Using Thermal Energy Mrs. Nell 8 th Grade Physical Science Chapter 6 Review

Thermal Energy on the Move Thermal energy travels as heat from a material at higher temperature to a material at lower temperature. There are three methods of thermal energy transfer… 1)Conduction 2)Convection 3)Radiation

1) Conduction One way thermal energy can move is by conduction. Conduction is the transfer of energy through matter by direct contact of particles. Conduction takes place in solids, liquids, and gases.

2) Convection Any material that flows is a fluid. Convection is the transfer of energy by the bulk movement of matter. The particles move around carrying energy with them.

Convection Currents in the Air Winds are examples of convection currents. Warm air at the equator is pushed upward. Cold, dense air at the poles sinks and moves away from the poles.

Convection Currents in the Ocean Some ocean currents are also convection currents.

3) Radiation In order for conduction or convection to take place, matter must be present. Since there is almost no matter in outer space, thermal energy has to reach Earth as radiant energy. Radiation is the transfer of energy in the form of waves. Some of the radiant energy is absorbed and some is reflected back.

Reducing Thermal Energy Flow Good insulators do not allow heat to move easily through them.

R-value In building materials, the R-value measures the resistance to heat flow. Outer walls of a home should have an R-value of at least 19, while the R- values of the roof should be ZoneAtticFloor

Using Heat to Stay Warm The simplest heating system is one in which fuel is burned right in the area to be heated. The energy is transferred to the air by conduction, convection, and radiation.

Radiator Systems A radiator is a large surface area designed to heat the air near it by conduction. Convection currents circulate the heat to all parts of the room. Sometimes radiators use electricity. Other radiators use pumped hot water or steam to heat.

Forced-Air Systems In a forced-air system, a furnace heats air, then blowers force the heated air through vents in each room. The air then circulates in the room by convection.

Electric Heating System Some buildings are heated entirely by electricity. Heating coils are in floors or ceilings. Air near the coils is heated by conduction and the rest of the room is warmed by radiation.

Passive Solar Heating Passive systems heat with solar energy, but use no fans or other mechanical devices. They have large windows on the south side. The radiant energy is stored and then converted to thermal energy later.

Active Solar Heating Active solar heating systems have solar collectors to absorb radiant energy from the sun. This radiant energy heats water or air, which is pumped throughout the house.

Heat Engines Heat engines are devices that convert thermal energy into mechanical energy. They burn fuel in a process called combustion, or rapid burning.

Internal Combustion Engine In an internal combustion engine, fuel is burned inside chambers called cylinders. Gasoline and diesel engines are internal combustion engines.

Four Stroke Engine 1)Intake stroke: carburetor sends gas droplets mixed with air in the cylinder. Intake valve opens and piston moves down. 2) Compression stroke: intake valve closes and piston moves up squishing the fuel-air 3)Power stroke: spark plug ignites the fuel-air. Hot gases force piston down. Energy is transferred to the wheels of the car through the crankshaft. 4) Exhaust stroke: The piston moves up. Waste from burned fuel-air pushed out exhaust valve.

Diesels are Slightly Different Diesels fuel is injected during the compression stroke. There are no spark plugs. The fuel-air is compressed until it becomes hot and ignites on its own. Diesel engines burn fuel much more efficiently than gasoline engines.

External Combustion Engine In an external combustion engine, fuel is burned outside the engine. The steam engines of early locomotives were external combustion engines.

Heat Movers A heat mover is a device that removes thermal energy from one location and transfers it to another location at a different temperature. Refrigerators and air conditioners are heat movers. A heat pump is a special two-way heat mover.

Why Do We Sweat? Sweating is important for our body to stay cool in hot weather. When a liquid changes to a gas, energy is absorbed from the liquid’s surroundings. Heat removed from your body as your perspiration evaporates, makes you cooler.

Humidity Humid days feel warmer than dry days at the same temperature, because more water vapor is in the air around you. Your sweat doesn’t evaporate as quickly, and some of the water vapor condenses on your skin (changes from a gas to a liquid). Dogs cool themselves by panting. They only sweat between their toes!

Energy From the Ocean Ocean waters are in constant motion. The mechanical energy of the moving water can be used to rotate turbines and generate electricity.

OTEC (Ocean Thermal Energy Conversion) OTEC is a process that uses heat engines to convert differences in ocean water temperature in tropical or subtropical regions into mechanical energy to drive turbines.

Is OTEC Practical? Presently OTEC plants aren’t very efficient, since they need a lot of energy to pump the water from ocean depths. More studies will need to be done to determine the environmental impact of pumping so much cold water to the surface in warm-water regions.