Chapter 11
Energy in Thermal Processes
Heat Compared to
Internal Energy
Important to distinguish between them
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 random translational, rotational and vibrational motion of the atoms or molecules
Also includes the intermolecular potential energy
Heat
Heat is a mechanism by which energy is transferred between a system and its environment because of a temperature difference between them
The system 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
1 cal = 4.186 J
This is called the Mechanical Equivalent of Heat
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
Specific Heat
Every substance requires a unique amount of energy per unit mass to change the temperature of that substance by 1° C
The specific heat, c, of a substance is a measure of this amount
Units of Specific Heat
SI units
J / kg °C
Historical units
cal / g °C
Heat and Specific Heat
Q = m c ΔT
ΔT is always the final temperature minus the initial temperature
When the temperature increases, ΔT and ΔQ are considered to be positive and energy flows into the system
When the temperature decreases, ΔT and ΔQ are considered to be negative and energy flows out of the system
Consequences 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
Calorimeter
One technique for determining the specific heat of a substance
A calorimeter is a vessel that is a good insulator that 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 or concealed
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.
Latent heat of fusion is used for melting or freezing
Latent heat of vaporization is used for boiling or condensing
Table 11.2 gives the latent heats for various substances
Graph of Ice to Steam
Warming Ice
Start with one gram of ice at –30.0º C
During A, the temperature of the ice changes from –30.0º C to 0º C
Use Q = m c ΔT
Melting Ice
Once at 0º C, the phase change (melting) starts
The temperature stays the same although energy is still being added
Use Q = m Lf
Warming Water
Between 0º C and 100º C, the material is liquid and no phase changes take place
Energy added increases the temperature
Use Q = m c ΔT
Boiling Water
At 100º C, a phase change occurs (boiling)
Temperature does not change
Use Q = m Lv
Heating Steam
After all the water is converted to steam, the steam will heat up
No phase change occurs
The added energy goes to increasing the temperature
Use Q = m c ΔT
Problem Solving Strategies
Use consistent units
Transfers in energy are given as Q=mcΔT for processes with no phase changes
Use Q = m Lf or Q = m Lv if there is a phase change
In Qcold = - Qhot be careful of sign
ΔT is Tf - Ti
Methods of Heat Transfer
Need to know the rate at which energy is transferred
Need to know the mechanisms responsible for the transfer
Methods include
Conduction
Convection
Radiation
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 flame vibrate with larger amplitudes
These collide with adjacent molecules and transfer some energy
Eventually, the energy travels entirely through the rod
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
Conduction, equation
The slab allows energy to transfer from the region of higher temperature to the region of lower temperature
Conduction, equation explanation
A is the cross-sectional area
L = Δx is the thickness of the slab or the length of a rod
P is in Watts when Q is in Joules and t is in seconds
k is the thermal conductivity of the material
See table 11.3
Good conductors have high k values and good insulators have low k values
Home Insulation
Substances are rated by their R values
R = L / k
More multiple layers, the total R value is the sum of the R values of each layer
Wind increases the energy loss by conduction in a home
Convection
Energy transferred by the movement of a substance
When the movement results from differences in density, it is called natural conduction
When the movement is forced by a fan or a pump, it is called forced convection
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 applications
Radiators
Upwelling
Cooling automobile engines
Algal blooms in ponds and lakes
Radiation
Radiation does not require physical contact
All objects radiate energy continuously in the form of electromagnetic waves due to thermal vibrations of the molecules
Rate of radiation is given by Stefan’s Law
Radiation example
The electromagnetic waves carry the energy from the fire to the hands
No physical contact is necessary
Radiation equation
P = σAeT
4P is the rate of energy transfer, in Watts
σ = 5.6696 x 10-8 W/m2 K4
A is the surface area of the object
e is a constant called the
emissivitye varies from 0 to 1
T is the temperature in Kelvins
Energy Absorption and Emission by Radiation
With its surroundings, the rate at which the object at temperature T with surroundings at To radiates is
Pnet = σAe(T4 – T4o)
When an object is in equilibrium with its surroundings, it radiates and absorbs at the same rate
Its temperature will not change
Ideal Absorbers
An ideal absorber is defined as an object that absorbs all of the energy incident on it
e = 1
This type of object is called a black body
An ideal absorber is also an ideal radiator of energy
Ideal Reflector
An ideal reflector absorbs none of the energy incident on it
e = 0
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
Resisting Energy Transfer
Dewar flask/thermos bottle
Designed to minimize energy transfer to surroundings
Space between walls is evacuated to minimize conduction and convection
Silvered surface minimizes radiation
Neck size is reduced
Global Warming
Greenhouse example
Visible light is absorbed and re-emitted as infrared radiation
Convection currents are inhibited by the glass
Earth’s atmosphere is also a good transmitter of visible light and a good absorber of infrared radiation