Thermodynamics

A lesson in heat (and the study of it)

Chapter 12

Kinetic Energy and AtomsKinetic Energy and Atoms

Everything is made of moving Everything is made of moving atoms/moleculesatoms/molecules

Moving things have kinetic energyMoving things have kinetic energy

Kinetic energy causes “warmth”Kinetic energy causes “warmth”

More warmth means more energyMore warmth means more energy

More energy/motion means More energy/motion means

more warmthmore warmth

What makes a hot body hot?

The kinetic-molecular theory: Matter is made up of tiny particles that are always in

motion There is a direct relationship between the speed in

which the molecules move and the temperature. The warmer it is, the faster the particles move. Think about the molecules in a solid as if they were

held together by springs. The springs are the electromagnetic forces that keep

the solid together.Thermal energy, then, is the overall energy of motion of the particles that make up an object.

Warmer objects have more thermal energy than colder bodies.

Temperature and HeatTemperature and Heat

If the temperature outside is 90 degrees – If the temperature outside is 90 degrees – why do some things feel hotter than why do some things feel hotter than others?others?

If the temperature outside is 30 degrees – If the temperature outside is 30 degrees – why do some things feel colder than why do some things feel colder than others?others?

HeatHeat

The energy that transfers from one object to The energy that transfers from one object to another because of a temperature difference another because of a temperature difference between thembetween them

ALWAYS goes from high temp to low temp; ALWAYS goes from high temp to low temp; warmer objects to cooler objectswarmer objects to cooler objects

Substances do not contain heat – they contain Substances do not contain heat – they contain internal energyinternal energy

Thermal Energy and Temperature:

How can you determine an object’s “hotness” (keep it to Physics!!!)?

Temperature!!! A property of an object Depends on the average kinetic energy of the

molecules in the object (not the number of particles). Think of it like this – A 1kg object and a 2kg object

can have the same temperatures.

The thermal energy in an object is proportional to the number of particles it has, but its temperature is not.

TemperatureTemperature

How hot or cold something is compared to a How hot or cold something is compared to a standardstandard

Measured by degrees on a scale – Celsius, Measured by degrees on a scale – Celsius, Fahrenheit or KelvinFahrenheit or Kelvin Celsius: 0-100Celsius: 0-100 Kelvin: Based on absolute zero; TKelvin: Based on absolute zero; Tcc +273 +273

What is absolute zero?What is absolute zero?

Thermal EquilibriumThermal Equilibrium

The temperatures of different objects in The temperatures of different objects in contact with each other will always seek contact with each other will always seek to to equal outequal out. .

Equilibrium and Thermometry

Understanding thermometers at a molecular level:

Your body is hotter than the thermometer, meaning the thermal energy of your body is higher causing the molecules in your body to move faster.

When you put the thermometer in your mouth, energy is transferred from the skin of your mouth to the thermometer by a process called conduction.

The thermal energy of your skin goes down and of the thermometer increases.

ExamplesExamples

Making a Making a snowballsnowball

Holding a cup of hot chocolateHolding a cup of hot chocolate

Thermal ExpansionThermal Expansion

When temperature increases, all forms of When temperature increases, all forms of matter will expand (one exception). Different matter will expand (one exception). Different materials expand at different rates.materials expand at different rates.

It’s easiest for gases to expand – why?It’s easiest for gases to expand – why?

How does this affect roads and buildings?How does this affect roads and buildings?

ThermostatsThermostats

ThermometersThermometersHow do you go about measuring temperature? If you think you are sick, you mom might take your

temperature, but what causes the liquid in the thermometer to rise?

Matter expands when its temperature increases and contracts when it decreases.

Place mercury, alcohol, or even water in a glass tube with a scale on the side and you can measure temperature by the expansion and contraction of the liquid.

Measuring HeatMeasuring Heat

Specific Heat (Unit of heat – calorie):Specific Heat (Unit of heat – calorie): Defined as the amount of heat required to raise the Defined as the amount of heat required to raise the

temperature of 1 gram of water by 1°C.temperature of 1 gram of water by 1°C. Kilocalorie =1000 calories ( this is what’s actually Kilocalorie =1000 calories ( this is what’s actually

used to measure food); they just write it as Calorie used to measure food); they just write it as Calorie (capital c).(capital c).

SI unit for heat – joules ( 1 calorie = 4.184 J)SI unit for heat – joules ( 1 calorie = 4.184 J)

Expansion of WaterExpansion of Water

The exception – ice cold water The exception – ice cold water contracts when heated (at first), then contracts when heated (at first), then it begins to expand. 4it begins to expand. 4°°C is the magic C is the magic number.number.

As water freezes and gets below As water freezes and gets below

ConductionConduction

Transfer of thermal Transfer of thermal energyenergy through through direct direct contactcontact of particles. All of particles. All mattermatter is composed of molecules in constant is composed of molecules in constant motionmotion, so all matter can “conduct” , so all matter can “conduct” heatheatThink about a Think about a soup spoonsoup spoon

Heat ConductorsHeat ConductorsMaterials that conduct heat wellMaterials that conduct heat well

SolidsSolids are better than liquids or gases are better than liquids or gases – why?– why?

Which metals are the best Which metals are the best conductors? Which is #1?conductors? Which is #1?

Metals have “loose” outer Metals have “loose” outer electronselectrons, , which allows them to conduct heat better which allows them to conduct heat better than other materials.than other materials.

