Section 4: Using Thermal Energy

Chapter 9:Heat and States of Matter

44 How is thermal energy used?

1. Fuel is burned in a furnace and heats a volume of air.

2. A fan blows the heated air through a series of large pipes called ducts.

3. The ducts lead to openings called vents in each room.

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HEATING SYSTEMS FORCED-AIR SYSTEMS

44 RADIATOR SYSTEMS

• A radiator is a closed metal container that contains hot water or steam.

• The thermal energy contained in the hot water or steam is transferred to the air surrounding the radiator by conduction.

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44

ELECTRIC HEATING SYSTEMS

• In an electric heating system, electrically heated coils placed in floors and in walls heat the surrounding air by conduction and convection.

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44 THERMODYNAMICS

• A SYSTEM is anything you can draw a boundary around.

• The energy transferred to a system is the amount of energy flowing into the system across the boundary.

• The work done on a system is the work done by something outside the system’s boundary.

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HEATING AND WORK INCREASE THERMAL ENERGY

44THE FIRST LAW OF THERMODYNAMICS

• According to the FIRST LAW OF THERMODYNAMICS, the increase in thermal energy of a system equals the work done on the system plus the thermal energy transferred to the system.

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• The increase in energy of a system equals the energy added to the system.

44CLOSED AND OPEN SYSTEMS

• A system is an OPEN SYSTEM if thermal energy flows across the boundary or if work is done across the boundary.

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• If NO thermal energy flows across the boundary and no outside work is done, the system is a CLOSED SYSTEM.

44THE SECOND LAW OF THERMODYNAMICS

• SECOND LAW OF THERMODYNAMICS states it is impossible for thermal energy to flow from a cool object to a warmer object unless work is done.

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44

CONVERTING THERMAL ENERGY TO WORK

• No device or process can convert thermal energy completely into work.

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44 HEAT ENGINES

• A device that converts thermal energy into work is called a HEAT ENGINE.

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• When thermal energy is converted into work, some thermal energy always is transferred to the surroundings.

44INTERNAL COMBUSTION ENGINES

• The heat engine in a car is an internal combustion engine.

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• Each cylinder contains a piston that moves up and down.

• Each up or down movement of the piston is called a stroke.

44 Internal Combustion Engines

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44 MOVING THERMAL ENERGY• Liquid coolant is pumped through an

expansion valve and changes into a gas.

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• The cold gas is pumped through pipes inside the refrigerator.

• The coolant absorbs thermal energy and the inside of the refrigerator cools.

44 ENTROPY• According to the laws of thermodynamics,

work can be converted completely into thermal energy, but thermal energy cannot be converted completely into work.

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• ENTROPY is a measure of how spread out, or dispersed, energy is.

• Entropy increases when energy becomes more spread out and less concentrated.

44ENTROPY ALWAYS INCREASES

• According to the entropy principle, all events that occur cause the entropy of the universe to increase.

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• The energy that becomes spread out is no longer useable.