Heat and Thermodynamics

• All matter is in constant motion, so the atoms and molecules of all matter have KE. Gases vibrate the most b/c there are no bonds

b/w the molecules Solids vibrate the least b/c there are the

strongest bonds• This KE causes the effect of warmth.

Whenever something becomes warmer, the average KE of its atoms or molecules has increased.

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Does a solid desk sitting in a classroom have kinetic energy?

No. The entire desk is not moving so the entirety of the desk does not have KE.

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Do the molecules in a solid desk sitting in a classroom have kinetic energy?

Yes! Even though the desk itself is not moving, the individual molecules are constantly moving. We do not see the desk moving because they are all vibrating back and forth over VERY small distances.

TemperatureTemperature: the measurement of the

average molecular KE of an object. As molecules gain KE, the temperature

increases As molecules lose KE, the temperature

decreases SI Unit: Kelvin (K) Ex: 0°C = 273 K = 32°F

• Temperature is the measure of the AVERAGE KE of the substance, not the total KE.

Ex: there is more KE in a bucketful of warm water than in a cupful of water, but if the temperature of the two samples is the same, the average KE of the two is the same.

• There are 3 commonly used temperature scales

1. Fahrenheit2. Celsius3. Kelvin

Temperature Scale

Freezing Point of Water

Boiling Point of Water

Where it is Used

Fahrenheit 32°F 212°F In the US

Celsius 0°C 100°C Most of the world

Kelvin 273 K 373 K In science (SI unit)

A Kelvin is the same size as a Celsius degree• If you increase the temperature by 1°C, you

increase it by 1 K• If you increase the temperature by 10°C, you

increase it by 10 K

Absolute zero: the lowest possible temperature. At this temperature all motion ceases, so the

substance has no kinetic energy. Scientists have never gotten any substance to

reach absolute zero. SI Unit: Kelvin (k) Ex: 0 K = -273°C

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Which has a greater average KE, a hot cup of coffee or swimming pool full of ice cold water?

The cup of coffee. As temperature is related to the average KE, the one with the higher temperature has the greater average KE.

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Which has a greater total KE?The swimming pool of ice cold water.

Even though the cup of coffee has a higher average KE, the swimming pool has more mass and therefore a greater overall KE.

HeatHeat: The energy transfer from one object

to another because of a temperature difference between them. Heat flows from the higher-temperature

substance into the lower-temperature substance.

Heat never flows on its own from a cold substance to a hot substance, just like water will never flow uphill by itself.

Ex: You feel cold holding a piece of ice because heat flows from your hand to the ice cube

Thermal Energy: the energy resulting from heat flow. When heat flows from one object or

substance to another it is in contact with, the objects are said to be in thermal contact.

Ex: When you hold a piece of ice, you lose thermal energy while the ice gains thermal energy

Thermal Equilibrium

Thermal Equilibrium: the state of two or more objects or substances in thermal contact when they have reached a common temperature. Objects have reached the same temperature Ex: when a piece of hot metal is added to a

cup of cool water, thermal energy flows between them until they reach the same temperature.

Internal Energy

Internal Energy: the grand total of all energies inside a substance. A substance contains internal energy, not

heat This energy is due to KE of the moving

molecules as well as the PE in the bonds holding the molecules together.

SI Unit: Joule (J) Ex: An iceberg has a larger internal energy

than a small cup of coffee

Thermal Expansion• When the temperature of a substance

increases this means the average kinetic energy of the substance increases, so the molecules move faster and further apart.

• This results in an expansion of the substance. Almost all forms of matter expand when

they are heated and contract when they are cooled

• Thermometers work off of this concept As the mercury or alcohol heats up, it

expands up the thermometer When the temperature decreases, the liquid

contracts down the tube• Not all objects have the same rate of

thermal expansion. Some objects expand more than others Liquids tend to expand more than solids

Bimetallic strip

• A bimetallic strip is two metals welded together, steel and copper

• When the strip is heated, it curves That is b/c the steel side expands at a

different rate compared with the copper side

Check Your UnderstandingCheck Your UnderstandingWhy is it advisable to allow telephone Why is it advisable to allow telephone

wires to sag when stringing between wires to sag when stringing between poles in summer?poles in summer?

Because if the lines were pulled tight in the summer, when they contract in the winter, they would snap.

Expansion of Water• Water is the exception to thermal

expansion• Water at the temperature of melting ice

contracts when the temperature is increased, and will continue to contract until it reaches 4°C. Water is most dense at 4°C This is why ice floats in water

• The ice is less dense than the water

What would happen if you put a can of soda in the freezer and leave it there overnight?

It will explode. As the water goes away from 4°C, the volume increases. As the pressure builds from the excess volume, the soda can will eventually explode.

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Heat Transfer

• Remember that heat is the transfer of energy

• There are 3 types of heat transfer Conduction Convection Radiation

• More than one type of heat transfer can occur at the same time, but usually one is more dominate than the rest

Conduction

Conduction: energy transfer from one object to another when the two are in direct contact. The materials MUST be touching. Heat moves from the warmer object to the

cooler object. Ex: a hot piece of metal burning your hand;

your hand melting a piece of ice

• Materials that conduct (move) heat well are called heat conductors. Objects will change their temperature easily Ex: metals, glass

• Wood would be considered a good insulator. An insulator delays the transfer of heat. Objects do NOT change their temperature easily Ex: Styrofoam, wood, air

• A poor conductor is a good insulator.• Liquids and gases in general are good insulators.

