Chapter 5 TEMPERATURE AND HEAT Dr. Babar Ali

CHAPTER OUTLINE Temperature & Heat Kinetic Theory of Matter Specific Heat Calculating Heat Phase Changes Latent Heat Heat Transfer Thermal Expansion Thermal Properties of Water

TEMPERATURE Temperature is a measure of how hot or cold a substance is. Thermometer is an instrument used for measuring temperature, and is based on thermometric properties of matter. Three scales are used for measuring temperature.

TEMPERATURE SCALES Fahrenheit 32 - 212 Celsius 0 - 100 Kelvin 273 - 373

Example 1: The melting point of silver is 961ºC. What is this temperature in Kelvin? TK = TC + 273 TK = 961 + 273 = 1234 K

Example 2: Pure iron melts at 1800 K. What is this temperature in Celsius? TC = TK - 273 TC = 1800 - 273 = 1527 C

Example 3: TC = [(TF + 40) ÷ 1.8]- 40 TC = [(5 + 40) ÷ 1.8]- 40 = -15 On a winter day, the temperature is 5 ºF. What is this temperature on the Celsius scale? TC = [(TF + 40) ÷ 1.8]- 40 See also Eq. 5.2 (a, b) TC = [(5 + 40) ÷ 1.8]- 40 = -15

KINETIC THEORY OF MATTER All matter is composed of tiny particles (molecules) that are in constant motion.

KINETIC THEORY OF MATTER Solid particles are closely packed with little motion. Solids have definite volume and definite shape. Liquid particles are loosely packed and have some motion. Liquids have definite volume, but indefinite shape. Gas particles are far apart from one another and have great motion. Gases have indefinite shape and volume.

KINETIC THEORY OF MATTER Increase in T increases the motion of molecules in any state. For example, evaporation of liquids results from loss of fast moving molecules @ surface.

TEMPERATURE & HEAT Heat is the Ethermal that is transferred from one body to another because of T difference between the bodies. Heat flow occurs from high T to low T. T (temperature) is the average Ekinetic of molecules in a substance. See, Kinetic Molecular Theory - Heat

CONCEPT CHECK 1 Consider a cup of hot coffee and a frozen lake. Which has higher T? Which has more heat? Hot coffee has a higher temperature than a frozen lake Frozen lake has more heat because it contains many more particles

CONCEPT CHECK 2 On a cold day when someone leaves a door open, it is often said that the person is letting in the cold air. Is this correct? No, heat travels from higher to lower T.  heat (H) is going out rather than cold in!

MEASURING HEAT Heat (H) is a physical quantity that can be measured. The science of heat measurement is known as calorimetry. Quantity of H is  to: 1) the amount of substance 2) its T, and 3) the nature of the substance. The SI unit of H is joules (J). Another convenient unit for H is calorie.

Concept Check It takes 5× as much heat to boil 5 kg of water than 1 kg. Mass and heat are directly proportional Which container requires more heat to reach the same temperature?

Concept Check Although the same amount of heat is added to both containers, the container with less water gets hotter. Mass and temp. are indirectly proportional Using the same amount of H, which container reaches a higher T?

SPECIFIC HEAT Different materials absorb or lose H differently. i.e., the filling of a hot apple pie may be too hot to eat, whereas the crust is not. The Specific heat of a substance is the amount of H required to T of 1 gram of that substance by 1 C.  Units are J/gC (or cal/gC)

Specific heat of substance Change in its temperature CALCULATING HEAT Heat changes are measured using a calorimeter. Conservation of E: the amount of heat lost by one body is gained by another. H gained or lost is based on three quantities: = Mass of substance × Specific heat of substance × Change in its temperature Heat H = m × c × T

Example 1: How much H must be supplied to 20 g of tin to raise its T from 25C to 100C? Specific heat (c) of tin is 0.055 cal/g ºC. m = 20 g ∆T = 75 ºC c = 0.055 cal/g◦C H = ??? H = m × c × ∆T H= (20 g)(0.055 cal/gºC)(75 ºC) H = 82.5 cal

