Physical Science Chapter 14 Thermal Energy & Heat

Section 1: Thermal Energy & Heat Objectives: Name the three common temperature scales Describe how thermal energy is related to temperature and heat Explain what specific heat of a substance is

Thermal Energy and Heat Temperature – a measure of the AVERAGE kinetic energy of the individual particles of a substance. Thermal energy – TOTAL energy of all of the particles Heat – thermal energy moving from a warmer object to a cooler object, trying to reach thermodynamic equilibrium.

B. Thermal Energy Thermal Energy the total energy of the particles in a material KE - movement of particles PE - forces within or between particles due to position depends on temperature, mass, and type of substance

B. Thermal Energy A B Which beaker of water has more thermal energy? B - same temperature, more mass 200 mL 80ºC A 400 mL B

Temperature: A measure of the average KE of the particles in a sample of matter Absolute zero- that temperature where the individual particles contain no more energy. The particles (atoms and/or molecules) cease vibrating. No movement occurs. 0o Absolute Zero -459o -273o

Specific Heat Specific Heat (Cp) amount of energy required to raise the temp. of 1 kg of material by 1 degree Kelvin units: J/(kg·K) or J/(kg·°C)

Specific Heat 50 g Al 50 g Cu Al - It has a higher specific heat. Which sample will take more energy or longer to heat to 100°C? 50 g Al 50 g Cu Al - It has a higher specific heat. Al will also take longer to cool down.

C. Heat Transfer heat gained = heat lost Calorimeter Coffee cup Calorimeter Calorimeter device used to measure changes in thermal energy in an insulated system, heat gained = heat lost

Q = m  T  Cp Specific Heat Q: heat (J) m: mass (kg) T: change in temperature (K or °C) Cp: specific heat (J/kg·K) – Q = heat loss + Q = heat gain T = Tf - Ti

Calculations Using Specific Heat How much heat is required to warm 230 g of water from 12°C to 90°C? GIVEN: m = 230 g Ti = 12°C Tf = 90°C Q = ? Cp= 4184 J/kg·K WORK: Q = m·T·Cp m = 230 g = 0.23 kg T = 90°C - 12°C = 78°C Q = (0.23kg)(78°C)(4184 J/kg·K) Q = 75,061 J

Calculations Using Specific Heat A 32-g silver spoon cools from 60°C to 20°C. How much heat is lost by the spoon? GIVEN: m = 32 g Ti = 60°C Tf = 20°C Q = ? Cp = 235 J/kg·K WORK: Q = m·T·Cp m = 32 g = 0.032 kg T = 20°C - 60°C = – 40°C Q = (0.032kg)(-40°C)(235J/kg·K) Q = – 301 J

C. Heat Transfer A B Why does A feel hot and B feel cold? Heat flows from A to your hand = hot. Heat flows from your hand to B = cold. 80ºC A 10ºC B

Heat Transfer Always trying to reach Thermodynamic Equilibrium Heat is transferred (moves) in only one direction: from a warmer object to a cooler object. Hot coffee cools to room temp because the heat of the coffee is transferred to the cooler temperature of the room. A cold glass of Iced tea soon warms up to the surrounding room temperature because the warmer temperature of the room’s surroundings is transferred to the colder glass of iced tea thereby warming it up. Heat is transferred in one of three ways: Conduction, Convection and Radiation Conductor – a material that transfers heat well: metal, tile, glass Insulator – a material that does not transfer heat well: air, carpet, wood

Conduction, Convection and Radiation Conduction – heat is transferred from one particle to the next particle w/out the particles actually moving or changing place. Examples include: a metal spoon in hot water gets hot or a pot gets hot as it sits on an electric stove. Convection – movement that transfers heat by movement of currents within the particles. The particles actually are moving and thereby transferring the heat. Examples include: a pot of boiling water sets up convection currents to move the hot water at the bottom of the pot being heated to the cooler water at the top of the pot and the convection zone in the sun. Radiation Zone – transfer of energy by electromagnetic waves. Examples include: the Sun’s energy traveling thru space and heating up the Earth w/out heating space itself, Heat lamps used at fast food restaurants, and the radiator of a car dissipating the heat of an engine.

