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PSCI II – Chapter 6– Thermal Energy (pages 156 - 189) Section 1 – Thermal Energy I. Temperature º We all know that hot means high temperature and cold means low temperature. º But how are they related. Matter in Motion º All matter is made up of tiny particles called atoms. º All materials (solid, liquid, and gas) are in constant motion. º Just like other objects in motion atoms have kinetic energy. º Cool objects are moving slower so they have less kinetic energy than warmer ones. Temperature º Defn. – related to the average kinetic energy of the atoms and molecules. º The faster the atoms and molecules move the more kinetic energy they have. º SI units are in Kelvin (K) – remember a change of one degree Kelvin is the same as one degree Celsius. II. Thermal Energy º Defn. – the sum of the kinetic and potential energy of all the molecules in an object. Thermal Energy and Temperature º When temperature of an object increases the average kinetic energy increases. Thermal Energy and Mass º The more mass means more kinetic energy simply because there are more molecules to move. º This is only if the temperature doesn’t change. III. Heat º Defn. – thermal energy that flows from something at higher temperature to something at lower temperature. º Heat is a form of energy so the units are in Joules. PSCI II – Chapter 6 – Page 1 of 6 IV. Specific Heat º As a substance absorbs heat its temperature changes depending on the nature of the substance. º Also depends on how much heat is added. º Specific Heat – the amount of heat that is needed to raise the temperature of 1kg of a substance by 1 degree or 1K. º Measured in joules per kilogram Kelvin [J/kgK]. Water as a Coolant º Look at the table on page 161. º Water has the highest specific heat and can absorb large quantities of heat with out a large temperature change. º Since it takes longer for something to heat up it also takes longer to cool down. V. Calculating Changes in Thermal Energy º Remember that the change in thermal energy is related to mass. º Change in thermal energy = mass x specific energy x change in temperature. º Change in temperature – Tfinal – Tinitial º Q is the change in thermal energy º m is the mass º C is the specific heat º Q = m x (Tfinal – Tinitial) x C º Usually I’ll give you C. º Example: The temperature of a 32 g silver spoon increases from 20C to 60C. If silver has a specific heat of 235 J/kgK what is the change in the thermal energy of the spoon. m 32 g 0.032kg T final 60C Tinitail 20C C 235 J kgK 301J Q 0.032kg 60C 20C 235 J kgK PSCI II – Chapter 6 – Page 2 of 6 Thermal Energy and Temperature Change º When heat flows into an object the thermal energy usually increases (final temperature is greater) and change is positive. º When heat flows out of an object the thermal energy usually decreases (final temperature is less) and change is negative. Measuring Specific Heat º Use a device called a calorimeter. º Basically the device can measure the temperature changes after something’s been heated. Section 2 – Transferring Thermal Energy I. Conduction º Defn. – transfer of thermal energy through matter by the direct contact of particles. º Think of a snowball melting in your hand. Transfer by Collisions º In your snowball the slower moving particles of your snowball come into contact with the faster moving particles of your hand. º As the particles collide energy is transferred. º One transfers energy to its neighbor and so on. Heat Conductors º Conductors can happen in solids, liquids and gases. º Solids are the best because the molecules are closer together. º Metal solids are even better. º Silver, copper, and aluminum are among the best. º Wood, plastic, and glass are among the poorest. II. Convection º Defn. – transfer of energy in a fluid by the movement of heated particles. º Heated particles move from one location to another. º They also expand but mass does not change. º Usually causes particles to move up because of less density. º Think of a lava lamp. Heat Transfers by Currents º The rising and sinking are called convection currents. º Convection currents transfer heat from warmer to cooler parts of the fluid. PSCI II – Chapter 6 – Page 3 of 6 Desert and Rain Forests º The atmosphere has convection currents. º These are what cause the weather. III. Radiation º Defn. – the transfer of energy by electromagnetic waves. º These waves can travel through space (called radiant energy). º One example is the sun. º Another one is a fire – if you stand next to a fire you feel heat. Radiant Energy and Matter º When radiant energy is transmitted some is absorbed and some reflected. º This depends on the type of material. º When radiant energy is absorbed thermal energy is increased. Radiation in Solids, Liquids, and Gases º Most important in gases since particles are more mobile. IV. Controlling Heat Flow º We do lots of things to protect our bodies from losing or gaining to much heat. º Almost all-living things have things that protect them from extreme temperatures. V. Insulators º Defn. – a material that doesn’t allow heat to flow through it easily. º An object that is a good conductor is a poor insulator. º A good insulator is a poor conductor. Section 3 – Using Heat I. Heating System º Almost all buildings in the US have some kind of heating system. º All heating systems require a heat source. º Oldest is burning wood or charcoal. º Heat is then transferred to surrounding area. Forced-Air Systems º Most common heating system. º Fuel is burned in the furnace and a system of fans blows warm air through vents in the building. PSCI II – Chapter 6 – Page 4 of 6 Radiator Systems º Before forced air systems. º Closed metal container with hot water or steam where heat is transferred through air by convection. º Radiator systems heat water in a central core where water is pushed as water cools it flows back to central core. Electric Heating Systems º Heating coils in ceiling and floors and heat through conduction. º Convection carries warm air to where it needs to be. º Less fuel is used but it still uses fuel. II. Solar Heating º Think of green houses. º Solar energy – energy from the sun. º Free and seemingly limitless. Passive Solar Heating º Solar energy heats a room inside a building but no mechanical device is used to transfer energy. Active Solar Heating º Solar collector – solar heating system device that absorbs radiant energy from the sun. º Solar collectors use radiant energy to heat water in the collectors and a pump pushes warmth through the house. III. Using Heat to do Work º Heat engine – an engine that converts thermal energy to mechanical energy. Internal Combustion Engines º Defn. – fuel is burned inside the engine in chambers or cylinders. º Four cycles or strokes. º Intake stroke – valve opens and piston moves down and a mix of air and gas go in. º Compression stroke – valve closes, piston moves up compressing mixture. º Power stroke – spark ignites fuel, gases expand pushing piston down. º Exhaust stroke – piston moves up pushing exhaust out. Why are Engines Hot? º During the power stroke lots of heat is produced. º Cooling systems are in cars to keep the systems cool. PSCI II – Chapter 6 – Page 5 of 6 IV. Heat Movers º Defn. – a device that removes thermal energy from one location and transfers it to another location at a different temperature. Refrigerators º Contains a coolant pumped through pipes. º Evaporates at low temperature. º When it changes to a gas it cools and as a result cools the air around it. º Then goes and gets condensed. Air Conditioners and Heat Pumps º Like a refrigerator but for homes. º A heat pump is a two way heat mover. º Acts as an air conditioner in warm weather and as a heater in cool weather. The Human Coolant º You use sweat to cool your body. º As sweat evaporates it carries heat away. Why do Humid Days Feel Hotter? º More water vapor is in the air so sweat doesn’t evaporate as much. PSCI II – Chapter 6 – Page 6 of 6