Thermal Energy:
Moore C10.1 -
C10.5, Silberberg 6.1
- Case
of the disappearing energy
- Friction
and heat
- Friction
depends on variables ie speed, but potential E depends only on position
and seperation
à no potential energy function
for friction
à friction is a macroscopic
phenomenon
- Thermal
energy
- Temperature
is measure of average kinetic energy of molecules
à Kaverage(per
molecule) = 3/2 kBT
- Absolute
zero = zero kinetic energy
- For
a substance, above relationship for thermal energy complicated by
rotation/vibration and intermolecular interactions
- Friction
and thermal energy
- Friction
and collisions transform kinetic energy into thermal energy in
macroscopic view
- Heat
and work
- Describe
energy transfer across a boundary
- Heat
is a result of temperature difference
- Work
is any other kind of energy flowing across boundary
- Atomic
level: heat is transfer of energy that makes use of chaotic molecular
motion; work is transfer of energy that makes use of organized motion
(Moore’s term k-work is more apt here)
- Boundary
is rather arbitrarily defined as difference between system and
surroundings
- System:
part of the world we’re interested in
- Surroundings:
where we make out measurements
- At
atomic level, it’s all still kinetic and potential energy!!
- Atomic
level: molecular
vibrations/rotations/translations (thermal motion) produce heat
- Microwaves,
IR radiation stimulate atomic motion
- Difference
between thermal energy and heat
- Heat
is energy in transit across boundary à
can absorb heat but not contain heat
- Thermal
energy is a form of internal energy that changes as the system’s
temperature changes
- Internal
energy
- Defined
in thermodynamic terms as total energy of the system; total kinetic and
potential energy of the molecules composing the system
DU = q + w
- In
energy terms, the change in energy (not absolute) is what’s important
DU = Ufinal - Uinital
- Change
in energy of the system is accompanied by and opposite change in energy
of surroundings
- Lose
energy to surroundings: Efinal < Einitial DE
< 0
- Gain
energy from surroundings Efinal > Einitial DE
> 0
- Change
is a transfer of energy as heat and/or work
- Heat
flows out of system à q is negative; heat
flows in à
q is positive
- Work
done by system à energy lost by system
as work à
w is negative
- Work
done on system à energy gained by system
à
w is positive
- W, q
> 0 if energy transferred to system
- W, q
< 0 if energy is lost from system
- See
fig S6.3 pg 223
- First
law of thermodynamics: total
energy of the universe is constant
DEuniverse = DEsystem
+ DEsurroundings
= 0
à
if system is isolated from surroundings, no change in internal energy takes
place!
·
Problems: S6.1
pg 226
·
6.8. A system
conducts 255 cal of heat to the surroundings while delivering 428 cal of
work. What is the change in internal
energy?
·
DE
= q + w = -255
cal + (-428
cal) = -683
cal