Chapter 9 Heat

What is hot? 1700s - Caloric – heat fluid that flows from hot to cold items till equal in each Fails to explain rubbing hands together Old school theory

What is hot? Today – Kinetic-Molecular theory – atoms or molecules are constantly in motion, the faster they move the hotter the body (more and high velocity collisions) How hot something is depends on the average KE of its particles Remember chemistry?

Internal Energy Energy of the particles’ motion is called Internal Energy (symbol U) Several types of particle motion energy Translational – (measured by temp) Rotational Vibrational

Energy flow between objects Energy flows between objects by collisions – fast ‘hot’ particles transfer momentum & energy to slow ‘cold’ particles

Thermal Equilibrium When both objects have same avg. KE (temp) then energy is flowing equally in both directions To get a temperature – must get thermal equilibrium between object & thermometer Demonstrate equlibrium, not no flow, equal flow. Pour water back and forth between two cups Therm-o-meter – needs thermal equilibrium to work, not instant when you put it in something

Thermal Expansion Objects expand when their temperature increases* We can calculate rate of expansion per degree temperature for different materials How Thermometers Work

Temperature Scales Fahrenheit – 1720s thermometer maker Celsius - 1742 Standard cold temp – salt water freezing Water freezes: 32° Water Boils: 212° Celsius - 1742 Based on water Water freezes: 0° Water Boils: 100° Kelvin - 1848 Starts at coldest possible temperature Water freezes: 273.15 Water Boils: 373.15

High Quality H2O Water boils at Water Freezes at 100oC 212oF 373 K 0oC Need to memorize these 6 values – then can convert anything

Absolutely Cool Absolute Zero Temperature at which KE is no longer present 0 K = -273.15oC Unreachable – Measuring temperature adds KE (thermal equilibrium with thermometer) Then how do we know it is the coolest? Do absolute zero graphing activity

Is It Getting Hot in Here Matter contains energy, not heat Paperclips HEAT is : Energy that is transferred from high temperature substances to low temperature substances Hand out paperclips – twist back & forth till it breaks – hold end – its hot! KNOW THIS DEFINITION!!!! Like water flowing downhill – energy flows from hot to cold

Heat Heat’s symbol is Q measured in Joules or calories or BTUs or kilocalories or Calories or Therms James Joule showed equivalence between thermal and mechanical energy - diagram

How does it flow Thermal Energy moves from high to low temperature Particle collisions transfer translational KE Rate directly proportional to Temperature difference Newton’s law of cooling R ~ ΔT Newton’s law of cooling

Too Hot to Handle Thermal Energy Transferred in 3 ways Conduction Convection Radiation Ask them if they know the 3 ways first…

Conducting Business Conduction Heat transferred through CONTACT – atoms of one material crash into atoms of other material Conductors are good at it Insulators are bad at it What is heat? What is temperature? Conduction is atoms crashing – momentum being transferred Do desk example – touch metal, touch top Show Hewitt videos – air is insulator and walking on hot coals Styrofoam, down jackets, house insulation, double pane windows, metal vs. plastic windows, vacuum bottles

Convection Convection Transfer of energy through fluids Not so much energy transfer as matter transfer Archimedes’ principle applied Convection currents produce winds Show Eureka Video – Show example on the board – lake wind examples – boiling water example – Sun example Works in all fluids!

Full of Cold Air Air is cooler at higher altitudes Less pressure so: gases are able to expand Lower density = less kinetic energy Air is very hot @ low altitudes This is why it is cold at higher altitudes – cold in Denver, on Everest, warm in Death valley & Jordan Valley Death Valley

You’re So Radiant Forms of radiant energy Microwaves UV IR Radio Visible Gamma Heat transfer from sun is radiation Energy transmitted via radiation is radiant energy Doesn’t have to be touching, doesn’t need air or fluid – only way energy can enter from outside the planet

Riding the Waves Good absorbers = Good Emitters Low temperature emit long wavelengths High temperature emit short wavelengths We absorb IR waves (heat lamp) Remember Movie at start of semester – this is black body radiation!!! Only depends on temperature Heat lamp – absorb & emit IR waves

Radiant Energy Absorbing and Reflecting are opposites Light colors reflect: radiant energy All wavelengths Dark colors absorb: Silver coffee & drink pitchers, black clothing, white clothing Coffee/Cream NTQ

Energy Conservation Energy still conserved ΔPE + ΔKE + ΔU = 0 Q (heat) is part of the U (internal energy)

How much heat? Materials require different amounts of heat to increase their temperature Specific Heat (C)– amount of energy needed to raise temperature of 1 kg of material 1 Kelvin Chart pg 314 for common materials

Heat equation Specific heat – math definition (C)= Heat (Q)/(mass*ΔTemp) Solve for the heat (Q) needed: Q = mCΔT

Practice with Heat How much energy is needed to increase the temperature of 45 grams of Silver 25 Kelvin? If 1.5 kg of water starts at 274 Kelvin, what temperature will it be at if 35,000 Joules of energy are added?

