Upload
percival-cain
View
217
Download
0
Tags:
Embed Size (px)
Citation preview
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.