Chapter 16 Thermal Energy & Heat

 Objectives:  1. Explain how heat and work transfer energy  2. Relate thermal energy to the motion of particles that make up a material  3. Relate temperature to thermal energy and to thermal expansion  4. Calculate thermal energy, temperature change, or mass using the specific heat equation

Def: heat is the transfer of thermal energy from one object to another because of temperature difference Heat flows spontaneously from hot objects to cold objects Def: temperature is a measure of how hot or cold an object is compared to a reference point On the Celsius scale, the reference points are the freezing point and boiling point of water

 Another reference point is called absolute zero  Absolute zero is on the Kelvin scale  It is the lowest temperature known  There are no negative values on the Kelvin scale  Temperature is related to the average kinetic energy of particles in an object due to their random motions through space

Recall that thermal energy is the total potential and kinetic energy of all the particles in an object Thermal energy depends on the mass, temperature and phase (solid, liquid, gas) of an object Def: thermal expansion is an increase in the volume of a material due to a temperature increase Thermal expansion occurs when particles of matter move farther apart as temperature increases

 Def: Specific heat is the amount of energy transferred as heat that will raise the temperature of 1 kg of substance by 1 K  Little c is the symbol for specific heat, & specific heat is a physical property  The units for specific heat are J/kg-K  We will assume that volume & pressure do not change in order to do calculations involving the formula energy = specific heat x mass x temperature change OR energy = cm∆t (the ∆ means change & is the final temperature minus the initial temperature)

 Objectives:  1. Describe conduction, convection and radiation & identify which one of these is occurring in a given situation  2. Classify materials as thermal conductors or thermal insulators  3. State the 3 laws of thermodynamics  4. Apply the first & second laws to given situations

 Def: conduction is the transfer of thermal energy with no overall transfer of matter  Conduction is the heat energy transfer between particles as they collide within a substance or between objects in contact (ex: roasting marshmallows using a wire coat hanger)  Conduction in gases is slower than in liquids and solids because the particles of gas collide less often

 Def: a thermal conductor is a material that conducts thermal energy well  Def: a thermal insulator is a material that conducts thermal energy poorly  Def: convection is the transfer of thermal energy when particles of a fluid move from one place to another  Convection is the heat energy transfer by movement of fluids with different temperatures (ex: warm air rising, cools, condenses, becomes dense and sinks)

 A convection current is the flow of liquid due to heated expansion followed by cooling and contraction  Convection currents are important in many natural cycles such as ocean currents, weather systems, and movements of hot rock in Earth’s interior in the asthenosphere under the lithospheric plates  Def: radiation is the transfer of energy by waves moving through space  Radiation does not require physical contact between objects

 Radiation does not involve movement of matter & can take place in a vacuum (ex: the radiation we receive from the sun is in a vacuum)  All objects radiate energy  As an object’s temperature increases, the rate at which it radiates energy increases  Q: What is radiation?

 Def: thermodynamics is the study of conversions between thermal energy and other forms of energy  The First Law of Thermodynamics states that energy is conserved  The Second Law of Thermodynamics states that thermal energy can flow from colder objects to hotter objects only if work is done on the system

 Def: waste heat is thermal energy that is not converted into work  Spontaneous changes and waste heat always make a system less orderly  The Third Law of Thermodynamics states that absolute zero cannot be reached  Scientists have gotten close, but have not been able to do this

 Objectives:  1. Describe heat engines and explain how they convert thermal energy to mechanical energy  2. Describe how different heating systems operate  3. Describe how cooling systems operate

 Def: a heat engine is any device that converts heat into work  The two main types of heat engines are the external combustion engine and the internal combustion engine  Def: the external combustion engine is an engine that burns fuel outside the engine  Ex: steam engine

 Def: an internal combustion is a heat engine in which fuel burns inside the engine  Ex: your car  Both types of engines have pistons that move either back and forth or up and down  Some waste energy is discharged

 The principles of transfer of heat allow us to design buildings that can be heated and cooled efficiently  Recall that total energy is conserved whether it is transferred due to work, heat or both  This is called the first law of thermodynamics  Secondly, energy transferred as heat always moves from an object of high temperature to an object at low temperature

 A heating system is any device or process that transfers energy to a substance to raise the temperature of the substance  Work can be done to raise temperatures  Mechanical processes are energy transfer done by work  Def: a central heating system heats many room from one central location (often the basement)  Most heating systems use convection to distribute thermal energy

 Hot water heating is usually by boiler, heating oil or natural gas  A pump circulates the water to radiators in each room that transfer energy by conduction  Most heating systems use these principles  A refrigerator is a heat pump  It transfer thermal energy from the cold food compartment to the warm room  A cooling system is a device that transfers heat energy out of an object to lower its temperature

 Air conditioners use a liquid refrigerant to decrease temperature and then transfer the warm air out  Def: A refrigerant is a substance used in cooling systems that transfers large amounts of energy as it changes state  Condensation transfers energy to the surroundings  Condensation uses a refrigerant as part of the process of transferring heat away