Chapter 6

Heat is transferred from one place to another by threedifferent processes

How Heat is Transferred

I. ConductionPConduction is the transfer of energy through

matter from particle to particle. It is thetransfer and distribution of heat energy fromatom to atom within a substance. Forexample, a spoon in a cup of hot soupbecomes warmer because the heat from thesoup is conducted along the spoon.Conduction is most effective in solids-but itcan happen in fluids.

6.1

Conduction

How Heat is Transferred

II ConvectionPConvection is the transfer of heat by theactual movement of the warmed matter. Heatleaves the coffee cup as the currents of steamand air rise. Convection is the transfer of heatenergy in a gas or liquid by movement ofcurrents. (It can also happen in some solids,like sand.)

PConvection Currents are created whenheated gas (ex. air) or liquid (ex.water) risesand cooler gas or liquid drops back down.

Convection

Hot water rises, cools,and falls.

Heated air rises, cools,then falls. Air near

heater is replaced bycooler air, and the cyclerepeats. This creates aconvection current.

What if coils were atthe bottom?

How Heat is Transferred

III. RadiationPRadiation - Electromagnetic waves that directlytransport ENERGY through space. Sunlight is a formof radiation that is radiated through space to our planetwithout the aid of gases,liquids or solids. The energytravels through nothingness!

PRadiation is also transmitted through air. This is thereason you can feel the heat of a campfire from theside.

How Heat is Transferred

Waves

PYou cannot see electromagnetic wavesbecause they are not made of matter like thewaves in the ocean.

PElectromagnetic waves can carry energythrough empty space or air.

PThe heat you feel from the Sun, a fire or thehot burner on your stove are transmitted byelectromagnetic waves.

How Heat is Transferred

Types of Electromagnetic Waves - radiationP Radio waves - carry radio and television signals

P Microwaves - carry information to and from satellites

P Infrared waves - mostly responsible for transmitting heat

P Visible light - electromagnetic waves that can be detectedby our eyes

P Ultraviolet waves - invisible to our eyes, these are the wavesfrom the Sun that cause sunburn

P X-rays - used to make images of bones

P Gamma Rays - radioactive waves. Can do serious damageto living tissue.

How Heat is TransferredAbsorbing Radiant Energy

P Different substances react differently to radiant energy

P If the substance absorbs radiant energy, it gets warmer

P If the substance reflects radiant energy, it does not get aswarm

P The darker the color, the more radiant energy that isabsorbed ex. A dark shirt will be warmer on a summerday than a light shirt.

P If radiant energy passes through a substance, very littleenergy is absorbed. Ex. glass

How Heat is Transferred

Absorbing Radiant Energy

PAll forms of electromagnetic radiation works thisway.

PThat is, when a surface absorbs any sort ofelectromagnetic radiation, it gets warmer

Conductors and Insulators

PConductor - materials which permit heatenergy to flow freely from atom to atom andmolecule to molecule. An object made of aconducting material will permit heat energy tobe transferred across the entire surface of theobject.

PThermalC onductivity - the rate at which asubstance conducts heat.

Examples of conductors - metals ,graphite,water and the human body

6.2

Conductors and Insulators

PInsulator - materials which do not allow thefree flow of heat energy from atom to atomand molecule to molecule. The particles ofthe insulator do not permit the free flow ofheat energy; subsequently heat is seldomdistributed evenly across the surface of aninsulator.

Example of insulators - a vacuum,wood,plastic, styrofoam, air

Conductors and Insulators

A Vacuum

A vacuum is a situation where there are no gas particlespresent. That is, all of the air is removed.

Since there are no particles present, neither conductionnor convection can occur since both these methods of heattransfer, require particles to take place.

A Thermos has a vacuum between the outside plastic andthe inside glass section. The inside glass section is alsosilvered to act as a mirror to reflect back any heatradiation.

Diagram of a Thermos

Conductors and Insulators

R-Value

PAll insulators are rated on their ability to resist heattransfer. This is called the R-value.

PThe higher the R-value, the better the insulatingproperties of the substance.

PWhen determining the R-value of the walls in a house,you must add up all the R-values of each layer.

PA single pane of glass does not have a very high R-value, but two panes with air or argon gas between themhas a much higher R-value.

Temperature vs Heat

P Average Kinetic energy is the average amountof heat energy of all particles in a substance.

P Total Kinetic Energy is the sum of the heatenergy of all the particles in a substance.

P Specific Heat Capacity is the amount of heatenergy needed to raise the temperature of1.00g of a substance by 1.00 degrees C.

6.3

Temperature vs Heat

Average Kinetic Energy

PExample of Average Kinetic Energy< Let’s say you have 100 particles in a liquid. Each

particle has 2 Joules of heat energy. To get theaverage you add up all the particles and divide by100 (that is add up 2 one hundred times to get 200Joules and divide by 100 to get an average kineticenergy of 2 Joules per particle for the liquid)

< Temperature is a measure of average kineticenergy.

Temperature vs Heat

Total Kinetic Energy

PExample of Total Kinetic Energy< To get the total kinetic energy of the same liquid,

you just add up the amount of energy in eachparticle. So at 2 Joules each for 100 particles thatwould give you a total kinetic energy of 200Joules.

< This is a measure of the amount of heat in asubstance.

Temperature vs Heat

P If this liquid had a temperature of 40 degreesCelcius and you were to pour half intoanother beaker, the temperature would staythe same, the average kinetic energy (100Joules divided by 50 = 2 Joules) would staythe same, but the total kinetic energy wouldbe half (50 particles at 2 Joules each equals100 Joules total)

Temperature vs Heat

Specific Heat Capacity

P The Specific Heat Capacity of water is higher thanmost substances.

P This is why the ground will heat up more than waterwhen they are exposed to the same amount of sunlight.

P Since heat can only be transferred through the groundby conduction (which is quite slow), the heat isdistributed better through water because 1) sunlightpenetrates deeper and 2) convection distributes the heatfaster in water.