Porous materials having many small air pockets are good insulators as well (the secret behind feathers’, fur’s, and wool’s perceived warmth).

Ice HotelIce Hotels and Igloos keep people warm because

ice and snow are very good insulators or heat.

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Touch a piece of metal and a piece of wood in you immediate vicinity. Which one feels colder?

Since wood is a poor conductor, the metal feels colder b/c it is a better conductor; heat easily moves out of your warmer hand into the cooler metal.

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Which is really colder?If the wood and metal are in the same

general area, they should have the same temperature (room temperature); thus neither is colder.

Check Your UnderstandingCheck Your UnderstandingWhy can fire walkers walk on red-hot coals

without getting injured?The coals are good insulators, and therefore

poor conductors, of heat. Even though the coals are red hot, they give up very little heat in a brief contact with a cooler surface (your foot). Things would be different if they were walking over red-hot metal shards.

Convection

Convection: heat transfer by movement of the atoms themselves from place to place In convection, heating occurs by currents in a

fluid. Ex: heat rises from downstairs to upstairs;

bubbles in boiling water rises to the top and rolls back to the bottom of the pot

Convection occurs in all fluids, liquids and gases alike. Convection will NEVER occur in a solid!

When the fluid is heated, it expands, becomes less dense, and rises. Cooler fluid then moves to the bottom and the process continues.

Check Your UnderstandingCheck Your UnderstandingYou can hold your fingers beside a flame

without harm, but not above the flame. Why?

Heat travels upward by air convection. Since air is a poor conductor, very little heat travels sideways.

Radiation

• Heat from the sun warms the Earth’s surface after passing through the atmosphere. As air is a poor conductor, conduction cannot

be responsible for this warming. Neither does it pass via convection, as convection begins only after Earth is warmed.

Both forms need molecules to transfer heat.

Radiation: heat transfer by electromagnetic waves. Radiation does NOT require a medium to

transfer heat Ex: The sun warms the earth

• Types of EM waves• Radio waves• Microwaves• Infrared waves• Visible light• Ultraviolet waves• X-rays• Gamma rays

Absorption of Radiant Energy

• Absorption and reflection are opposite processes.

• A good absorber of radiant energy reflects very little radiant energy, including light. Because of this, a good absorber appears

dark.• A perfect absorber reflects no radiant

energy and appears perfectly black. No visible light is reflected. Ex: Your eye’s pupil, a Black Hole in space

Check Your UnderstandingIf you paint the inside of a box white and

cut a hole on one side to see inside, the hole appears black. Why?

Radiant energy that enters an opening has little chance of leaving before it is completely absorbed. That’s why holes appear black.

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Why are most coffee mugs painted white on the inside?

B/c white is good reflector of radiant energy (light). You want the radiant energy to be reflected back into your coffee, keeping it hotter for a longer time.

Emission of Radiant Energy

• Good absorbers are also good emitters poor absorbers are poor emitters poor absorbers are good reflectors

• Light colored objects in contact with dark colored objects eventually reach thermal equilibrium. The warmer, darker object must give up (emit) radiant energy to the cooler, lighter object.

Check Your UnderstandingIs it more efficient to paint a heating

radiator black or silver?

Black. The radiator painted silver would be a poor emitter and poor absorber or energy. Black would increase the contribution of heat from the radiator.

Thermodynamics

Thermodynamics: the study of heat and its transformation into mechanical energy. The foundation of thermodynamics is the

law of conservation of energy and the fact that heat flows from a warmer object to a cooler object

1st Law of Thermodynamics

The First Law of Thermodynamics: whenever heat is added to a system, it transforms to an equal amount of some other form of energy. Conservation of energy; energy cannot be

created nor destroyed, just change from one form to another

Ex: Food changes from chemical energy to body heat and/or kinetic energy

Object/Process

Energy Conversions

Automobile Engine

Chemical Kinetic

Heater/Furnace Chemical HeatHydroelectric Gravitational Potential

ElectricalSolar Optical ElectricalNuclear Nuclear Heat, Kinetic, OpticalPhotosynthesis Optical ChemicalFood Chemical Heat, KineticBattery Chemical Electrical

• When energy is added to a system (any group of atoms, molecules, particles, or objects), this energy does one or both of two things:

1. increase the internal energy of the system if it remains in the system

2. does external work if it leaves the system

Equation:

heat = increase in + external work doneheat = increase in + external work done added internaladded internal by the systemby the system energyenergy

Check Your UnderstandingIf 10 J of energy is added to a system

that does no external work, by how much will the internal energy of that system be raised?

10J

If 10J of energy is added to a system that does 4J of external work, by how much will the internal energy of that system be raised?

10J – 4J = 6J

Second Law of Thermodynamics

The second law of thermodynamics: heat will never flow by itself from a cold object to a hot object. In order for heat to flow from a cold object to

a hot object, work must be put into it (refrigerators work off of this idea)

Ex: heat flows from your hand to a piece of ice (from hot to cold)

Entropy• Natural systems tend to proceed toward

a state of greater disorder. Entropy: the measure of the amount of

disorder. As disorder increases, entropy increases. The second law states that for natural

processes, in the long run, entropy always increases.

Ex: liquids evaporating, ice melting, a messy room

• You can’t get more energy out of a system then what you put in it.