Thermal energy Ch. 6 mostly. Transferring thermal NRG There are three mechanisms by which thermal energy is transported. 1. Convection 2. Conduction 3.

Thermal energy

Ch. 6 mostly

Transferring thermal NRG

• There are three mechanisms by which

thermal energy is transported.

•  1.  Convection    • 2.  Conduction     •  3.   Radiation  

Convection

• Convection: Transfer of energy by bulk movement of matter (movement of a fluid)

• Fluid: Any material that flows, gas or liquid

• Warm land is cooled during the day, while cooler land is warmed at night.

Conduction

• Conduction: The transfer of energy through direct contact of particles.

• Particles transfer their kinetic energy to other particles but remain in the same place.

• Vs. Convection where particles move taking energy with them.

• Conduction and convection require a medium for energy to flow.

• Will conduction work in a vacuum?

• How does a thermos work?

conduction

Radiation

• Radiation: the transfer of energy in the form of waves.

• No medium is required

• Ex. Light, radio, x-rays, heat?

Don’t forget

• Temperature - the measure of how hot something is, measured in degrees Celsius;

• Heat - a flow of energy from a hot object to a cold object.  Heat is measured in joules (J).

•  

All objects above 0K emit radiation

Insulation

• Insulation: substances that do not allow heat to easily move through them.

• R values indicate a substances resistance to heat flow.

• The higher the R value the greater the resistance.

• Where and why do we use insulators?

Conductors

• Air is a good insulator• Items often use air

pockets to provide insulation

• More dense objects usually conduct better then less dense objects

• Why?

• Metal and dull objects tend to absorb radiated energy

• Shiny objects tend to reflect radiant energy

Staying Warm

• Radiator: A system with a large surface area used to transfer heat by conduction

• Larger surface area allows for more heat conduction

• Currents within home carry heat by convection

Heat types

• Hot water: heated by fuel and pumped through a building

• Forced Air: Air heated and blown through ducts

• Electrical heating: uses electric heaters

Solar energy: Energy from the sun

• Solar panels collect sunlight and convert it to heat or electricity

• Passive solar heating uses windows but no mechanical devices

• Active solar heating uses solar collectors

• Solar collectors: A device used to collect solar energy

• Why use solar energy?• Why don’t we use it

Heat Engines

• Heat engines: Convert thermal energy into mechanical energy

• Combustion: Rapid burning

• Where does the fuel burn?

• Internal combustion engine: Fuel burns inside the engine, this is what your car has

External Combustion (Steam) Engine:

• Fuel is burned to heat water to make steam in a boiler outside the engine.

• When boiled to make steam, water expands 1000 times.

• Steam is confined and builds up a pressure that can exert a force on a piston (or turbine blade).

• Efficiency using pistons is low (<10%).• Widely used until the 1960’s to power trains and

ships.• Steam turbines are used today mainly to

produce electricity.

Intake stroke: Fuel is drawn into the cylinder

Compression stroke: The fuel is compressed

Power Stroke: The compressed fuel is ignited by a spark plug

Exhaust stroke: The piston pushes the exhaust out of the cylinder

Howstuffworks "Internal Combustion"

• At least two valves are used: Intake and exhaust

• More valves help improve efficiency

• How would the world be different without the internal combustion engine?

Other parts

• Carburetor mixes fuel and air together

• Fuels injectors inject fuel into the cylinder at the completion of the compression phase

• Diesel engines: Do not require an ignition system (No spark plugs) Instead compression to ignite the fuel.

• Gas heats up as it is compressed and cools as it expands

• Where else could we use this process?

Heat Mover• Heat mover: transfers thermal

energy from one place to another (ex. Regrigerator, air conditioner)

• The compressor compresses a gas driving up its temperature. The temp. is higher then the outside air so heat flows out.

• As the gas cools it becomes liquid and passes through an expansion valve.

• Rapid expansion causes the liquid to expand and evaporate making it cold.

• The thermal energy in the refrigerator now flows to the coils filled with gas. The gas returns to compressor and the cycle begins again.