Chapter 14 Heat and Temperature

Intro: Discussion A person from Seattle tells his friend from Florida that the weather in Seattle is somewhat warm. When the friend arrives for a visit, he finds that he is uncomfortably cool wearing the shorts he packed. What would be a more effective way for the person from Seattle to explain the weather?

Temperature So, what kind of energy does temperature measure? The degree of “hotness” or “coldness” of an object, related to the average kinetic energy of an object’s atoms or molecules What makes something hot? Particles that make up matter are in constant motion They have kinetic energy When you heat something the particles move faster So, what kind of energy does temperature measure?

Kinetic Energy Kinetic Energy Temperature transferred as particles collide Particles are always in motion no matter what state they are in. What is happening to the coffee? What else is being affected? Kinetic Energy Temperature

Measuring Temperature Thermometers- mechanical or electrical device for measuring temperature. How thermometer works: Most materials expand when heated. Liquid thermometers have a large bulb hooked to a narrow tube. When the liquid expands it rises up the tube. Won’t work if too hot or cold

Measuring Temperature Metals expand when heated Different metals expand different amounts Hook two metals together to make a thermometer that turns Bimetallic Thermometers Used in thermostats Iron Copper Cold Hot

Limits of Temperature Upper Limit No upper limit exists. Plasma found in starts= millions of degrees C Lower Limit Definite limit called absolute zero. Molecules will slow down SO much, they will essentially stop moving Out of energy, so they can’t get any colder.

Temperature Scales Three different scales: 1. Fahrenheit- the one we use 2. Celsius- metric standard 3. Kelvin- starts at absolute zero but same degree size as Celsius Fahrenheit scale. In the US we know at 32 ⁰F water freezes and 212 ⁰F water boils.

Measuring Temperature The absolute temperature scale is called the Kelvin scale. Absolute zero is 0 K. The melting point of ice is 273 K, and the boiling point of water is 373 K. There are no negative numbers on the Kelvin scale.

0⁰C 32⁰F 273 K

100⁰C 212⁰F 373 K 0⁰C 32⁰F 273 K

100⁰C 212⁰F 373 K 0⁰C 32⁰F 273 K -273⁰C -492⁰F 0 K Notice anything else about the scales?

Heat We know that temperature is the hot or cold nature of something (based on the kinetic energy of its molecules), So what is heat? Heat is the thermal energy transferred from one thing to another due to a temperature difference. Always moves from high temperature to low temperature. Faster molecules (high temperature) hit slower molecules (low temperature) and speed them up

Heat Temperature tells you direction of heat transferring. If you touch a hot stove, thermal energy enters your hand because the stove is warmer than your hand. When you touch a piece of ice, thermal energy passes out of your hand and into the colder ice. Temperature tells you direction of heat transferring.

14.2 Energy Transfer Heat can be transferred 3 different ways: Conduction Convection Radiation

Energy Transfer: Conduction Transferred of energy by direct contact Works well in some solids, then liquids, and least in gases. Conductors- materials that allow heat to pass through them Most metals Insulators- materials that don’t let heat pass through them well Rubber, plastics, glass, air

Energy Transfer: Convection Transferring energy by moving fluids Liquids and gases are fluids When heated they expand, become less dense They rise, replaced by cooler denser fluids Make a circular flow called a convection current Clip

Water movement Cools at the surface Convection current Hot water rises Cooler water sinks

Why is it windy at the seaside?

Energy Transfer: Radiation Energy transferred by electromagnetic waves Ex: infrared radiation, visible light, ultraviolet rays Can travel through empty space When wave hit object they make the molecules move faster.

Emission experiment Four containers were filled with warm water. Which container would have the warmest water after ten minutes? Shiny metal Dull metal Dull black Shiny black The __________ container would be the warmest after ten minutes because its shiny surface reflects heat _______ back into the container so less is lost. The ________ container would be the coolest because it is the best at _______ heat radiation. shiny metal radiation dull black emitting

Absorption experiment Four containers were placed equidistant from a heater. Which container would have the warmest water after ten minutes? Shiny metal Dull metal Dull black Shiny black The __________ container would be the warmest after ten minutes because its surface absorbs heat _______ the best. The _________ container would be the coolest because it is the poorest at __________ heat radiation. dull black radiation shiny metal absorbing