Thermal Energy & States of Matter Kinetic Molecular Theory Thermal Expansion

Kinetic Molecular Theory KMT Tiny, constantly moving particles make up all matter. The kinetic energy (motion) of these particles increases as temperature increases. These particles are colliding with each other and the walls of their container.

States of Matter Solids low KE - particles vibrate but can’t move around atoms held tightly into place definite shape & volume

States of Matter Liquids higher KE - particles can move around but are still close together indefinite shape definite volume

States of Matter Gases high KE - particles can separate and move throughout container indefinite shape & volume move more quickly than particles that make up solids

The Fourth State of Matter Plasma very high KE - particles collide with enough energy to break into charged particles (+/-) gas-like, indefinite shape & volume stars, fluorescent light bulbs, TV tubes

Changes of State Heat of fusion-melting Freezing solid to liquid. energy required to change a substance from the solid phase to the liquid phase at its melting point is known as the heat of fusion some attractive forces are broken Freezing liquid to solid melting point = freezing point

Changes of State Heat of vaporization energy required for the liquid at its boiling point to become a gas. all attractive forces are broken EX: steam burns, sweating, and… the drinking bird

Heating Curves Gas - KE  (temp. ) Boiling – Heat of Vaporization Liquid - KE  (temp. ) Melting – Heat of Fusion Solid - KE  (temp. )

Heat & Phase Changes During a phase change, the temperature remains the same. The energy added to the system is used to change from one phase to the next…. heat = 500 g x 340 J / g = 170,000 J = 170 KJ 57,500 J = Mass x 2,300 J / g = 57,500 J /2,300 J/g = 25 J/J/g = 25 g

Changes of State Condensation Sublimation gas to liquid solid to gas EX: dry ice, freeze drying, iodine

Changes of State (Phase Changes)

Thermal Expansion Most matter expands when heated & contracts when cooled.  Temp causes  KE. Particles collide with more force & spread out. EX: thermostats (bimetallic strip)

Converting Heat to Work A device that converts heat into work is a heat engine A car’s engine converts the chemical energy in gasoline into heat.

Converting Heat to Work The engine then transforms some of the thermal energy into work by rotating the car’s wheels. However, only about 25 percent of the heat released by the burning gasoline is converted into work, and the rest is transferred to the engine’s surroundings.

Internal Combustion Engines The heat engine in a car is an internal combustion engine in which fuel is burned inside the engine in chambers or cylinders Each cylinder contains a piston that moves up and down. Each up-and-down movement of the piston is called a stroke. Automobile and diesel engines have four different strokes.

Internal Combustion Engines Intake Compression Power Exhaust

External Combustion Engines Combustion happens outside the engine Heat turns water into steam energy Steam expands and pushed a piston

Friction and the Efficiency of Heat Engines Almost ¾ of the heat produced in an internal combustion engine is not converted into useful work. Friction between moving parts causes some of the work done by the engine to be converted into heat.

Cooling Systems A refrigerator does work as it moves heat from inside the refrigerator to the warmer room. The energy to do the work comes from the electrical energy the refrigerator obtains from an electrical outlet.

Refrigerators A refrigerator contains a coolant that is pumped through pipes on the inside and outside of the refrigerator. The coolant is a special substance that evaporates at a low temperature. Liquid coolant is pumped through an expansion valve and changes into a gas. When the coolant changes to a gas, it cools.

Refrigerators The cold gas is pumped through pipes inside the refrigerator, where it absorbs thermal energy. As a result, the inside of the refrigerator cools. The gas then is pumped to a compressor that does work by compressing the gas. This makes the gas warmer than the temperature of the room. The warm gas is pumped through the condenser coils.

Refrigerators Because the gas is warmer than the room, thermal energy flows from the gas to the room. Some of this heat is the thermal energy that the coolant gas absorbed from the inside of the refrigerator. As the gas gives off heat, it cools and changes to a liquid. The liquid coolant then is changed back to a gas, and the cycle is repeated.

Air Conditioners and Heat Pumps An air conditioner operates like a refrigerator, except that warm air from the room is forced to pass over tubes containing the coolant. The warm air is cooled and is forced back into the room. The thermal energy that is absorbed by the coolant is transferred to the air outdoors.

Nite … Nite…. All Done!