Heat Transfer

Objectives:

1.    Describe how heat energy causes molecules to move.

2.     List examples of heat energy transfer by conduction, convection, and radiation.

Notes:

Heat:

Heat energy is created due to the internal motion of molecules in a substance.     Heat can more correctly be explained as the amount of kinetic energy in a substance.  Therefore, the amount of heat in a substance depends on the mass of the object.  Heat transfer is defined as the movement of heat from a warmer substance to a cooler one.  This is technically known as moving down a temperature gradient.  There are three types of heat transfer.

(blueox.uoregon.edu/~courses/dlivelyb/ ph161/globwarm_an.gif)

Conduction

 Transferred by direct contact of molecules
 Molecules in hot substances move fast Collide with cooler, slower molecules and transfer energy

(www.gcse.com/energy/images/ conduction.gif)

·        Conductors – transfer heat well (metals: silver, copper, aluminum, iron)

·        Insulators – bad conductors of heat (nonmetals:  glass, wood, rubber, & plastics)

Convection

 Liquids & gases The molecules in heated gas or liquid move farther apart and become less dense. The less dense gas of liquid rises Created a circular current

(www.bluffton.edu/~bergerd/NSC_111/ images/waterconvect.gif)

 Heat transferred through empty space via infrared radiation

(www.min.uc.edu/nuclear/ ibt/ht2.jpg)

Measuring Heat

 Can measure temperature not heat Unit is the calorie (cal) or joule (J)
 1cal = 4.19J a cal is the amount of heat needed to raise 1g of water 1 degree

The specific heat   of a substance is the amount of heat (cal) needed to raise a substance 1oC .

 Can be used to calculate the amount of heat gained or lost by a substance measured using a bomb calorimeter Is the ability of a substance to absorb heat
 High specific heat – slow to heat up and slow to cool down (& visa versa)
Can be measured using a calorimeter
 Mass · DT · Specific heat

Heat PE can be stored in the bonds of fuels and food