Heat and Temperature

How do we measure temperature? Think about using a thermometer….. How does the thermometer know how hot the substance is? The molecules of the substance bump into the thermometer and transfer energy. How often and how hard they bump into the thermometer are directly related to their speed. Temperature turns out to be related to the average speed of the molecules in a substance Temperature is not a measure of the total amount of energy in an object.

Kinetic molecular theory Gas molecules move in a straight line at a constant speed until they collide with another molecule or with a wall. Molecules bounce off one another or off a wall as if they are billiard balls. When you heat gas molecules, they move faster. When you cool gas molecules, they move slower.

Temperature and KE It can be shown that the kinetic energy of the gas is proportional to the temperature of the gas

Put these bottles with the coin on top in the freezer for a half hour. What happens when you take the bottles out of the freezer and put it on a table at room temperature? The coin is pushed up off the bottle. Why? Heating air molecules makes them move faster and push harder, so the coin moves up

Temperature scales Fahrenheit Celsius Kelvin Conversion formulas – Fahrenheit to Celsius – Celsius to Fahrenheit – Celsius to Kelvin

Temperature is a measure of a) The total amount of internal energy in an object b) The total amount of thermal energy in an object c) How much heat something might be able to give off d) The average speed that the molecules in an object are moving The answer is d. Temperature is related to the average speed of the molecules in a substance Temperature is NOT thermal energy and it is NOT heat A very small hot object may not contain much thermal energy A very large cool object may contain a lot of thermal energy

Thermal Energy vs. Temperature The temperature of an object or substance is directly related to the average kinetic energy (which depends on the square of the speed) of the molecules in the object or substance. Thermal, or internal, energy is a measure of the total kinetic and potential energy within an object. Thermal energy and temperature are not the same thing. The thermal or internal energy, U, depends on the number of moles of gas (amount of gas) and the temperature of the gas

Thermal Energy vs. Temperature Temperature A measure of hotness or coldness of an object Based on average molecular kinetic energy Thermal Energy Based on total internal energy of molecules Doubling amount at same temperature doubles thermal energy

Thermal Energy Thermal Energy = is a measure of the total kinetic and potential energy in an object Thermal Energy is not heat….

So what is heat? 50 degrees C 20 degrees C Heat Heat is energy that flows….

Heat is…? A form of energy associated with the motion of atoms or molecules and capable of being transmitted through solid and fluid media by conduction, through fluid media by convection, and through empty space by radiation. Heat always flows from warmer to colder objects

Measures of heat English system British thermal unit (BTU) - energy needed to raise the temperature of 1 lb of water 1 degree Fahrenheit Mechanical equivalence J = 1 cal Metric units calorie (cal) - energy needed to raise temperature of 1 g of water 1 degree Celsius kilocalorie (kcal, Calorie, Cal) - energy needed to raise temperature of 1 kg of water 1 degree Celsius

Specific heat Variables involved in heating Temperature change Mass Type of material – Different materials require different amounts of heat to produce the same temperature change – Measure = specific heat Summarized in one equation

Phase changes Two responses of matter to heat 1.Temperature increase within a given phase – Heat goes mostly into internal kinetic energy – Specific heat 2.Phase change at constant temperature – Related to changes in internal potential energy – Latent heat

Phase changes Solid/liquidLiquid/gasSolid/gas Latent heatFusionVaporizationSublimation Temperature (Direction ->) Melting pointBoiling pointSublimation Temperature (Direction <-) Freezing point Condensation point Sublimation

Evaporation and condensation Individual molecules can change phase any time Evaporation: – Energy required to overcome phase cohesion – Higher energy molecules near the surface can then escape Condensation: – Gas molecules near the surface lose KE to liquid molecules and merge

Sample conductivities MaterialRelative conductivity Silver0.97 Iron0.11 Water1.3x10 -3 Styrofoam1.0x10 -4 Air6.0x10 -5 Vacuum0

Phase Diagram for Water Phase diagrams show the relationship between phase, temperature and pressure Notice that water can be “melted” by changing the temperature or the pressure By increasing the pressure, you can cross the ice/water line (melt ice)

Heat flow Three mechanisms for heat transfer due to a temperature difference – Conduction – Convection – Radiation Natural flow is always from higher temperature regions to cooler ones

Conduction Heat flowing through matter Mechanism – Hotter atoms collide with cooler ones, transferring some of their energy – Direct physical contact required; cannot occur in a vacuum Poor conductors = insulators (Styrofoam, wool, air…)

Convection Energy transfer through the bulk motion of hot material Examples – Space heater – Gas furnace (forced) Natural convection mechanism - “hot air rises”

Radiation Radiant energy - energy associated with electromagnetic waves Can operate through a vacuum All objects emit and absorb radiation Temperature determines – Emission rate – Intensity of emitted light – Type of radiation given off Temperature determined by balance between rates of emission and absorption – Example: Global warming

Seasons and the Sun The angle at which sunlight strikes the earth, which varies by location, time of day, and season, is an important factor in the amount of heat energy received at any location on the globe. Seasonal change in the angle of sunlight, caused by the tilt of the earth's axis, is the basic mechanism that results in warmer weather in summer than in winter This diagram illustrates how sunlight is spread over a greater area in the polar regions so it is weaker

Global Warming and the Greenhouse Effect A greenhouse works by trapping radiation. Short wave infrared waves and visible light are going in and are turning into long wave infrared waves, which cannot escape through the glass. They just reflect around and get absorbed by their surroundings and the greenhouse (or your car or the Earth)heats up.

Global Warming and the Greenhouse Effect The ability of the atmosphere to capture and recycle energy emitted by the Earth surface is the defining characteristic of the greenhouse effect. The Earth’s average surface temperature of 14 °C (57 °F) would otherwise be about -19 °C (-2.2 °F) in the absence of the greenhouse effect. Global warming, a recent warming of the Earth's lower atmosphere, is believed to be the result of an enhanced greenhouse effect due to increased concentrations of greenhouse gases in the atmosphere. Global warming greenhouse gases

How molecules “trap” heat Most of the infrared absorption in the atmosphere can be thought of as occurring while two molecules are colliding. The molecules/atoms that constitute the bulk of the atmosphere: oxygen (O 2 ), nitrogen (N 2 ) and argon (Ar); do not interact with infrared radiation significantly.oxygennitrogenargon In the Earth’s atmosphere, the dominant infrared absorbing gases are water vapor, carbon dioxide, and ozone (O 3 ).water vaporcarbon dioxide ozone Molecules vibrate when they interact with infrared radiation

Greenhouse Gases The most powerful greenhouse gases are – water vapor, which causes about 36–70% of the greenhouse effect on Earth. (Note clouds typically affect climate differently from other forms of atmospheric water.) water vaporaffect climate – carbon dioxide, which causes 9–26% carbon dioxide – methane, which causes 4–9% methane – ozone, which causes 3–7% ozone On earth, the most abundant greenhouse gases are, in order of relative abundance: – water vapor water vapor – carbon dioxide carbon dioxide – methane methane – nitrous oxide nitrous oxide – ozone ozone – CFCs CFCs

Global Warming Dr. Todd Albert Department of Geography School of Earth, Environment, and Society Bowling Green State University Changing Earth’s Climate by

Sources Physical Science, 6e Chapter 4 Heat and Temperature, Bill W. Tillery