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Ch. 16 - Thermal Energy & Heat

Heat --> transfer of thermal energy from oneobject to another because of adifference in temperature (heatmoves from hot objects to coldobjects)

Temperature --> based on the average kineticenergy of the particles in an object due to their random motionthrough space

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Heat is transferred as a high-energy particle collides with a low-energy particle ... the high-energy particle loses energy to the low-energy particle.

Thermal Energy --> depends on the mass, temperature and phase (solid, liquid or gas) of an object

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Which would have a greater temperature, a cup of boiling water or a large pot of boiling water?

They would have the same temperature!

Which would have more thermal energy?

The large pot because of the greater mass!

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Thermal expansion --> an increase in the volume of a material due to a temperature increase; occurs when particles of matter move farther apart as temperature increases

Gases expand more than liquids or solidsbecause the force of attraction amongthe gas particles is weaker.

Thermometers & thermostats both usethermal expansion to operate.

The only exception to thermal expansion is water as is nears its freezing point.

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Over most temperature ranges, water increases in volume as its temperature increases. Between 0oC and 4oC water actually contracts as it gets warmer (the hydrogen bonding between water molecules causes them to arrange themselves in such a way that the molecules occupy a greater volume). Thus, ice is less dense than water and floats!! Its also why ponds freeze from the top down.

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Specific heat --> the amount of heat needed to raise the temperature of 1 gram of a material by 1 degree Celsius; the lower a material‛s specific heat, the more its temperature rises when a given amount of energy is absorbed by a given mass

Units for specific heat = joules/(gram * oC)

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To calculate the specific heat (c) of an object:

Q = (m)(c)( T)

Q = Heat absorbed by an objectm = mass in gramsc = specific heat in J/goC T = oC

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Calorimeter --> an instrument used to measure change in thermal energy; uses the principle that heat flows from a hotter object to a colder object until both reach the same temperature.

A known amount of water is added to the calorimeter. The mass of aluminum is measured. Aluminum is heated and then placed in the water. The calorimeter is sealed. As the aluminum cools off, the water is stirred to distribute the thermal energy evenly. The water heats up until both the water and aluminum are at the same temperature. T is measured and then you use the Q = mc T equation (since you know the mass of water, the specific heat of water and the T to calculate Q.) You then can use your calculated Q, the mass of the aluminum and your T to determine the specific heat of aluminum.

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16.2 - Heat & Thermodynamics

Methods of Heat Transfer:

1. Conduction --> transfer of thermal energy with no overall transfer of matter; occurs within a material or between materials that are touching; particles collide and energy is transferred

***Because particles in gas are further apart & collide less, conduction in gas is slower than in liquids and solids.

***Conduction is usually faster in metals because of the electrons that are free to move about. These electrons will collide with one another & with other atoms to transfer the thermal energy.

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Thermal conductor = conducts thermal energy well; copper, aluminum & silver are all good conductors

Thermal insulator = conducts thermal energy poorly; air, wood, wool fabric & foam cups are good insulators

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2. Convection --> transfer of thermal energy when particles of a fluid move from one place to another

***A convection current occurs when a fluid circulates in a loop as it alternately heats up and cools down.

***Convection currents help warm rooms and play an important role in oceanic current systems, weather systems and movements of molten rock in the earth‛s interior.

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3. Radiation --> the transfer of energy by waves moving through space; doesn‛t require air to occur; heat lamps and the sun heat by radiation.

***All objects radiate energy. The greater the kinetic energy, the greater the temperature and the more heat that is radiated.

***Radiation itself has no temperature because it does not consist of matter. However, matter can absorb radiation, change it to thermal energy and then emit heat (which would cause an increase in temperature.) Matter can also emit radiation.

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Thermodynamics --> the study of conversions between thermal energy and other forms of energy

1st Law of Thermodynamics = All energy is conserved; it can change forms but it is never lost

2nd Law of Thermodynamics = Thermal energy can flow from colder objects to hotter objects only if work is done on the system

***Refrigerators use this principle to remove thermal energy from food (thus cooling it) by moving the heat that is removed from the food through coils at the bottom/back of the refrigerator.

***A heat engine is any device that converts heat into work; efficiency is always less than 100% with waste heat always being produced.

***Work will always produce some waste heat.

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3rd Law of Thermodynamics = Absolute zero can not be reached (we have reached 3 billionth of a Kelvin above absolute zero!).

***Because the efficiency of a heat engine increases with a greater difference between the temperature inside the engine and the temperature outside the engine, a heat engine could be 100% efficient if we could reach absolute zero.

