Heat

Chapter 9

9.1 Heat EnergyObjectives• Relate heat energy to moving molecules.• Distinguish between the Celsius and

Kelvin temperature scales.• Compare temperature, internal energy,

and heat energy.• Define operationally a temperature

change.

9.1 Heat Energy

9.1 Heat Energy

• You experience heat all the time, and you use words such as hot, cold, and heat often.

• In science, heat has a different and very specific meaning.

Molecules in Motion• Recall that all matter is made of molecules

that are in constant motion.• Matter that moves has kinetic energy.• The measurement of the average kinetic

energy of the molecules in a substance is called temperature.

• If two objects with different temperatures come into contact, energy flows from the object with the higher temperature to the one with the lower temperature.

Molecules in Motion• Energy that is transferred from one substance

to another is called heat energy.• Heat is energy that flows between objects

that have different temperatures.• The official SI unit of heat is the joule.• When heat energy transfers to a substance, it

adds to the internal energy of the substance.• Internal energy is the total amount of energy

a substance contains.

Measuring Temperature

• Temperature is a measure of the average movement of the molecules in a substance.

• Volume doesn't affect temperature.

Measuring Temperature

• How do you measure temperature?• You measure temperature with a thermometer.• As a thermometer heats up, the molecules in its

liquid begin to move faster and farther apart. The liquid expands and rises in the tube.

• The scale indicates the temperature reading.• In SI, two different scales are used for

temperature readings; the Celsius scale and the Kelvin scale.

Celsius Temperature Scale• The Celsius scale is

commonly used to measure temperature.

• The Celsius scale was developed by Anders Celsius, a Swedish astronomer.

Kelvin Temperature Scale• Another SI temperature

scale is called the Kelvin scale.

• The Kelvin scale, also called the absolute scale, is named for its originator, Lord Kelvin.

• Kelvin thermometers are used primarily in the physical sciences.

Kelvin Temperature Scale• The Kelvin scale is used to measure the

temperature of super-cold substances.• The Kelvin scale identifies the temperature

where molecules in a substance are so cold they don't move. They have no kinetic energy.

• This temperature is called absolute zero.• The Kelvin temperature scale is used in science

because it relates directly to energy.• For example, I cm3 of a substance at 200 K has

twice the kinetic energy of I cm3 of the same substance at 100 K.

SI Temperature Scale

Check and Explain pg. 2121.

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9.2 Transfer of Heat

Objectives

• Identify three ways heat is transferred.• Explain how heat transfer is measured.• Calculate the amount of heat transferred

between two substances.• Infer how heat transfer affects climate.

Transfer of Heat

• Heat is transferred by conduction, convection and radiation.

Conduction

• Heat energy flows from a warm substance to a cool substance.

• When solid substances are in contact, heat energy transfers by conduction.

• Conduction is the transfer of heat energy throughout a substance, or when one substance comes in contact with another.

Conduction• Some materials conduct

heat better than others.• For example, a cloth pot

holder conducts heat poorly. It is an insulator.

• Wood, plastic, and glass are also insulators.

Convection• Heat can also be transferred by a process

called convection.

• Convection is the transfer of heat energy by the movement of a fluid, such as a liquid or a gas.

Convection• Heat energy flows through a fluid in a pattern called

a convection current.• Convection currents form because heated fluid

expands and is less dense than surrounding fluid, which is cooler.

• The difference in the density causes warm fluid to rise and cooler fluid to sink.

• The result is a convection

current that moves heat

energy through the fluid.

Convection

Radiation

• Every time you feel the sun warm your skin, you experience radiation.

• Radiation is the transfer of energy by infrared waves.

• When infrared waves strike your skin, the molecules in your skin vibrate faster

and become warmer.

Radiation• Radiation differs from conduction and

convection because matter isn't needed to transfer energy by radiation.

• Recall that conduction depends on the collision between the molecules of a substance.

• Convection depends on the expansion of a fluid when molecules collide.

• Radiation can occur in a vacuum, where no molecules of matter are present.

Radiation• Any form of energy that is transferred

by radiation is called radiant energy.• Radiant energy and radiation shouldn't

be confused with harmful radioactivity, or nuclear radiation.

• When any object gets hot, it gives off radiant energy.

Radiation• Matter can reflect or absorb radiant energy.

• The earth and its atmosphere reflect and absorb radiant energy from the sun.

Measuring Heat Transfer• Which one would you

rather stir a hot stew with?

• Not only is wood a poor conductor of heat, its temperature increases less than many other substances when it absorbs heat energy.

22 g

22 g

Measuring Heat TransferSpecific Heat• The effect of heat energy on a substance‘s

temperature is a physical property of that substance.

• Each substance requires a different amount of heat to raise its temperature 1°C.

• The amount of heat 1 g of a substance must absorb to raise its temperature 1°C is called the specific heat of the substance.

• Specific heat can be used to identify a substance.

Measuring Heat TransferCalorie• You know that the official SI unit of heat is the

joule.• Another unit still commonly used to measure heat

is the calorie.• One calorie equals 4.186 J.• One calorie is the amount of heat needed to raise

the temperature of 1 g of water 1°C.• Five calories of heat are needed to raise the

temperature of 1 g of water soc.

Solar Radiation• Heat energy from the sun reaches

the earth by radiation.

• Solar radiation is seen as ‘sunshine’ and felt as ‘heat’.

Heat in the Atmosphere• Radiant energy from the sun warms the earth's

surface.• The earth's surface transfers heat to the air by

conduction.• Convection currents form as air is heated by the

earth's surface.

Heat in the Atmosphere• Warm air expands and rises. It displaces the

cool, dense air at higher elevations. The dense air sinks.

• The warm rising air carries moisture to higher elevations. As the air cools, the moisture condenses and falls to the earth as rain or snow.

• Convection currents in the atmosphere are called convection cells. Convection cells are responsible for the wind and rain patterns over the earth's surface.

Check and Explain pg. 2201.

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9.3 Heat & Matter

Objectives• Describe the role of heat in phase changes.• Identify three examples of thermal

expansion.• Generalize about what occurs at the boiling

point or melting point of a substance.• Interpret data to identify an unknown

substance by its boiling point.

Heat and Phase Changes• Recall that matter can change phases from

solid to liquid to gas.• When heat is applied to ice, the water molecules

in the ice vibrate faster. The ice melts as it changes from a solid to a liquid. The temperature doesn't change until all the ice melts. When all the ice is melted, additional heat causes the temperature of liquid water to rise again until it reaches 100°C. At 100°C, a phase change occurs and the water evaporates, or changes to its gaseous phase.

Heat and Phase Changes• Phase changes also occur when heat leaves

matter.

Boiling Point• The boiling point is the temperature at which a

substance changes from a liquid phase to a gas phase.

Melting & Freezing Point• The melting point is the

temperature at which a solid becomes a liquid.

• The melting point of water is 0°c and the freezing of water is also 0°c

• Melting point is an important physical property of a substance.

• Scientists use melting-point tables to help identify unknown substances.

Thermal Expansion• An increase in the volume of a substance due to

heat is called thermal expansion.• Thermal expansion occurs in solids, liquids, and

gases.

Check and Explain pg. 2261.

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