16.2 Heat and Thermodynamics
Newton’s cradle helps to visualize conduction. One
ball strikes the rest, and most of the kinetic energy is
transferred to one ball on the end.
16.2 Heat and Thermodynamics
II. Heat and Thermodynamics
A. Conduction- the transfer of thermal energy with no overall
transfer of matter.
1. Conduction occurs within a material or between materials
that are touching.
2. In conduction, collisions between particles transfer
thermal energy, without any overall transfer of matter.
a. Conduction in gases is slower than in liquids and
solids because the particles in a gas collide less often.
16.2 Heat and Thermodynamics
3. Thermal conductor -a material that conducts thermal
energy well.
a. Metals are good thermal conductors.
1) When a frying pan is on a hot stove, the bottom of
the metal pan heats first and the metal handle last.
The flames do not directly heat the handle.
b. Tile is a better conductor than wood.
1) A tile floor feels colder than a wooden floor when
both floors are at room temperature.
2) The tile transfers thermal energy more rapidly away
from your skin.
16.2 Heat and Thermodynamics
Conduction
The arrows show how thermal energy is
conducted away from the heat source in a
metal frying pan.
16.2 Heat and Thermodynamics
4. Thermal insulator- material that conducts thermal energy
poorly.
a. Air is a very good insulator. Wool garments and
plastic foam cups use trapped air to slow down
conduction.
16.2 Heat and Thermodynamics
B. Convection -the transfer of thermal energy when particles of a
fluid move from one place to another.
1. A convection current occurs when a fluid circulates in a
loop as it alternately heats up and cools down.
a. Air at the bottom of an oven heats up, expands, and
becomes less dense. The hot air rises.
b. Rising hot air cools as it moves away from the heat
source.
c. As a result, the coolest air is at the top of the oven.
2. Convection currents are important in many natural cycles,
such as ocean currents, weather systems, and movements
of hot rock in Earth’s interior.
16.2 Heat and Thermodynamics
Convection
A. Passing sandbags along a line is like
transferring thermal energy by convection.
B. The arrows show convection of air in an oven.
16.2 Heat and Thermodynamics
C. Radiation-the transfer of energy by waves moving through
space.
1. All objects radiate energy. As an object’s temperature
increases, the rate at which it radiates energy increases.
a. When you stand to the side of a charcoal grill, heat
reaches you without convection or conduction.
b. The sun warms you by radiation on a clear day. The
space between the sun and Earth has no air to transfer
thermal energy.
c. Heat lamps used in restaurants are another example of
radiation.
16.2 Heat and Thermodynamics
Radiation
The heating coil on a
stove radiates thermal
energy.
The changing color of the
red arrows indicates that
the farther you are from
the coil, the less radiation
you receive.
16.2 Heat and Thermodynamics
D. Thermodynamics- The study of conversions between thermal
energy and other forms of energy
1. The first law of thermodynamics states that energy is
conserved.
a. Energy cannot be created or destroyed, but it can be
converted into different forms.
b. Added energy increases the thermal energy of a
system or does work on the system. In either case,
energy is conserved.
16.2 Heat and Thermodynamics
Thermodynamics
James Prescott Joule (1818-1889) carefully
measured the energy changes in a system.
Joule's system included a falling weight that
turned a paddle wheel in a container of
water.
Joule found that the work done by the falling
weight almost exactly equaled the thermal
energy gained by the water.
16.2 Heat and Thermodynamics
Thermodynamics
Pushing on the pump does
work on the system.
Some of the work is
converted into thermal
energy, which heats the air in
the pump and the tire.
16.2 Heat and Thermodynamics
2. The second law of thermodynamics states that thermal
energy can flow from colder objects to hotter objects only
if work is done on the system.
a. Thermal energy flows spontaneously only from hotter
to colder objects.
1) A refrigerator must do work to transfer thermal
energy from the cold food compartment to the
warm room air.
2) The thermal energy is released by coils at the
bottom or in the back of the refrigerator.
16.2 Heat and Thermodynamics
b. Heat engine - any device that converts heat into work.
1) The efficiency of a heat engine is always less than
100 percent.
2) Thermal energy that is not converted into work is
called waste heat.
3) Waste heat is lost to the surrounding environment.
16.2 Heat and Thermodynamics
Thermodynamics
Spontaneous changes will always make a
system less orderly, unless work is done on
the system.
For example, if you walk long enough, your
shoelaces will become untied. But the
opposite won't happen; shoelaces don't tie
themselves. Disorder in the universe as a
whole is always increasing.
16.2 Heat and Thermodynamics
Thermodynamics
The efficiency of a heat engine increases with
a greater difference between the high
temperature inside and the cold temperature
outside the engine.
A heat engine could be 100 percent efficient
if the cold outside environment were at
absolute zero (0 Kelvin). This would violate
the third law of thermodynamics.
16.2 Heat and Thermodynamics
3. The third law of thermodynamics states that
absolute zero cannot be reached.
16.2 Heat and Thermodynamics
Thermodynamics
The third law of
thermodynamics states
that absolute zero cannot
be reached.
This physicist uses a laser
to cool rubidium atoms to
3 billionths of a Kelvin
above absolute zero.
16.2 Heat and Thermodynamics
Assessment Questions
1. What form of energy transfer requires the motion
of particles of a fluid?
a.
b.
c.
d.
conduction
convection
radiation
insulation
16.2 Heat and Thermodynamics
Assessment Questions
1. What form of energy transfer requires the motion
of particles of a fluid?
a.
b.
c.
d.
conduction
convection
radiation
insulation
ANS: B
16.2 Heat and Thermodynamics
Assessment Questions
2. What happens in every case in which energy is
added to a system?
a.
b.
c.
d.
Temperature increases.
Work is done on the system.
All of the energy can be accounted for as work or heat.
An identical amount of energy is removed from the
system.
16.2 Heat and Thermodynamics
Assessment Questions
2. What happens in every case in which energy is
added to a system?
a.
b.
c.
d.
Temperature increases.
Work is done on the system.
All of the energy can be accounted for as work or heat.
An identical amount of energy is removed from the
system.
ANS: C
16.2 Heat and Thermodynamics
Assessment Questions
3. Thermal energy can move from a cooler object to
a warmer object when
a.
b.
c.
d.
the warmer object is larger.
the cooler object has more thermal energy.
energy is transferred by radiation.
work is done on the system.
16.2 Heat and Thermodynamics
Assessment Questions
3. Thermal energy can move from a cooler object to
a warmer object when
a.
b.
c.
d.
the warmer object is larger.
the cooler object has more thermal energy.
energy is transferred by radiation.
work is done on the system.
ANS: D
16.2 Heat and Thermodynamics
Assessment Questions
4. According to the third law of thermodynamics, it is
impossible
a. to cool an object to absolute zero.
b. transfer thermal energy from a cooler object to a
warmer object.
c. convert energy from one form to another.
d. account for all of the energy in a system.
16.2 Heat and Thermodynamics
Assessment Questions
4. According to the third law of thermodynamics, it is
impossible
a. to cool an object to absolute zero.
b. transfer thermal energy from a cooler object to a
warmer object.
c. convert energy from one form to another.
d. account for all of the energy in a system.
ANS: A
16.2 Heat and Thermodynamics
Assessment Questions
1. All metals are good thermal insulators.
True
False
16.2 Heat and Thermodynamics
Assessment Questions
1. All metals are good thermal insulators.
True
False
ANS:
F, conductors