Holt Physics, Chapter 10 Section 2
DEFINING HEAT
Heat and Energy
 Heat – energy transferred between objects
because of a difference in their temperatures.
 It is helpful to shift attention to the behavior of atoms
and molecules.
 Mechanics can be used to explain much of what is
happening at the molecular, or microscopic, level.
 This in turn accounts for what you observe at the
macroscopic level.
 Energy is transferred between substances as
heat.
 Macroscopic viewpoint: always from higher to
lower temperature.
 The direction of heat travel can be explained at
the atomic level.
 Higher temperature juice has higher avg. KE than
the water.
 Collision of particles transfers energy from high to
low.
 Net transfer continues until avg. KE is the same
between the juice and the water.
 With same avg. KE, juice and water at thermal
equilibrium (i.e., at the same temperature).
 The transfer of energy
as heat alters and
object’s temperature.
 When two object are in
thermal contact, there is
no net gain or loss of
energy, even though
energy is transferred.
 Difference between
heat and temperature.
 The amount of heat
transferred depends on
the temperature
difference between the
two objects.
 Greater the difference,
greater the amount of
energy transferred as
heat.
 Concepts of heat and
temperature can help
explain hot and cold
sensation.
 Because heat, like work, is energy in transit, all heat
units can be converted to joules (J)
 Heat is symbolized with the letter Q.
Heat and Work
 Hammer nail into wood,
then pry out.
 Nail feels hot.
 Work done on nail was
resisted by friction.
 Friction increases internal
energy of nail  temp.
increases.
 Temperature difference
between hand and nail
results in transfer of heat.
 Deforming solids can
increase internal energy.
 Total energy is conserved
 Not all work done to overcome fiction is converted
to mechanical energy.
 If changes in internal energy are taken into
account, along with changes in mechanical
energy, energy in a closed system is a conserved
property