physical science ch. 6 (part ii): thermal energy
TRANSCRIPT
Physical Science
Ch. 6 (Part II): Thermal Energy
Home Heating Systems
• There are several different ways to provide heat for a home.
Three of the most common are forced air systems, radiator systems, and solar heating.
• Forced air systems heat up air in a furnace, and then blow this heated air through vents into your house.
• Pro's: relatively inexpensive, can be pollution free, safe
• Con's: may not heat evenly, price fluxuation
• Furnaces may be wood-burning, natural gas, or electric.
• Radiator systems heat water in a boiler, and then pump that water or steam through a system of pipes to the radiators located throughout the building.
• Radiator systems are more commonly seen in older buildings, although they are available in newer structures as well.
• Pro's: hold their heat for a longer period of time, more even distribution of heat
• Con's: more costly to operate, more potential operational problems, older radiators can cause burns
• Solar heating is where the energy of the sun is used to heat your home.
• Passive solar heating is simply using direct sunlight to heat a room or a building.
• Active solar heating uses solar panels to heat up water in pipes. A fan then blows air over these heated pipes. The heated air is then blown through the building through vents. The hot water could also be piped directly to radiators to heat the house.
Solar Heating
• Pro's: inexpensive (after initial cost), pollution free, renewable, can be stored, can warm large buildings efficiently
• Con's: unreliable, not practical in all areas, initial expenses
Heat Movers
• A heat mover is a device which removes heat from an area at a lower temperature and gives it to an area at a higher temperature.
• Ex.: refrigerator, air conditioner
• Hollow tubes carry freon (or other refrigerants such as ammonia or halomethanes) which is pumped throughout the refrigerator. The freon collects heat from the inside (through evaporation) and releases it through the coils on the back of the refrigerator (by condensing).
• Now, suppose you go on vacation and when you come back all your food is spoiled because your refrigerator is not working. What might be some things that could be wrong with it?
• A heat engine is a device which converts thermal energy into mechanical energy.
In other words, it changes heat to work.
• Internal combustion engines, such as a car engine, burn a fuel inside the engine to produce the thermal energy.
• An external combustion engine, like an old steam locomotive, burns the fuel outside the engine.
• Thermodynamics is a branch of science which deals with the relationships between heat, thermal energy, and work.
• Laws of thermodynamics govern the operations of combustion engines since these engines convert thermal energy to work.
• Car engines are called 4-stroke internal combustion engines because there are 4 distinct steps which occur in sequence which power the car.
• Stroke 1 - The Intake Stroke
The piston inside the cylinder moves down, and fuel enters the cylinder (either from the carburetor or the fuel injector) and mixes with air.
• Stroke 2 - The Compression Stroke
• The piston moves back up and squeezes the fuel/air mixture into a smaller area. This makes it more combustible.
• Stroke 3 - The Power Stroke
• The spark plug ignites the mixture creating an explosion which forces the piston downward. This actually powers the engine.
• Stroke 4 - The Exhaust Stroke
• The piston moves up, pushing left over fumes and exhaust out the exhaust valve.
• A radial engine piston configuration, commonly seen in smaller or older aircraft.
• The pistons are all connected to a central crank shaft which pushes them up and down as it rotates. The crankshaft is turned by the force of the power stroke.
• A 2-stroke engine commonly seen in boat motors and lawnmower engines.
• The turning of the crankshaft powers the motor and drives the axles which turn the wheels.
Stepping on the gas provides more fuel and allows the car to accelerate.
Topics for Discussion
• Efficiency
• Octane
• Diesel engines
• Potential problems
• Maintenance and upkeep
• Exhaust systems
• piston rings
Your family goes on vacation, driving from Kansas City to Disney World and back (a total distance of about 2,000 mi.).
Upon returning home, you notice that your car, which averaged 30 mi/gallon on the same trip last year, averaged only 22 mi/gallon.
Come up with 3 possible
reasons (related to how
the car operates) that might
explain this decrease in
gas mileage.
• We have discussed how leaving the door of a running refrigerator open should not effect the overall temperature of a room, since the heat is just moved from 1 area (front of the fridge) to another (back of the fridge). In reality though, there will be a slight increase in the overall room temperature. What do you think would cause this?
• You wake up on a cold winter morning and the temperature in your house is 48 degrees Fahrenheit because your forced air heating system suddenly stopped working. What are some possible causes of this?
• Three beakers containing the same amount of water each have 2 drops of red food coloring dropped in. One beaker is at 30 degrees, one at 50, and one at 70. After 30 min. each beaker had pink water. If the water was still in each beaker, which one turned pink first, or did they turn at the same rate? Explain.