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- Energy in Thermal Processes
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- Distinguish between heat and internal energy.
- Be able to work problems dealing with specific heat.
- Discuss the major methods of heat transfer.
- Understand the concepts of calorimetry.
- Explain energy absorption and emission by radiation.
- Understand thermal resistance, thermal conductivity and calculate R
values.
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- 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
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- 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
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- 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
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- 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
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- James Joule described the reversible conversion of heat energy and
work. The calorie is defined as the amount of energy needed to raise
the temperature of one gram of water at 14.5° one degree Celsius. The
SI unit for work and energy is the Joule.
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- Energy is required to heat substances and to produce phase changes.
Phase changes require larger amounts of energy than the amounts needed
to heat the same substance as a solid, liquid, or gas. Physical and
chemical changes always involve energy transfer, either in the form of
work or heat, between a system and its surroundings.
- According to the law of conservation of energy: heat lost = heat gained
- Heat energy is measured in calories (cal) or joules (J) using a calorimeter.
The joule is the SI unit of energy, although calories are commonly used.
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- 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
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- An apparatus used for measuring heats of combustion. It consists of a
strong container in which the sample is sealed with excess oxygen and
ignited electrically. The heat of combustion at constant volume can be
calculated from the resulting rise in temperature. The apparatus is used
to calculate the calorific values of foods and the energy content of a
sample of biomass, required for constructing a pyramid of energy.
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- 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
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- SI units
- Historical units
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- 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
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- How much heat is lost when a solid Al ingot with a mass of 4110g cools
from 933 K to 298 K?
- Q = m c ΔT
- Q = 4110g (933K-298K) (.903 J/g . K)= 2,360,000 J.
- Remember, the surroundings will gain an equal amount of heat.
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- 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
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- 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
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- During a phase change, the amount of heat is given as
- 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
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- 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
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- 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
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- Once at 0º C, the phase change (melting) starts
- The temperature stays the same although energy is still being added
- Use Q = m Lf
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- 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
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- At 100º C, a phase change occurs (boiling)
- Temperature does not change
- Use Q = m Lv
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- 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
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- 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
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- Need to know the rate at which energy is transferred
- Need to know the mechanisms responsible for the transfer
- Methods include
- Conduction
- Convection
- Radiation
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- 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
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- 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
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- 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
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- The slab allows energy to transfer from the region of higher temperature
to the region of lower temperature
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- 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
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- Substances are rated by their R values
- 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
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- R values are indicators of the effectiveness of insulation to block the
transfer of heat by radiation.
- Materials which have a high thermal conductivity have, by definition, a
low thermal resistance - they are poor heat insulators. On the other
hand, materials with a low thermal conductivity have a high thermal
resistance - they are good heat insulators. Good insulating materials
therefore should have a high thermal resistance or R value.
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- R values can be calculate by this equation: R = L/K where L is the
material’s thickness and K is the thermal conductivity of the material.
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- Different materials transfer heat by conduction at different rates -
this is measured by the material's thermal conductivity.
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- 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
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- 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
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- Radiators
- Upwelling
- Cooling automobile engines
- Algal blooms in ponds and lakes
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- 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
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- The electromagnetic waves carry the energy from the fire to the hands
- No physical contact is necessary
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- P = σAeT4
- P 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 emissivity
- T is the temperature in Kelvins
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- 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
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- An ideal absorber is defined as an object that absorbs all of the energy
incident on it
- This type of object is called a black body
- An ideal absorber is also an ideal radiator of energy
- An ideal reflector absorbs none of the energy incident on it
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- 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
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- 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
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- 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
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