Chapter 6. Temperature ◦ Is something hot or cold? ◦ Relative measure.

Temperature And Heat

Chapter 6

Temperature◦ Is something hot or cold?◦ Relative measure

Temperature

Temperature◦ Molecules of all substances are in constant motion◦ Depends on the kinetic energy of molecules in a

substance

Temperature

Temperature◦ Greater the temperature◦ Greater the motion of its molecules◦ Measure of the average kinetic energy of the

molecules of a substance

Temperature

Thermometer◦ Measuring instrument

Utilizes the physical properties of materials to accurately determine temperature

◦ Thermal expansion Expand with increasing temperature Contract with decreasing temperature

Temperature

Temperature scales◦ Reference points◦ Ice points

Freezing point of water◦ Steam point

Boiling point of water

Temperature

Temperature scales◦ Fahrenheit

Ice point 32 Steam point 212

◦ Celsius Ice point 0 Steam point 100

Temperature

Temperature scales◦ Kelvin

Absolute temperature scale Zero temperature is the absolute lower limit Absolute zero

Temperature

Heat◦ Flows from higher to lower temperature◦ Energy in transit because of a temperature difference◦ When heat is added to a body, internal energy

increases

Heat

Heat◦ SI unit Joule (J)◦ Common unit calorie (cal)

Heat necessary to raise one gram of pure water by one Celsius degree

Heat

Kilocalorie◦ Food calorie (Cal) is equal to one kilocalorie

Heat

British thermal unit (BTU)◦ British system unit of heat◦ Heat necessary to raise one pound of water one

Fahrenheit degree

Heat

Nearly all matter expands when heated and contracts when cooled

Water is an important exception◦ Expands when frozen

Heat

Specific heat◦ Heat necessary to raise the temperature of one

kilogram of a substance one Celsius degree◦ Specific to a particular substance◦ Greater the specific heat◦ Greater its capacity for heat

Specific Heat And Latent Heat

Recall: heat will flow from Hot to Cold ie: from high heat content to low heat

content Until they reach thermal equilibrium

Thermal Equilibrium Heat energy lost = heat energy gained

Specific Heat - Mixtures

hot substance coldsubstance

hot substance coldsubstance

hot substance coldsubstance

hot substance coldsubstance

(hot)

(cold)

(hot)

(cold)

(hot)

(cold)

What’s going to happento the temperatureof the small block?

What’s going to happento the temperatureof the large block?

What’s going to happento the temperatureof the small block?

(warm)

(warm)

(warm)

(warm)

(warm)

(warm)

(warm)

(warm)

Heat is a form ofenergy

that flowsfrom one object

to another(warm)

(warm)

(always fromhot to cold)

(warm)

(warm)

Heat transfer occurs in mixtures too. Consider mixing hot and cold water

Specific Heat – Mixtures

By mixing together, the energy from one is gained by the other.

The result is an increase in T for one and a decrease for the other

Specific Heat – Mixtures

Heat gained = heat lost

Specific Heat – Mixtures

Heat gained Heat lost

mc∆T = Q Q= mc∆T

mc∆T = mc∆T

Practice 500. g of water at 35 ºC is added to 400. g

of water at 85 ºC. What is the temperature of the resulting mixture once equilibrium has been reached?

Specific Heat –Mixtures

Phases of matter◦ Solids◦ Liquids◦ Gases

Latent heat◦ Heat associated with a phase change

Specific Heat And Latent Heat

Latent heat of fusion◦ Heat required to change solid into liquid

Specific Heat And Latent Heat

Latent heat of vaporization◦ Heat necessary to change a liquid into a gas

Specific Heat And Latent Heat

Sublimation◦ Changing directly from the solid to the gaseous phase

Deposition◦ Changing directly from the gaseous to the solid phase

Specific Heat And Latent Heat

Boiling◦ Process by which energetic molecules escape from

liquid◦ Energy is gained from heating◦ Increasing pressure increases the boiling point of

water

Specific Heat And Latent Heat

At mountain altitudes◦ Decreased air pressure◦ Boiling point of water lower that at sea level

