object increases, the thermal energy of the the object also increases–Ex: beakers w/different water

levels

Heat

• -is thermal energy that flows from something at a higher temp to something at a lower temp

• Heat is a form of energy, so it is measured in Joules

• Heat ALWAYS flows from warmer to cooler materials

• Ex: homemade ice cream

Specific Heat

• Definition:-the amount of heat needed to raise the temperature of 1 kg of some material by 1 degree C or 1 degree Kelvin

• Measured in J/kg (joules per kilogram Kelvin)

Water and Specific Heat

• Water is useful as a coolant since it has a higher specific heat than other substances

• It can absorb heat w/out a large change in temperature (ex: lakes)

• Ex: cooling system in cars• Change in thermal energy = mass X

change in temp X specific heat

• To measure specific heat, a CALORIMETER can be used—see pg. 163

Section 2: Transferring Thermal Energy

• Conduction: the transfer of thermal energy through matter by the direct contact of particles (mostly through solids)

• Occurs because all matter is made of atoms and molecules that are in constant motion– Transfer by collision

grabbing a handful of snow

• Heat can be transferred by conduction from one material to another or through one material

• Good Heat conductors: silver, copper and aluminum

• Poor Heat conductors: wood, plastic, glass and fiberglass

• Convection: the transfer of energy in a fluid by the movement of heated particles

• (in conduction particles collide w/ea other and transfer energy)

• In Convection: more energetic fluid particles move from one location to another and carry their energy with them

• Convection transfers heat from warmer to cooler parts of the fluid

• As a result, particles move faster, fluids expand and density increases

• Ex: Lava Lamp—pg. 165

• Radiation: the transfer of energy by electromagnetic waves (through air)– These waves can travel through space,

even when no matter is present

Radiant energy: energy that is transferred by radiation

ex: when you stand near a fire and feel warm

• When radiation strikes a material, some of the energy is absorbed, some is reflected and some may be transmitted through the material

• In a solid, liquid or gas, radiant energy can travel through the space between molecules and is absorbed and re-emitted by other molecules

• Therefore, radiation usually passes more easily through gases than through solids or liquids since particles are farther apart

• Controlling heat flow:– Insulators: materials that do not allow heat

to flow through easily

GOOD CONDUCTORS = BAD INSULATORS

Gases are usually better insulators than solids or liquids

Ex: jacket, fiberglass insulation in buildings, thermos bottle, etc.—pg. 176

Section 3: Using Heat

• Heating systems: all heating systems require some source of energy – Ex: wood, coal, burning fuel, – Forced air system: Fuel is burned in a furnace and

heats a volume of air– A fan then blows the warm air through a series of

large pipes/ducts– Cool air returns to the furnace through additional

vents

• Radiator systems: a closed metal container that contains hot water and steam

• Thermal energy contained in the hot water or steam is transferred to the air surrounding the radiator by conduction– Ex: small scale—electric radiators

• Electric Heating Systems: uses electrically heated coils placed in ceilings and floors to heat the surrounding air by conduction

• Convection then distributes the heated air through the room

• Solar Heating: • In passive solar heating systems, solar

energy heats rooms inside a building, but no mechanical devices are used to move heat from one area to another– Ex: greenhouse– Homes often have a wall of windows on the

south side of the house for passive solar heating

• In active solar heating, solar collectors absorb radiant energy from the sun

• The absorbed radiant energy heats water in pipes and a pump circulates the warm water to radiators in rooms of the house

• Some systems have large, insulated tanks for storing heated water

• Solar panels absorb heat to heat water in pipes

Heat Engine

• -an engine that converts thermal energy into mechanical energy

• Internal combustion engine—burns fuel inside the engine in chambers or cylinders

• (more cylinders = more power)• Some of the power produced is given off

as heat, therefore engines need cooling systems

• Heat Movers:– A device that removes thermal energy from

one location and transfers it to another location at a different temperature

– Ex: Refrigerators use a coolant (liquid that evaporates at a low temperature) to cool the inside of the fridge as it changes state between liquid and gas

• Air Conditioners :• Operate similar to a refrigerator• warm air from the room is forced to pass over

tubes containing coolant • Works like a heat engine in reverse using

mechanical energy supplied by a compressor motor to move thermal energy from cooler to warmer areas.

• HEAT PUMPS: a two-way heat mover• In warm weather, operates like an air

conditioner, and in winter, operates like a heater

• The Human Coolant• Your body uses evaporation to keep its

internal temperature constant• As you sweat, it evaporates and carries

away heat, making you cooler• Thermal energy that is lost by your body

becomes part of the thermal energy of your evaporated sweat