Example 2: Calculate the specific heat of a solid if 1638 J of heat raises the temperature of 125 g of the solid from 25.0 to 52.6 C. m = 125 g ∆T = 27.6 ºC c = ??? H = 1638 J c = 0.475 J/g ºC

PHASE CHANGES Gas Liquid Solid When matter releases or absorbs E without a change in T, phase change occurs Gas Evaporation Condensation Sublimation Liquid Deposition Melting Freezing Solid Arrows up: E in Arrows down: E out

T vs. H For Water When heat is added to ice, it absorbs the H without a ΔT  causing a phase change Similarly, when heat is added to hot water, a phase change occurs without an  in T

LATENT HEAT OF FUSION The quantity of H required to melt 1 g of solid is called the latent heat of fusion (Lf). Lf for ice is 80 cal/g

LATENT HEAT OF VAPORIZATION The quantity of H required to evaporate 1 g of liquid is called the latent heat of vaporization (Lv). Lv for water is 540 cal/g

HEAT DURING A PHASE CHANGE The amount of H, released or absorbed during phase change, depends on the amount of substance and its latent heat. H = m Lf H = m Lv and

Example 1: How much heat is required to melt 50 g of ice at 0C? H = m × Lf m = 50g Lf = 80 cal/g H = ??? H = 50 g × 80 cal/g H = 4000 cal

Example 2: How much heat is required to vaporize 50 g of water at 100C? H = m × Lv m = 50g Lv= 540 cal/g H = ??? H = 50 g × 540 cal/g H = 27000 cal

Example 3: Calculate the amount of H required to change 20 g of ice at 0C to water at 10C. Solid-Liquid Liquid

Example 3: = m × Lf = 20 g × 80 cal/g = 1600 cal = m × c × ∆T = 20 g × 1.0 cal/gºC × 10 ºC = 200 cal Htotal = 1600 cal + 200 cal = 1800 cal

HEAT TRANSFER 1. Conduction 2. Convection 3. Radiation When there is a T difference between two objects, heat flows from warmer  cooler Heat (Energy) Transfer occurs by one of 3 methods: 1. Conduction 2. Convection 3. Radiation

1. Conduction Conduction is transfer of heat by contact between two objects through molecular collisions Metals are good conductors of heat, while glass and wood are poor conductors (insulators).

2. Convection Convection is transfer of heat by actual motion of molecules Liquids and gases transfer heat mainly by convection.

3. Radiation Radiation is transfer of heat without molecules Heat from the sun reaches the earth through space by radiation.

THERMAL EXPANSION All matter expands when heated, due to  molecular motion which causes them to separate from each other. the more molecular motion, the more expansion (volume) Gases > Liquids > Solids

THERMAL EXPANSION OF SOLIDS Solids expand upon  in T, based on three factors: 1) increase in temperature 2) the original length 3) type of material

THERMAL EXPANSION OF SOLIDS A practical use of thermal expansion of metals is use of bimetallic strip in operation of a thermostat.

THERMAL EXPANSION OF WATER When water at 0C is heated, it first contracts and then expands Water has its smallest volume at 4C.  water has its highest density @ 4ºC.

More on WATER Thermal behavior of water is important in preserving aquatic life in colder climates. As water cools on surface, it sinks due to its high density, allowing for water circulation @ T < 4C, it becomes less dense and rises to the surface Freezing occurs at top, allowing for marine life to exist below the surface.

THERMAL CIRCULATIONS The difference in the specific heat of water and land cause the land to warm up more quickly during the day. The less dense warm air rises, and is replaced by cool air from the sea, causing a sea breeze.

THERMAL CIRCULATIONS During the night, the land cools faster than the sea, reversing the process, and causing a land breeze.

THE END

Some Symbols Used  all  there is or there exists D Change  infinity  multiplication  proportional to  less than or equal to  greater than or equal to  plus or minus  degrees  therefore ≈ approximately equal to  decreases  increases E – Energy H – Heat e- - electron p+ - proton F. Morales Unit 1