Calorimetry – Finding Specific Heat Energy conservation says the total energy must remain the same. If we isolate a hot object with a cold one, we can be sure all the energy flows to the cold from the hot. If we know the ΔT of each and the Specific Heat of one, we can find the specific heat of the other.

Practice Calorimetry A 1.2 kg mass of unknown metal cools from 115°C to 27°C in 600. ml of water that started at 22°C. What is the specific heat of the metal? A 2.0 kg block of copper metal that is 125°C is put in 550 ml of 22°C water. What is the final temperature of the water and copper?

Water Water Everywhere Water has a very high specific heat capacity Advantages Useful for cooling Useful for warming Holds a lot of energy in its temperature U of I – chilled water cooling – send cold water out in summer – heat of air absorbed into it Hot water heat – same idea – opposite direction – house I grew up in – this building – hot water bottle in bed Cars use ‘coolent’ to remove heat from the engine – basically water- talk about anti-freeze later

Controlling world climates Deserts Why are summer & winter late? Why do people want to live in California? Deserts – hot during day – cold @ night – sand has low capacity – water high Thermal Lag – water on earth takes time to warm up/cool down – swim in August, not June, cold in February, not December Live by a big ocean – California & Florida & England mild – Middle of Continent is hot & cold – kansas – cooler by the lake - Chicago

Let’s Expand Our Horizons Most matter expands when heated Gases fastest, then Liquids then Solids Hot-air balloons Ring & Sphere demo Bi-metal strip Thermal rate of expansion Hot air balloons – air spreads out, less dense – weight is less – buoyancy overcomes Bi-metal strip – different things different coefficents of expansion

Expansion examples How thermostats work Railroads, power lines Bridge joints Thermal pipe joints Show example of thermostat – basically long piece of metal Why railroads are built like they are – click-clack

Exceptions to the Rule Water is different from other substances Water expands when temperature is above or below 4oC Water is smallest and most dense at 4oC This is why Ice floats – draw graph of density vs. Temperature -

Phase Transitions Material changes phase, energy change comes from other internal energy, not KE translational…. so NO TEMPERATURE CHANGE Latent heat (L) – energy transferred during phase change (Joules/kg) Q = mL (heat = mass*latent heat)

Solid – Liquid transition Melting/freezing point – temperature at which the transition occurs Latent Heat of Fusion (Lf)– energy required to melt a solid into liquid – Also energy given off when liquid freezes to solid

Liquid-Gas transition Boiling/condensing point – temperature at which the transition occurs Latent Heat of Vaporization (Lv)– energy required to vaporize a liquid into gas Also energy given off when gas condenses to liquid

Water transitions Lf = 3.33*105 Joules/Kg Lv = 2.26*106 Joules/Kg Chart pg 318 for more common substances Graph of Water transitions

Transition problems How much energy is needed to convert 1.5 liters of 273 K ice to 273 K water? How much energy is needed to heat 2.5 kg of 350 K water into 373 K steam?

More transition problems How much water at 360 K could be turned into steam with 25,000 Joules of energy? How much energy is released as .5 kg of 373 K steam is cooled and frozen into 273 K ice? What energy is required to turn a 5.0 kg block of ice at -10°C into 110°C steam?

It’s Time to Chill Out Freezing Freezing temperature can be altered Change from a liquid to a solid state Freezing temperature can be altered Change in pressure Adding salt or sugar

You’re Getting Warmer Boiling Changing from liquid to gas Occurs beneath the surface Cooling process - molecules having the highest KE escape

Boiling Dependent on temperature and pressure Pressure of vapor within the bubbles > pressure of surrounding liquid Temperatures below boiling point, vapor pressure is not great enough

Boiling Higher Atmospheric Pressure Increases Boiling Point molecules are required to move faster to exert increased pressure within the bubble

Boiling Lower Pressure (high altitudes), Decreases boiling point Pressure Cooker Vapor builds up inside sealed cooker Pressure on the surface is increased Raises boiling point Increased Temp. cooks the food

2 Phases for the price of 1 Boiling and Freezing can occur at the same time Regelation Melting takes place under pressure Freezing occurs when pressure is reduced

Triple Point – all 3 phases @ once