Energy in Thermal Processes

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Energy in Thermal Processes
Chapter Eleven Energy in Thermal Processes

Read and take notes on pages: 352-353

Read and take notes on pages: 366-367

Energy Transfer When two objects of different temperatures are placed in thermal contact, the temperature of the warmer decreases and the temperature of the cooler increases The energy exchange ceases when the objects reach thermal equilibrium The concept of energy was broadened from just mechanical to include internal Made Conservation of Energy a universal law of nature Introduction

Heat Compared to Internal Energy
Important to distinguish between them They are not interchangeable Heat involves a transfer of energy Section 11.1

Internal Energy Internal Energy, U, is the energy associated with the atoms and molecules of the system Includes kinetic and potential energy associated with the random translational, rotational and vibrational motion of the particles that make up the system Also includes any potential energy bonding the particles together Section 11.1

Link to Bright storm on Heat

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 Section 11.1

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 Section 11.1

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 = J This is called the Mechanical Equivalent of Heat Section 11.1

James Prescott Joule 1818 – 1889 British physicist
Conservation of Energy Relationship between heat and other forms of energy transfer Section 11.1

Link to Bright storm on Heat Transfer

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 Section 11.5

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 Section 11.5

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 Section 11.5

Conduction, cont. The rate of conduction depends on the properties of the substance 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 Section 11.5

Conduction, equation The slab of material allows energy to transfer from the region of higher temperature to the region of lower temperature A is the cross-sectional area Section 11.5

Conduction, equation explanation
A is the cross-sectional area Through a rod, Δx = L 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 for some conductivities Good conductors have high k values and good insulators have low k values Section 11.5

Read and take notes on page: 368

Read and take notes on pages: 371-374

Convection Energy transferred by the movement of a substance
When the movement results from differences in density, it is called natural convection When the movement is forced by a fan or a pump, it is called forced convection Section 11.5

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 Section 11.5

Convection applications
Boiling water Radiators Upwelling Cooling automobile engines Algal blooms in ponds and lakes Section 11.5

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 Section 11.5

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 Section 11.5

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 Section 11.5

Radiation equation P = σ A e T4
The power is the rate of energy transfer, in Watts σ = x 10-8 W/m2.K4 Called the Stefan-Boltzmann constant A is the surface area of the object e is a constant called the emissivity e varies from 0 to 1 T is the temperature in Kelvins Section 11.5

Energy Absorption and Emission by Radiation
The rate at which the object at temperature T with surroundings at To radiates is Pnet = σ A e (T4 - To4) When an object is in equilibrium with its surroundings, it radiates and absorbs at the same rate Its temperature will not change Section 11.5

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 Section 11.5

Ideal Reflector An ideal reflector absorbs none of the energy incident on it e = 0 Section 11.5

Clothing Black fabric acts as a good absorber White fabric is a better reflector Thermography The image of the pattern formed by varying radiation levels is called a thermogram Body temperature Radiation thermometer measures the intensity of the infrared radiation from the eardrum Section 11.5

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 Section 11.5

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 Section 11.6

The Laws of Thermodynamics
Chapter Twelve The Laws of Thermodynamics

Read and take notes on pages: 385-387

First Law of Thermodynamics
The First Law of Thermodynamics tells us that the internal energy of a system can be increased by Adding energy to the system Doing work on the system There are many processes through which these could be accomplished As long as energy is conserved Introduction

Video # 1 First Law of Thermodynamics & Internal Energy Khan Academy
The next four videos are long but a MUST WATCH!!!!!! The concepts being covered I find to be very difficult, these videos when watched together, do a great job explaining the first law of Thermodynamics

Video # 2 More on Internal Energy (Khan Academy)

Video # 3 Work from Expansion (Khan Academy)

PV Diagrams and Expansion Work
Video # 4 PV Diagrams and Expansion Work (Khan Academy)

Second Law of Thermodynamics
Constrains the First Law Establishes which processes actually occur Heat engines are an important application Introduction

Work in Thermodynamic Processes – Assumptions
Dealing with a gas Assumed to be in thermodynamic equilibrium Every part of the gas is at the same temperature Every part of the gas is at the same pressure Ideal gas law applies Section 12.1

Work in a Gas Cylinder The gas is contained in a cylinder with a moveable piston The gas occupies a volume V and exerts pressure P on the walls of the cylinder and on the piston Section 12.1

Work in a Gas Cylinder, cont.
A force is applied to slowly compress the gas The compression is slow enough for all the system to remain essentially in thermal equilibrium W = - P ΔV This is the work done on the gas where P is the pressure throughout the gas Section 12.1

More about Work on a Gas Cylinder
When the gas is compressed ΔV is negative The work done on the gas is positive When the gas is allowed to expand ΔV is positive The work done on the gas is negative When the volume remains constant No work is done on the gas Section 12.1

Work By vs. Work On The definition of work, W, specifies the work done on the gas This definition focuses on the internal energy of the system Wenv is used to denote the work done by the gas The focus would be on harnessing a system’s internal energy to do work on something external to the gas W = - Wenv Section 12.1

The pressure remains constant during the expansion or compression This is called an isobaric process The previous work equation can be used only for an isobaric process Section 12.1

PV Diagrams Used when the pressure and volume are known at each step of the process The work done on a gas that takes it from some initial state to some final state is equal in magnitude to the area under the curve on the PV diagram This is true whether or not the pressure stays constant Section 12.1

PV Diagrams, cont. The curve on the diagram is called the path taken between the initial and final states The work done depends on the particular path Same initial and final states, but different amounts of work are done Section 12.1

First Law of Thermodynamics
Energy conservation law Relates changes in internal energy to energy transfers due to heat and work Applicable to all types of processes Provides a connection between microscopic and macroscopic worlds Section 12.2

First Law, cont. Energy transfers occur
By doing work Requires a macroscopic displacement of an object through the application of a force By heat Due to a temperature difference Usually occurs by radiation, conduction and/or convection Other methods are possible All result in a change in the internal energy, DU, of the system Section 12.2

First Law, Equation If a system undergoes a change from an initial state to a final state, then DU = Uf – Ui = Q + W Q is the energy transferred between the system and the environment W is the work done on the system DU is the change in internal energy Section 12.2

First Law – Signs Signs of the terms in the equation Q W DU
Positive if energy is transferred into the system Negative if energy is removed from the system W Positive if work is done on the system Negative if work is done by the system DU Positive if the temperature increases Negative if the temperature decreases Section 12.2

Notes About Work Positive work increases the internal energy of the system Negative work decreases the internal energy of the system This is consistent with the definition of mechanical work Section 12.2

Read and take notes on pages: 388-389

Read and take notes on pages: 390-391