General Physics 1

• C10 – Thermal Energy• C11 – Energy in Bonds

General Physics 2

Disappearing Energy

• Contact friction interaction doesn’tchange potential energy because theseparation between the two objectsdoesn’t change

• Kinetic energy of object decreases, butwhere does the energy go?

General Physics 3

• All objects are constructed of a hugenumber of tiny molecules

• These molecules are in ceaselessrandom motion

• An object’s temperature is a measure ofthe intensity of motion

Thermal Energy

TkK Bavg2

3=

General Physics 4

Friction and Thermal Energy

• When a surface moves past another other, theirsurface molecules often become entangled.

• The entangled molecules are stretched away fromtheir normal positions

• A small amount of kinetic energy is transformed topotential energy in the bonds between the molecules.

• When the molecules snap back, they oscilate• The energy is transmitted to the rest of the

molecules in the objects

General Physics 5

• Heat and work are defined to describe energytransfer across a system boundary

• Heat is any energy that crosses the boundarybetween two objects because of the temperaturedifference between them

• Work is any other kind of energy flowing across anobject’s boundary

• The change in internal energy of an object is theheat added to and the work flowing into the system

Heat and Work

WQU +=!

General Physics 6

Specific Heat

McdTdUth=

• A small change in an object’s thermal energyis proportional to the mass, the specific heat,and the temperature change

General Physics 7

General Physics 8

Example C10.3

• Imagine that we place a 50-gblock of aluminum with aninitial temperature of 100°Cinto an insulating coveredStyrofoam cup containing250 g of water at 22°C.Heat will flow from thealuminum to the water untilboth are at the sametemperature. What is thattemperature?

General Physics 9

Potential Energy Diagrams

• Scan p 194

General Physics 10

Bonds

• Typical potential energycurve for a bond.

• When bonds form,energy is released

• When bonds break,energy is absorbed

General Physics 11

Latent Heat

mLUmLU

mLUmLU

lath

lath

+=!"=!

"=!+=!

L is the latent heat, m is the mass

For vaporization or melting

For condensation or freezing

General Physics 12

Group Problems

• C10B.6• C10S.2• C11B.5• C11S.5