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.