What are some poor conductors?What are some poor conductors? Wood, plastic, glass, fiberglass, paperWood, plastic, glass, fiberglass, paper Also called what?Also called what?

QuestionsQuestionsIf you pulled a wooden spoon and a metal If you pulled a wooden spoon and a metal spoon out of the freezer – which one is spoon out of the freezer – which one is colder?colder?

Is snow a good conductor or an insulator? Is snow a good conductor or an insulator? It’s not a source of heat, but it prevents heat It’s not a source of heat, but it prevents heat

from escapingfrom escaping

Do insulators keep heat in or keep cold Do insulators keep heat in or keep cold out?out?

ConvectionConvection

Transfer of energy in Transfer of energy in fluidsfluids by the by the movement or flowmovement or flow (currents) of (currents) of particles in a substance.particles in a substance.

Solids do not “flow”, but Solids do not “flow”, but liquidsliquids and and gasesgases do. do.

Convection currents produce winds.Convection currents produce winds.

As air As air expandsexpands – it – it coolscools

When air is When air is compressedcompressed – it – it heats upheats up

RadiationRadiationTransfer of energy through electromagnetic waves, goes through gas and atmosphereSince there are no fluids (like air and water) in space, convection is not possible for transferring the heat from the sun.

Radiation brings heat to our planet and convection works to transfer the heat once it’s inside our atmosphere.When radiation strikes, some of the energy is absorbed, some is transmitted, and some is reflected (the atmosphere surrounding the Earth acts to reflect some of the Sun’s radiant energy. Otherwise we would burn up.

Types of Radiant Energy – Types of Radiant Energy – Electromagnetic WavesElectromagnetic Waves

http://www.colorado.edu/physics/2000/waves_particles/

http://imagers.gsfc.nasa.gov/ems/waves3.html

Objects at Objects at low low temperatures emit temperatures emit longlong wavelengths wavelengths

Objects at Objects at highhigh temperatures emit temperatures emit shortshort wavelengths wavelengths

Infrared waves are extremely short, and are absorbed Infrared waves are extremely short, and are absorbed

by our skin producing a heatby our skin producing a heat sensation sensation

Heat = infrared radiationHeat = infrared radiation

Emission of Radiant EnergyEmission of Radiant Energy

Good reflectors are poor Good reflectors are poor absorbersabsorbersLight colored objects Light colored objects reflect more light and reflect more light and heat than dark colored heat than dark colored objectsobjectsGood Good absorbersabsorbers are also are also good good emittersemittersPoor absorbers are poor Poor absorbers are poor emittersemitters

Specific Heat

The amount of energy required to raise the temperature of a unit mass, one temperature unit. Measured in J/kg * K Heat gained or lost by an object as its

temperature changes depends on the mass, the change in temperature and the specific heat of the substance.

Q = mC∆T (C is NOT celsius here)

Change of State

Three states are solids, liquids, and gases.

Melting point is the temperature at which a substance goes from a solid to a liquid.

Boiling point is the temperature at which a substance goes from a liquid to a gas.

Amount of energy needed to melt 1kg of a substance is the heat of fusion. (Q=mHf)

Thermal energy needed to vaporize 1kg of a liquid is the heat of vaporization. (Q=mHv)

First Law of Thermodynamics

The total increase in the thermal energy of a system is the sum of the work done on it and the heat added to it. Just a restatement of the law of conservation of energy – energy cannot be created nor destroyed, but changed into other formsDevice to convert thermal energy to mechanical energy is a heat engine. Refrigerators remove thermal energy from a colder body and add it to a warmer body through mechanical energy.A heat pump is a refrigerator that can be run in two directions.

The Second Law of Thermodynamics:

Entropy: measure of the disorder in a system or how much particles want to move from high pressure to low pressure.2nd Law: natural processes go in a direction that maintains or increases the total entropy of the universe.

Predicts that heat flows spontaneously only from a hot body to a cold body.

Newton’s Law of CoolingNewton’s Law of CoolingThe rate of cooling of an object depends on how The rate of cooling of an object depends on how much hotter the object is than the surroundingsmuch hotter the object is than the surroundings

A warm home will lose heat to the cold outside at A warm home will lose heat to the cold outside at a greater rate when there is a larger difference a greater rate when there is a larger difference between the inside and outside temperaturesbetween the inside and outside temperatures

The rate of cooling is proportional to the The rate of cooling is proportional to the temperature differencetemperature difference between the object and its between the object and its surroundingssurroundings

Rate of cooling ~ Rate of cooling ~ T T

Newton’s Law of CoolingNewton’s Law of Cooling

The larger the temperature difference of an The larger the temperature difference of an object the object the fasterfaster it cools. it cools.

Temperature

time

20° C

40° C

60° C

The Greenhouse EffectThe Greenhouse Effect

Your assignment – write a 3 paragraph Your assignment – write a 3 paragraph summary about Global Warming and the summary about Global Warming and the Greenhouse Effect using your textbook and Greenhouse Effect using your textbook and one external source (must be cited).one external source (must be cited). Discuss the causes, effects, and future predictions Discuss the causes, effects, and future predictions

about this phenomenaabout this phenomena How can we decrease Global Warming?How can we decrease Global Warming? How is global warming similar to the inside of your car How is global warming similar to the inside of your car

heating up?heating up? Due FridayDue Friday