Thermal Energy & Thermodynamics
Heat Heat – The transfer of thermal
energy from one object to another because of a temperature difference.
Flows spontaneously from hot objects to cold objects
Heat Heat – The transfer of thermal
energy from one object to another because of a temperature difference.
Flows spontaneously from hot objects to cold objects
Temperature Measure of how hot or cold an
object is compared to a reference point.
Celsius Scale: Boiling and freezing points of water
Kelvin Scale: Absolute zero
Zeroth Law of Thermodynamics
If objects A and B are separately in thermal equilibrium with a third object, C, then A and B are in thermal equilibrium with each other.
Allows a definition of temperature
Temperature from the Zeroth Law Two objects in thermal equilibrium
with each other are at the same temperature
Temperature is the property that determines whether or not an object is in thermal equilibrium with other objects
Thermometers Used to measure the temperature of an
object or a system Make use of physical properties that
change with temperature Many physical properties can be used
volume of a liquid length of a solid pressure of a gas held at constant volume volume of a gas held at constant pressure electric resistance of a conductor color of a very hot object
Celsius Scale Temperature of an ice-water mixture is
defined as 0º C This is the freezing point of water
Temperature of a water-steam mixture is defined as 100º C This is the boiling point of water
Distance between these points is divided into 100 segments or degrees
Kelvin Scale When the pressure of a gas goes to
zero, its temperature is –273º C This temperature is called absolute zero This is the zero point of the Kelvin scale
–273º C = 0 K To convert: TC = TK – 273
The size of the degree in the Kelvin scale is the same as the size of a Celsius degree
Pressure-Temperature Graph
All gases extrapolate to the same temperature at zero pressure
This temperature is absolute zero
Some KelvinTemperatures Some
representative Kelvin temperatures
Absolute zero has never been reached
Comparing Temperature Scales
Temperature Determined by the kinetic energy
of the particles in an object.
More thermal energy = faster motion of particles
Faster moving particles lose energy to slow moving particles in collisions
Thermal Energy Depends on:
Mass Temperature Phase
Thermal Energy vs. Temperature Which has more thermal energy, a
beaker of hot water or the water in my water bottle?
Thermal energy depends on mass
Kinetic Theory of Gases The number of molecules in the
gas is large and the average separation between them is large compared to their dimensions
The molecules obey Newton’s laws of motion, but as a whole they move randomly
Kinetic Theory of Gases – cont. The molecules interact only by
short-range forces during elastic collisions
The molecules make elastic collisions with the walls
The gas under consideration is a pure substance, all the molecules are identical
Specific Heat The amount of energy needed to raise
the temperature of one gram of a material by one degree Celsius.
Water – 4.18 J/g·oC Plastic (polypropylene) – 1.84-2.09
J/g·oC Iron – 0.449 J/g·oC
Specific Heat
In setting up an aquarium, the heater transfers 1,200kJ of heat to 75,000g of water. What is the increase in the water’s temperature?
A Consequence of Different Specific Heats Water has a high
specific heat compared to land
On a hot day, the air above the land warms faster
The warmer air flows upward and cooler air moves toward the beach
Heat Compared to Internal Energy Important to distinguish between
them They are not interchangeable
They mean very different things when used in physics
Internal Energy Internal Energy, U, is the energy
associated with the microscopic components of the system Includes kinetic and potential energy
associated with the motion and position of the atoms or molecules
Heat Heat is the transfer of energy
between a system and its environment because of a temperature difference between them The symbol Q is used to represent the
amount of energy transferred by heat between a system and its environment
Units of Heat Calorie
An historical unit, before the connection between thermodynamics and mechanics was recognized
A calorie is the amount of energy necessary to raise the temperature of 1 g of water from 14.5° C to 15.5° C .
A Calorie (food calorie) is 1000 cal
Units of Heat, cont. US Customary Unit – BTU BTU stands for British Thermal Unit
A BTU is the amount of energy necessary to raise the temperature of 1 lb of water from 63° F to 64° F
1 cal = 4.186 J This is called the Mechanical
Equivalent of Heat
Calorimeter One technique for determining the
specific heat of a substance A calorimeter is a vessel that is a
good insulator which allows a thermal equilibrium to be achieved between substances without any energy loss to the environment
Calorimetry Analysis performed using a calorimeter Conservation of energy applies to the
isolated system The energy that leaves the warmer
substance equals the energy that enters the water Qcold = -Qhot Negative sign keeps consistency in the sign
convention of ΔT
Phase Changes A phase change occurs when the physical
characteristics of the substance change from one form to another
Common phases changes are Solid to liquid – melting Liquid to gas – boiling
Phases changes involve a change in the internal energy, but no change in temperature
Latent Heat During a phase change, the amount of
heat is given as Q = ±m L
L is the latent heat of the substance Latent means hidden L depends on the substance and the nature
of the phase change Choose a positive sign if you are adding
energy to the system and a negative sign if energy is being removed from the system
Latent Heat, cont. SI units of latent heat are J / kg Latent heat of fusion, Lf, is used for
melting or freezing Latent heat of vaporization, Lv, is
used for boiling or condensing
Sublimation Some substances will go directly
from solid to gaseous phase Without passing through the liquid
phase This process is called sublimation
There will be a latent heat of sublimation associated with this phase change
Graph of Ice to Steam
Methods of Heat Transfer Need to know the mechanisms
responsible for the transfer Methods include
Conduction Convection Radiation
Conduction The transfer of thermal energy
with no transfer of matter.