Specific Heat And Latent Heat

Evaporation◦ Slow phase change of liquid to gas◦ Major cooling mechanism

Evaporation of perspiration has a cooling effect Energy is lost

Specific Heat And Latent Heat

Evaporation◦ Moving air promotes evaporation by carrying away

water molecules◦ Cooling effect

Specific Heat And Latent Heat

Relative humidity◦ Amount of moisture in the air◦ Humid environment◦ Little evaporation of perspiration◦ Hot feeling

Specific Heat And Latent Heat

Methods of heat transfer◦ Conduction◦ Convection◦ Radiation

Heat Transfer

Conduction◦ Transfer of heat by molecular collisions◦ Kinetic energy of molecules is transferred from one

molecule

Heat Transfer

Thermal conductivity◦ Measure of a substance’s ability to conduct heat

Metals◦ Good conductors

Liquids and gases◦ Poor conductors◦ Molecules farther apart◦ Less collisions

Heat Transfer

Convection◦ Transfer of heat by the movement of a substance, or

mass, from one position to another◦ Movement of heated air or water

Heat Transfer

Radiation◦ Process of transferring energy by means of

electromagnetic waves◦ Dark colors absorb radiation better than light colors

Heat Transfer

Three common phases of matter◦ Solid◦ Liquid◦ Gas

Phases Of Matter

Solid◦ Definite shape and volume◦ Crystalline solid

Molecules arranged in a particular repeating pattern

Phases Of Matter

Heating◦ Molecules gain kinetic energy◦ Vibrate about their positions in the lattice◦ Move farther apart◦ Lattice becomes distorted

Phases Of Matter

Liquid◦ Slight lattice structure◦ Molecules relatively free to move◦ Definite volume but no definite shape

Phases Of Matter

Liquid◦ Individual molecules gain kinetic energy when heated◦ Liquid expands as a result◦ Gas phase

Occurs when molecules are completely free from each other

Phases Of Matter

Gas◦ Rapidly-moving molecules◦ Exert little or no force on another except when they

collide

Phases Of Matter

Continued heating of a gas◦ Molecules and atoms ripped apart by collisions with

one another Plasma

◦ Hot gas of electrically charged particles◦ Ionosphere◦ Lightning strikes◦ Neon and fluorescent lamps

Phases Of Matter

Kinetic theory◦ Pressure rises as temperature increases◦ Gases diffuse in air

The Kinetic Energy Of Gases

Kinetic theory◦ A gas consists of molecules moving independently in

all directions at high speeds◦ Higher the temperature, the higher the average speed

The Kinetic Energy Of Gases

Ideal gas◦ Molecules are point particles (have no size at all)◦ Interact only by collision

Collisions exert only a tiny force on the wall

The Kinetic Energy Of Gases

Pressure◦ Force per unit area, p = F/A◦ SI unit is N/m2 or pascal (Pa)

The Kinetic Energy Of Gases

Pressure◦ Directly proportional to number of gas molecules

present◦ Directly proportional to the Kelvin temperature◦ Inversely proportional to volume

The Kinetic Energy Of Gases

The ideal gas law

The Kinetic Energy Of Gases

Thermodynamics◦ Dynamics of heat

Production Flow Conversion to work

Thermodynamics

Heat added to a closed system◦ Internal energy◦ Doing work

Thermodynamics

First law of thermodynamics◦ Positive heat values (heat is added to the system)◦ Positive work values (work done by the system)◦ Negative values indicate the opposite

Thermodynamics

Heat engine◦ Converts heat into work◦ Some input energy is lost or wasted

Thermodynamics

Second law of thermodynamics◦ Impossible for heat to flow spontaneously from a

colder body to a hotter body◦ No heat engine operating in a cycle can convert

thermal energy completely into work

Thermodynamics

Third law of thermodynamics◦ It is impossible to attain a temperature of absolute

zero◦ Would require all of the heat to be taken from an

object

Thermodynamics

Heat Pump◦ Uses work input to transfer heat◦ Low temperature to high temperature◦ Reverse of a heat engine

Thermodynamics

Entropy◦ Does a process take place naturally?◦ The entropy of an isolated system never decreases◦ Measure of the disorder of a system

Thermodynamics

Entropy◦ Disorder increases as natural processes take place◦ Total entropy of the universe increases in every

natural process

Thermodynamics