Thermal Energy & Heat

Chapter 6Sections 1 & 2

Temperature & Thermal Energy

Section 1

What is Temperature?

When we think of temperature we think of how hot or cold something is.

Temperature – the measure of the average value of kinetic energy of molecules in random motion.

Think of a glass of water…Its made of molecules that are in

random motion.Random = different speeds – some

fast, some slow

If they are moving, the molecules have KE.

Temperature measures the average value of their movement.

The more kinetic energy the molecules have, the higher the temperature.High temp = faster moleculesLow temp = slower molecules

Ex. Water molecules move faster in a cup of hot water than they do in cold water.

Thermal Expansion

When molecules speed up, they move farther apart.The object expands (stretches)

When molecules slow down, they move close together.The object contracts (shrinks)

Solids: In the summertime power lines will sag, whereas

in the winter they will be pulled more tightly.

Liquids:You probably been told never to put a glass

bottle of soda in the freezer. Why? It can explode and you will have glass all over you freezer.

Gases:Place a balloon in a very warm room and it will

expand, then place it in a very cold room and it will shrink.

The amount of expansion or contraction depends on:The type of material

Liquids expand more than solids

The degree to which temperature changesThe greater the temperature

change, the more expansion or contraction

Measuring Temperature

Thermometer – a device used to measure temperatureConsists of a liquid such as

mercury or alcohol sealed inside a narrow tube with markings symbolizing degrees.

Temperature changes cause the liquid to rise and fall as the liquid expands and contracts.

Temperature Scales

There are three common scales:FahrenheitCelsiusKelvin

Each is divided into regular intervals

Fahrenheit Scale

The most common in the U.S.Divided into degrees

Fahrenheit (oF)Freezing point of water = 32oFBoiling point of water = 212oFSpace between boiling and

freezing is 180 equal degrees.

Celsius ScaleMost commonly used in the rest

of the worldDivided into degrees Celsius (oC)Freezing point of water = 0oCBoiling point of water = 100oCSpace between boiling and

freezing is 100 equal degreesoC are bigger than oF

Kelvin ScaleThe SI Unit for temperatureDivided into Kelvins (not degrees!)Boiling point of water = 373 KFreezing point of water = 273KAbsolute zero = 0 K (the lowest

possible temperature an object can have!)

Kelvin and Celsius degrees are the same size!

Temperature Conversions

Fahrenheit CelsiusoC = (5/9)(oF – 32)

Celsius FahrenheitoF = (9/5)(oC) + 32

Please Excuse My Dear Aunt Sally!

Temperature Conversions

Kelvin can only be calculated if you know the object’s temperature in oC.K = oC + 273oC = K - 273

Thermal Energy

Is the sum of the KE and PE of all molecules in an object.

All molecules have PE that can be converted into KEThey exert attractive forces to

each otherPE increases as molecules are

farther apart.

Temperature Vs. Thermal EnergyHow hot or cold something

feels does not necessarily give you enough information to determine thermal energy.

Thermal energy is related to quantity of the molecules!

Examples

A glacier and an ice cube have the same temperature, but the glacier has much more thermal energy due to its size.

Compare a glacier to a boiling pot of water. Which has a higher temperature? Which has more thermal energy?

Heat

Section 2

HeatIs thermal energy that is transferred

from one object to another when the objects are at different temperatures

The amount of heat that is transferred when two objects are brought into contact depends on the difference in temperature between the two objects.

Heat continues to be transferred until equilibrium is reached (both objects are at the same temperature).

Transfer of Heat

Thermal energy always moves from warmer to cooler objects.

Heat never flows from a cooler object to a warmer object.

The warmer object loses thermal energy and becomes cooler while the cooler object gains thermal energy and becomes warmer until their temperatures are equal.

There are three ways heat can be transferred:ConductionRadiationConvection

Conduction

The transfer of thermal energy by direct contactEx: The bottom of a pot on a stove is

heated by direct contact between it and the heat source

Occurs when particles in a material collide with neighboring particles. Energy is passed from molecule to molecule during these collisions.

Occurs most easily in solids and liquids where atoms and molecules are closer together

As a result, heat is transferred more rapidly by conduction in solids and liquids than in gases.

Radiation

The transfer of energy in the form of electromagnetic wavesEx: You feel the warmth of the Sun

when standing on the beach.These waves can carry energy

through empty space as well as through solids, liquids, and gases.

ConvectionThe transfer of thermal

energy by the movement of molecules from one part of a material to anotherEx: water boiling in a pot on

the stoveOccurs most easily in liquids

or gases where molecules can move much more easily than in a solid

Forms convection currents

Convection Current

A continuous cycle of heating and rising, cooling and sinkingA substance is heated which

makes it less dense and causes it to rise. When it rises it cools, which causes it to become more dense and sink. The process continually repeats itself.

Natural ConvectionNatural convection occurs when a

warmer, less dense fluid is pushed away by a cooler denser fluid.

Ex: Wind movement near a lake or ocean is caused when air over land is heated and becomes less dense. Denser cool air rushes in , pushing the warm air up. The cooler air is then heated by the land and the cycle is repeated. You feel the cooler air rushing in as wind.

Forced Convection

Occurs when an outside force, such as a fan, pushes a fluid, such as air or water, to make it move and transfer heat

Ex: Computer use fans to keep electronic components from getting too hot.

Thermal Conductors

Any material that easily transfers heat

If a material has some electrons that are not held tightly by the nucleus, they are free to move around and bump into each other, transferring thermal energy.

The best thermal conductors are metals, such as gold and copper.

Thermal Insulators

Any material in which heat does not flow easily

Liquids and gases are usually better insulators than solids because their particles are farther apart and cannot transfer thermal energy by conduction as easily as a solid.

Air is a good insulator. Many insulating materials contain air spaces.

Good conductors are poor insulators and vice-a-versa.

Specific Heat

The amount of heat needed to raise the temperature of 1 kg of substance by 1 °C

More heat is needed to change the temperature of a material with a high specific heat than one with a low specific heat.

Ex: Sand heats up and cools down faster than water.Hotter than the water during the

dayCooler than the water at night

Thermal Pollution

The increase in the temperature of a body of water caused by adding warmer water

Sources:Hot water produced by power plants

that is released into a body of waterRain falling on hot asphalt that runs

off into waterways

Effects: Contains less oxygenCan cause some organisms to

become more sensitive to chemical pollutants, parasites, & disease

Reducing thermal pollution:Use cooling towers to cool warm

water before it is released into waterways