Between particles in the same material
Between materials that are in contact with each other.
Conduction The transfer can be viewed on an
atomic scale It is an exchange of energy between
microscopic particles by collisions Less energetic particles gain energy during
collisions with more energetic particles Rate of conduction depends upon the
characteristics of the substance
Conduction example The molecules vibrate
about their equilibrium positions
Particles near the stove coil vibrate with larger amplitudes
These collide with adjacent molecules and transfer some energy
Eventually, the energy travels entirely through the pan and its handle
Conduction, cont. In general, metals are good conductors
They contain large numbers of electrons that are relatively free to move through the metal
They can transport energy from one region to another
Conduction can occur only if there is a difference in temperature between two parts of the conducting medium
Thermal Conductors and Insulators Thermal Conductor – A material that
easily conducts thermal energy Metal
Thermal Insulator – A material that does not conduct thermal energy well. Wood Air Rubber
Convection The transfer of thermal energy
when particles of a fluid move from one place to another
Convection Current – Fluid that circulates in a loop as it heats up and cools down.
Convection example Air directly above
the flame is warmed and expands
The density of the air decreases, and it rises
The mass of air warms the hand as it moves by
Convection Current Example The radiator warms
the air in the lower region of the room
The warm air is less dense, so it rises to the ceiling
The denser, cooler air sinks
A continuous air current pattern is set up as shown
Global Ocean Currents
Radiation The transfer of energy by waves
moving through space.
All objects radiate energy
The higher an object’s temperature, the more energy it radiates.
Radiation Radiation does not require physical
contact
All objects radiate energy continuously in the form of electromagnetic waves due to vibrations of the molecules
Radiation example
The electromagnetic waves carry the energy from the fire to the hands
No physical contact is necessary Cannot be accounted for by conduction
or convection
Applications of Radiation Clothing
Black fabric acts as a good absorber White fabric is a better reflector
Thermography The amount of energy radiated by an object
can be measured with a thermograph Body temperature
Radiation thermometer measures the intensity of the infrared radiation from the eardrum
1st Law of Thermodynamics Energy is conserved.
Law of Conservation of Energy
First Law, Equation If a system undergoes a change
from an initial state to a final state, then U = Uf – Ui = Q + W Q is the energy transferred to the
system by heat W is the work done on the system U is the change in internal energy
First Law – Signs Signs of the terms in the equation
Q Positive if energy is transferred to the system by
heat Negative if energy is transferred out of the system
by heat W
Positive if work is done on the system Negative if work is done by the system
U Positive if the temperature increases Negative if the temperature decreases
2nd Law of Thermodynamics Thermal energy will flow from a
warm object to a cold object.
To get energy to flow from cold to warm, additional energy must be inputted into the system.
3rd Law of Thermodynamics Absolute zero cannot be reached.
-273.15oC All molecular motion stops
End of the universe!
Thermal Expansion The thermal expansion of an object is a
consequence of the change in the average separation between its constituent atoms or molecules
At ordinary temperatures, molecules vibrate with a small amplitude
As temperature increases, the amplitude increases This causes the overall object as a whole to
expand
Linear Expansion For small changes in temperature
, the coefficient of linear expansion, depends on the material
o o oL L T or L L T T
Applications of Thermal Expansion – Bimetallic Strip
Thermostats Use a bimetallic strip Two metals expand differently
Since they have different coefficients of expansion
Area Expansion Two dimensions
expand according to
is the coefficient of area expansion
,2
oA A t
Volume Expansion Three dimensions expand
For liquids, the coefficient of volume expansion is given in the table
3,solidsfortVV o
Unusual Behavior of Water
As the temperature of water increases from 0ºC to 4 ºC, it contracts and its density increases
Above 4 ºC, water exhibits the expected expansion with increasing temperature
Maximum density of water is 1000 kg/m3 at 4 ºC