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Section 1: Temperature, Thermal Energy, and Heat 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
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Chapter 5Thermal 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 thefreezing 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:
Example: convert 10 oF to oCSolution:
oo F - 32C =
1.8
oo
o
o
o o
F - 32C = 1.8
10 - 32C = 1.8
-22C = 1.8
C = -12.2
Section 1: Temperature, Thermal Energy, and Heat
• Converting Celsius to Fahrenheit:Equation:Example: convert 20 oC to oF Solution:
• Converting Celsius to Kelvin:
• Converting Kelvin to Celsius:
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.
o oF= 1.8 C + 32
o
o
o o
o o
F = (1.8 x20) 32F = 36 + 32F = 68
F= 1.8 C + 32
oK= C+273
oC = K - 273
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:
• 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 negativeExample: 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
Where:Q = change in thermal energy (J)m = mass of the material (kg)c = specific heat of the material (J/kgoC)T = change in temperature
o
oi
of
m = 0.05kgJc = 235
kg CT 20 CT = 60 CQ = ?
f i
o o
o
T = T - TT = 60 C - 20 CT = 40 C
Q = mc T
Q = 0.05 kg
J(235
kg oCo)(40 C )
Q = 470J
𝐐=𝐦𝐜∆𝐓
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.