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Thermodynamics --> the study of conversions between thermal energy and other forms of energy

1st Law of Thermodynamics = All energy is conserved; it can change forms but it is never lost

2nd Law of Thermodynamics = Thermal energy can flow from colder objects to hotter objects only if work is done on the system

***Refrigerators use this principle to remove thermal energy from food (thus cooling it) by moving the heat that is removed from the food through coils at the bottom/back of the refrigerator.

***A heat engine is any device that converts heat into work; efficiency is always less than 100% with waste heat always being produced.

***Work will always produce some waste heat.

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3rd Law of Thermodynamics = Absolute zero can not be reached (we have reached 3 billionth of a Kelvin above absolute zero!).

***Because the efficiency of a heat engine increases with a greater difference between the temperature inside the engine and the temperature outside the engine, a heat engine could be 100% efficient if we could reach absolute zero.

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16.3 - Using Heat

2 main types of heat engines:1. External combustion engine2. Internal combustion engine

External combustion engine --> fuel is burned outside of the engine; steam engines are external combustion engines

Hot steam enters on the right side & the valve slides closed trapping the steam inside. The steam expands and cools as it expands, pushing the piston to the left as it does so. Thus heat is converted into work

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Internal combustion engine --> a heat engine in which the fuel is burned inside the engine; a car engine is an internal combustion engine

***Most ICE‛s use pistons that move up and down inside cylinders. Each up or down motion of a piston is called a stroke. The piston is connected to a crankshaft that converts the up & down motion into rotary motion which is then used to turn the wheels.

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How a cylinder engine works:

1. A mixture of air and gasoline vapor enters the cylinder in the intake stroke.

2. The piston then compresses the gas mixture in the compression stroke.

3. The spark plug then ignites the mixture.

4. The gas is heated under pressure and then rapidly expands and drives the piston down in the power stroke. This is when work is actually done that can be used by the car.

5. Finally, during the exhaust stroke, gas leaves the cylinder and the cycle is repeated.

Remembering our laws of thermodynamics, what must be produced if work is done?

Waste heat.

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The cooling system of a car removes the waste heat to the radiator where it is then transferred to the atmosphere.

What might happen to a metal engine if the cooling system stops working?

Because of thermal expansion, the engine could be severely damaged or even crack!

Internal combustion engines are only about 33% efficient!

Hybrid engines are more efficient. Read p. 488 and write a paragraph describing how a hybrid automobile works.

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Heating Systems

Central Heating Systems --> heats many rooms from 1 central location; uses convection to distribute thermal energy; use electrical energy, natural gas, oil or coal as an energy source

Hot-Water Heating Systems -->

1. Water is heated by burning natural gas or oil using a boiler system 2. The circulating pump then carries the hot water to radiators in each room3. Hot water transfers thermal energy to the radiator by conduction (water in contact with the metal)4. As the pipes heat up, they heat the room air near the radiator by conduction & radiation5. The warm air then rises & convection currents form. 6. The water in the pipes has now cooled down because its thermal energy was transferred to the room air so the water returns to the boiler where it is heated again.

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Steam Heating Systems --> similar to the above but steam is used instead of hot water; often found in older buildings.

Electric Baseboard Heating system --> Uses electrical energy to heat a room; uses a conductor to convert electrical energy to thermal energy which then heats the air via conduction and eventually, convection.

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Forced-Air Heating --> Uses fans to circulate warm air through ducts to different rooms of a building. Air vents are usually located near the floor so after the warm air enters the room, it is further spread via convection currents.

Our school uses a combination of a hot water heating system with a forced-air heating system.

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Cooling Systems

Heat pumps --> a device that reverses the normal flow of thermal energy; uses a refrigerant - a fluid that vaporizes (becomes a gas) when it absorbs heat & condenses (becomes a liquid) when heat is removed.

***In order for the cold refrigerant to be moved through the refrigerator, a motor must do work to push it through the tubing. Because work is done, the 2nd Law of Thermodynamics is satisfied.

***The heat removed from the food & the heat produced by the motor is released into the room. Leaving the frig door open doesn‛t cool the room!!!

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Air Conditioning Systems

All air conditioning systems use some type of refrigerant.

1. The compressor raises the temperature & pressure of the refrigerant causing it to vaporize.

2. Because the temperature of the condenser coil is higher than the outside air temperature, heat will flow spontaneously from the coil to the outside air (which is why its so hot if you stand near the outside vent of an air-conditioning unit.)

3. As thermal energy is removed from the coil, the refrigerant cools and condenses into a liquid.

4. The liquid refrigerant than flows through the expansion valve & decreases in temperature.

5. As the cold refrigerant flows through the evaporator coil, it absorbs thermal energy from the warm room.

6. The fan sends cold air back into the room.

7. The refrigerant becomes a vapor because of the heat it absorbed and the process starts again.

Look at the diagram again...what is the net flow of warm air in an air conditioning system?

From inside the room to the outside!

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