Transcript Chapter 5 Thermal Energy

Chapter 5
Thermal Energy
• Section 1: Temperature, Thermal Energy,
and Heat
• Section 2: Conduction, Convection, and
Radiation
• Section 3: Using Thermal Energy
Section 1: Temperature, Thermal Energy,
and Heat
Temperature
• All matter is made of tiny particles—atoms or molecules
• These particles are in constant, random motion
• Because they are in motion, these particles have kinetic
energy
• Temperature - the measure of the average kinetic energy of
all the particles in that something
Temperature scales
• When we talk about the air temperature or the temperature
of something, we typically use a scale based on the physical
properties of water
The Fahrenheit and Celsius scales
• Fahrenheit scale - 180o between
the freezing and boiling points
• Celsius scale - 100o between the
freezing and boiling points
• A 1o change on the Celsius scale = a
1.8o change on the Fahrenheit scale
Section 1: Temperature, Thermal Energy,
and Heat
The Kelvin Scale
• In science the Kelvin temperature scale is used
• Based on the concept of absolute zero
• Absolute zero – temperature at which the random motion of
the particles in something ceases
• 0 K = absolute zero = 273oC
Note: that the degree symbol is not used when expressing
temperature on the Kelvin scale
Converting Between Temperature Scales
• Converting Fahrenheit to Celsius:
Equation:
o
o
C=
F - 32
1.8
Example: convert 10 oF to oC
o
Solution: o
F - 32
C=
1.8
10 - 32
o
C=
1.8
-22
o
C=
1.8
o
C = -12.2o
Section 1: Temperature, Thermal Energy,
and Heat
• Converting Celsius to Fahrenheit:
Equation: o F= 1.8 o C + 32
Example: convert 20 oC to oF
Solution: o F= 1.8 o C + 32
F = (1.8 x20)  32
o
F = 36 + 32
o
F = 68o
o
• Converting Celsius to Kelvin: K=oC+273
• Converting Kelvin to Celsius: oC = K - 273
Note: There is no direct conversion from Fahrenheit to Kelvin,
you must first convert the temperature in Fahrenheit to
Celesius and then convert Celsius to Kelvin.
Section 1: Temperature, Thermal Energy,
and Heat
Thermal energy – the sum of the kinetic and potential energies
of all the particles in something
• Energy is transferred by collisions between particles
• Particles within something exert an attractive force on each
other; this is the source of potential energy
• Thermal energy and temperature are related:
 When the temperature of an object increases, the average
KE of the particles increases
 So, as the temperature increases the thermal energy
increases
• Thermal energy and mass:
 As long as the temperature does not change, when the mass
of an object increases its thermal energy increases
Heat – thermal energy that flows from something at a high
temperature to something at a lower temperature
• Heat always flows from hot to cold (2nd Law of
Thermodynamics)
• Specific heat – the amount of heat required to raise the
temperature of 1-kg of a substance 1oC or 1K
 Different materials have different specific heats
Section 1: Temperature, Thermal Energy,
and Heat
• The amount of thermal energy changes when heat flows into
or out of an object
 The heat flow, or change in thermal energy can be
calculated:
The equation:
Where:
𝐐 = 𝐦𝐜∆𝐓
Q = change in thermal energy (J)
m = mass of the material (kg)
c = specific heat of the material (J/kgoC)
T = change in temperature
• Q can be positive or negative
 If heat flows into an object its temperature increases, so Q is
positive
 If heat flows out of an object its temperature decreases, so
Q is negative
Example: a 0.05-kg silver spoon is heated so that its
temperature increases from 20oC to 60oC. What is the change
in the thermal energy of the spoon?
Solution
m = 0.05kg
J
c = 235 o
kg C
Ti  20oC
Tf = 60oC
Q=?
T = Tf - Ti
T = 60oC - 20oC
T = 40 C
o
Q = mcT
Q = 0.05 kg (235
Q = 470J
J
o
kg C
)(40 oC )
Section 2: Conduction, Convection, and
Radiation
The 2nd Law of Thermodynamics states that energy (heat)can
only move in one direction: from an object or substance at a
higher temperature to an object or substances at a lower
temperature. This movement of energy is called heat
transfer.
There are three methods of heat transfer:
1. Conduction – the transfer of energy through matter by
the direct contact of particles
• Example: holding an ice cube in your hand. Heat flows
from your hand into the ice. Result: your hand gets
cooler and the ice starts to melt.
• On the molecular level, particles within one substance
will collide with each other and so transfer energy. In
the case of two different substance, particles from each
substance collide with each other and transfer energy
• Conduction can occur in solids, liquids, and gases, but
solids are generally better conductors than liquids or
gases, and metals are better conductors than
nonmetals (wood, plastic, glass)
Section 2: Conduction, Convection, and
Radiation
2. Convection – the transfer of energy by the motion of the
heated particles in a fluid
• Remember, a fluid is any substance that flows, so a fluid
can be either a liquid or a gas
• In convection, the more energetic fluid particles move
from one location to another, and carry energy with them
• As the particles in the fluid move faster, they get farther
apart, and the density of the “hot” fluid is less than the
density of the surrounding fluid. So, the mass of the “hot”
fluid is constant but its volume increases
• Because the “hot” fluid is less dense, it rises. As it rises it
starts to lose heat. As the fluid continues to lose heat it
contracts, the volume decreases, and the density
increases. Eventually, the fluid loses enough heat that its
density is great enough to cause it to sink. This rising and
sinking action is a convection current
• Examples: the flow of magma in plate tectonics, ocean
currents, lava lamps
Section 2: Conduction, Convection, and
Radiation
3. Radiation – the transfer of energy by electromagnetic waves
• Example: energy from the Sun
• When radiation strikes a material, some of the energy is
absorbed, some is reflected, and some is transferred
through the material
• Unlike conduction and convection which require a medium
in order to transfer energy, radiation requires no medium
Section 3: Using Thermal Energy
Heating systems create and control thermal energy and move it
from one place to another
• Types of heating systems include forced-air, radiator, electric,
and solar heating
 Solar collector – a device that transforms radiant energy
from the Sun into thermal energy
• Thermodynamics – the study of the relationships between
thermal energy, heat, and work
• 1st Law of Thermodynamics – if the mechanical energy of a
system is constant, the increase in thermal energy of that
system equals the sum of the thermal energy transfers into
that system and the work done on that system
 The 1st Law says that in a system total energy is constant
but can change form when work is done on the system
• 2nd Law of Thermodynamics – energy spontaneously
spreads from regions of higher concentration to regions of
lower concentration
 Or: energy flows from objects or regions at a higher
temperature to objects or regions at a lower temperature
• Heat engine – a device that converts some thermal energy
into mechanical energy
• Internal combustion engine – a heat engine that burns fuel
inside a set of cylinders
• Refrigerators and air conditioners do work to transfer
thermal energy. They reduce the temperature of a system by
moving energy from one place to another.