Specific Heat Some materials heat up easily Others require a large amount of energy to change their temperature Specific heat measures the amount of energy required to raise the temperature 1 kg of a substance by 1 degree Kelvin Table on page 485 (Learn to use it) Water has a high specific heat of 4186 J/kg K

Specific Heat The amount of energy required to change the temperature of substance identifies type of conductor. Metals have a low specific heat

Calculating Specific Heat Energy = Specific heat x mass x change in temp Energy (Q) = cmDT mass= kg energy= Joules Temperature= K c= specific heat Q c m DT Specific heat is the “price” to change 1 kg of a substance.

Practice Problems: Specific Heat 1. How much energy must be transferred as heat to 200 kg of water in a bathtub to raise the water’s temperature from 25 °C to 37 °C? energy = 4186 x 200 kg x 12 k energy = cmDT DT= 37 °C- 25 °C DT= 12 K energy = 10,000,000 J or 1.0 x 107 J m= 200 kg c= 4,186 J/kg x K energy = ?

Practice Problems: Specific Heat 2. How much heat does it take to change the temperature of 3 kg of water by 75 K? energy = cmDT energy = 4186 x 3 kg x 75 k DT= 75 K energy = 900000 J or 9.0 x 105 J m= 3 kg c= 4,186 J/kg x K energy = ?

Practice Problems: Specific Heat 3. How much energy is needed to increase the temperature of 0.755 kg of iron from 283 K to 403 K? energy = 449 x 0.755 kg x 120 k energy = cmDT energy = 40,700 J DT= 403 K -283 K DT= 120 K m= 0.755 kg c= 449 J/kg x K energy = ?

1st Law of Thermodynamics The word thermodynamics stems from Greek for “movement of heat.” First Law of Thermodynamics: When thermal energy transfers as heat, it does so without net loss or gain. The energy lost from one place is gained by the other. Whenever heat flows into or out of a system, the gain or loss of thermal energy equals the amount of heat transferred. (You can’t get something from nothing because energy and matter are always conserved).

2nd Law of Thermodynamics Energy transfers as heat always moves from a hot to cold objects. (You can’t break even; you can’t return to the same energy state because entropy always increases). Entropy is the tendency for natural systems to become disorganized over time. Any system that is left to itself will fall apart. This means Entropy increases. Usable energy decreases with every transfer or energy transformation.

Thermodynamics Work increases average kinetic energy Mechanical Process is the process in which energy is transferred by work. Bow drill

Heat Engines Heat engines convert chemical energy to mechanical energy through the process of combustion. Internal combustion engines Fuel burns inside the engine A carburetor is the part of the engine in which liquid gasoline becomes vaporized. 4 strokes: Intake, Compression, Power, and exhaust Automobile Cylinder Animation

Chapter 3.2 Changes of State What happens when a substance changes from one state of matter to another? What happens to mass and energy during physical and chemical changes?

Temperature vs. Time Adding energy either raises T or changes state, not both at the same time.

Energy and Changes of State The identity of a substance does not change during a change in state The ability to change or move matter As you add energy to a liquid, the temperature goes up separating molecules Some changes of state require energy Melting, evaporation and sublimation melting sublimation evaporation

Energy and Changes of State Evaporation The change of state from a liquid to a gas Boiling Point- The temperature at which a liquid boils.

The temp in the room is about 22°C and 70°F. The temperature of boiling water is 100oC on the celsius scale and 212oF on the Fahrenheit scale. The temp in the room is about 22°C and 70°F. Are the following temperatures hot or cold? 65°F

Sublimation The process in which a solid changes directly into a gas Ex. Dry ice (Carbon dioxide in the solid form) changes directly from a solid to a gas Video Clip

Melting Point The temperature at which a substances changes from solid to liquid. Melting point depends on the pressure. 32 degrees F 0 degrees C

Energy is released in some changes of state Freezing and condensation Freezing Condensation

Solid Liquid Gas sublimation Freezing Condensation Melting Boiling or evaporation Solid Liquid Gas Freezing Condensation

Temperature The temperature of a substance does not change during a change of state. For example, if you add energy to ice at 0oC, the temperature will NOT rise until all of the ice has melted.