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Temperature and Heat. Learning Goals Define temperature and convert from one scale to the other (Celsius, Kelvin, Fahrenheit) Define heat and understand its connection to energy Distinguish between specific heat and latent heat Explain how specific heat relates heat and temperature
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Temperature and Heat • Learning Goals • Define temperature and convert from one scale to the other (Celsius, Kelvin, Fahrenheit) • Define heat and understand its connection to energy • Distinguish between specific heat and latent heat • Explain how specific heat relates heat and temperature • Describe and give examples of three methods of heat transfer • Identify the phases of matter and the molecular arrangements of each • Describe how temperature and molecular speed of gas are related • Solve exercises involving pV=nRT
Temperature • Measure of average kinetic energy of molecules • Measured with thermometer • Relies on physical property of a substance and how it changes with temperature • Dilation/contraction mainly • Three scales • Fahrenheit: 32 (freeze) – 212 (boil) • Celsius: 0 (freeze) – 100 (boil) • Kelvin: Celsius + 273
Conversion Between Units • TK = TC+273 • TF = (9/5)TC + 32 • TC = (5/9)(TF - 32) A typical cold day in Quebec is -30ºC. What is the equivalent temperature on the Fahrenheit scale? In the Kelvin scale?
Heat • Energy transferred from hotter to colder substance because of temperature difference • Transfer of kinetic energy between molecules of two substances • Always goes from the molecules of the substance having MORE kinetic energy to the ones having LESSHENCEfrom HOTTER to COLDER!
Calories = Heat! • Heat is measured in JOULES (it’s energy!) but can be measured in calories • 1 calorie = amount of heat to raise the temperature of 1kg of water by 1ºCelsius. • Food Calorie = 1 kilocalorie • Amount of energy generated when you burn food
Effect of Heat Transfer on Matter • Change in temperature • Change in length or volume (thermal expansion) • Bridges • Thermostats • Etc. • Change in phase • Solid, liquid gas
Phases • Solid • Definite shape and volume • Liquid • Definite volume • Gas • No definite shape or volume
Change in Temperature: Specific Heat Capacity • The same amount of heat transferred to different substances doesn’t mean the same increase in temperature • SPECIFIC HEAT (c): amount of heat (H) necessary to raise the temperature (DT) of one kg (m) of the substance by one degree Celsius.
Example • How much energy does it take to heat 0.50kg of water from 20.ºC to 30.ºC?
Change in Phase: Latent Heat • Heating can change the PHASE of a substance • Energy is used to break bonds between molecule instead of raising temperature
Latent Heat • Amount of energy necessary to change 1kg of substance from one phase to another • Solid to liquid: latent heat of fusion, at the melting (or freezing) point • Liquid to gas: latent heat of vaporization, at vaporization (or condensation) point • Solid to gas: latent heat of sublimation, at sublimation (or deposition) point • While changing phase, the TEMPERATURE STAYS CONSTANT!
Latent VS Specific Heat • Each phase transition has a different latent heat 2100J/kg.Co 40.65 kJ/kg 4186 J/kg.Co 333.55 kJ/kg 2100 J/kg.Co
Examples of latent heat use • A dog opens its mouth to cool down • Hurricanes • Sweating • Condensation
Heat Transfer • Heat: transfer of energy through • Conduction: through molecular collisions • Convection: movement of a substance from one position to another • Radiation: by means of electromagnetic radiation (light!)
Kinetic Theory of Gases • Closed receptacle contain a finite number of gas molecules • Gases take the WHOLE volume of the enclosure they’re in • The faster the molecules move, the higher the temperature of the gas • Molecules hitting with the wall create pressure • Force per unit area, measured in Pascal, Pa • 1 Pa = 1N/m2 • High heels: higher pressure • Flat shoe: lower pressure
pV = NRT • If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules • If volume and number of gas molecules are kept constant, pressure p is proportional to temperature T • If the number of molecules and the temperature are kept constant, pressure p is inversely proportional to volume V
If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules Higher Pressure More Molecules
If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules Higher Temperature Higher Pressure
If temperature and volume are held constant, pressure p is proportional to the number N of gas molecules Higher Pressure Smaller Volume
Numerical Examples • if you don’t add any more gas in your experiment, then • p1V1/T1 = p2V2/T2 • To do calculations • Pressure: in Pascal • Volume: in m3 • N: in moles • R = 8.314 472 m3·Pa/(K·mol) • T : in KELVIN!
Numerical Examples • A cylinder of gas is at room temperature (20°C). The air conditioner breaks down, and the temperature rises to 35°C. What is the new pressure of the gas relative to its initial pressure? • What are the constant quantities? • What has changed? • -V, the volume, is not changing • -p is changing, so T will be changing • -T increased from 273+20 =293K to 273+35 = 308K, that’s an increase of a factor 308/293=1.05 • -The pressure will hence increase by a factor 1.05.
Example • A constant volume and mass of helium gas at 27°C is heated so that the pressure of the gas doubles. What is the new temperature of the gas in degrees Celsius? pV = NRT • In that problem, V and N stay constant • p changed as T was changed! • If p is doubled, T had to be doubled too!
T was 27°C • In Kelvin: 273+27°C = 300K • Double that • Tx2 = 600K • Transform back in Celsius • 600K – 273 = 327°C
If the temperature of a quantity of ideal gas increases, then the • the number of molecules must increase. • pressure must increase. • volume must increase. • the product of the pressure and volume must increase.
Homework Chapter 5 • Short-answer questions • 5, 7, 11, 19, 23 • Exercises • 2, 20, 22 • Additional Homework Assignment: NOT GRADED • Multiple-choice questions: 2, 4, 5, 6, 9, • Fill in the blanks: 1, 2, 8, 9, 10 • Visual connection • Exercises: 1, 3, 19, 21