ENVIR215 Spring Lecture 2 Basic Concepts of Energy

Transcription

1 Lecture 2 Basic Concepts of Energy In the lab, you are working on a variety of experiments that will help you to understand some basic principles of energy measuring energy, energy conversion and storage, energy transfer, energy concentration etc. Energy Scientists define energy as the capacity of an entity to do work and define work as the application of a force through a distance. We can also describe this in non-scientific terms (e.g., moving furniture). Conservation of Energy Energy is neither created or destroyed; it changes from one form to another and can transfer from one bit of matter to another. The 1 st law of thermondynamics is a keystone idea in physics and can be expressed Change in internal energy = net heating (or cooling) + mechanical work done on a system Forms of Energy Mechanical Energy kinetic (motion) and potential Thermal Internal thermal energy (Hidden form of mechanical energy) Electrical Electrostatic and electromagnetic Chemical Bonds between atoms Nuclear Based on changing the nucleus of an atom, splitting (fission) and combining (fussion) of atoms. Ultimate source of all the energy on earth (hydrogen atoms combined to make helium in the sun). Our Energy Usage Prologue of McNeil (p ) outlines a brief history of human s energy use. The terms energy production and consumption are used to describe societies usage but these are not scientific - 1 st law of thermodynamics tells us that energy cannot be created or destroyed. Concept of energy slaves (McNeil, p. 15). 1

2 The graph above shows global energy use from It was prepared in the UK by the Royal Commission on Environmental Pollution. Graphs are like this are very important in science and in society ( does anybody remember Ross Perot). Class will discuss briefly what this plot shows us. Want to look at the plot in detail. If we look at the axis labels we note that the plot has units TW for the left axis and kw. Understanding this sort of notation is very important for this class. We want to discuss ideas we need to understand the numbers and quantities involved Energy Units We are all familiar with units of length and with the fact that we can quote lengths in different units (meters, yards, kilometers, miles, astronomical units. There are many different kinds of units for energy Scientists (at least the young ones) use the SI units (Système International d'unités) of Joules which is written with the shorthand J 1 Joule is equal to the kinetic energy of a two-kilogram mass moving at the speed of one meter per second. We can see that the Joule is defined in terms of several other units. The units of length, time, and mass are basic units in the SI system and are defined 1 meter (m) distance traveled by light in a vacuum in 1/299,792,458 of a second. 1 second (s) - the time interval equal to 9,192,631,770 periods of the radiation corresponding to the transition between the two hyperfine levels of the ground state of the cesium 133 atom 1 kilogram (kg) - defined as the mass of the International Prototype Kilogram, a platinum-iridium cylinder kept at Sèvres, France, near Paris. Also the mass of 1000 cm3 of water at its temperature of maximum density (39ºF). There are many other units of energy. Calorie (cal) - The amount of heat required to raise the temperature of a 1 gram of water, at or near the temperature of maximum density, one degree Celsius Food or large calories (Cal or kilocalories) Erg - Energy unit in the centimeter-gram-second cgs system BTU (British thermal unit) - the quantity of heat required to raise 1 pound of water by 1 degree Fahrenheit. Electron-volt Energy accelerating an electron through a potential difference of 1 Volt We can also think as a barrel of oil, a gallon of gasoline or a cubic foot of natural gas as units of energy 2

3 Expressing very large and small quantities Often we want to describe quantities that are very small or very large relative to the basic unit. We can write small or large numbers in scientific notation e.g., the speed of light is m/s = 3 x 10 8 m/s. The spacing between atoms in a piece of copper is m = 2.3 x m. The three numbers that appear in scientific notation are the coefficient, base which is always 10, and the exponent). Scientific notation is explained nicely at Another site of interest is In SI units we can also avoid writing small or large numbers by placing prefixes before the units. Power In science, we are often interested in the rate at which things happen, which allows us to figure out how long things take. Rates are expressed in terms of a quantity per unit time 3

4 the rate at which our car covers distance or its speed in units of length/time (meters per second or m/s) the rate at which water flows into the bath in units of volume/time (meters cubed per second or m 3 /s) The rate at which energy is generated or consumed (that is how much energy is generated or consumed per unit time) is termed the power The SI unit for power is the Watt (W) 1 Watt is 1 Joule per second or 1 J/s There are many other units of power (e.g., horsepower (1 hp = 750 W), cal/s, foot-pound force per second). In normal conversation, we often use the terms power and energy almost interchangeably without thinking which one they really mean but it is important to understand the difference Unit Conversion You will come across all sorts of energy and power units in this class Just to get things really confused, one common unit for energy is the kilowatt hour (kwh) which is the energy obtained from a maintaining a power of 1000 W for 1 hour (3,600,000 J). We will give you a handout which will help you understand how to convert from 1 energy unit to another. There are also handy converters available on the web. For example. works well if you can put up with all the popup advertisements. (As a practical example, we can use the converter to help us compare the costs of heating a home with electricity, oil and natural gas. Heating oil costs $2/gallon and 1 gallon yields 1.5 x 10 8 J; natural gas costs $10 per 10 6 BTU; and electricity costs $0.10 per kwhr). Global Energy Use We can know go back to our original graph and we can see left hand axis has units of TW (pronounced terra Watts) which are W or trillion Watts. The current day global energy consumption exceeds W. The right hand axis puts that in more meaningful numbers by dividing by the world s population. Each person uses on average a little under 2 kw = 2 x 10 3 W (equivalent to about 2 horses or 20 slaves working around the clock). In the prologue of his book McNeil talks about energy slaves. In the US our 4

5 average energy use is 10 kw so we all have 100 energy slaves working for us (each slave would have to Climb Mount Rainier twice every day to average 100 W).. Energy Storage Our utilization of energy is dependent on two properties. First need to be able to store energy so that we can access it and change it to the form we need when we need it. Some sources of energy are very rich while others are poor. Nuclear energy is very rich. Chemical energy sources are also rich Hydrocarbons kj/g H kj/g (but only 1/3 the energy of methane per unit volume) Foods 2-17 kj/g g candy bar provides about 1.5 x 10 6 J or J Other sources of energy are relatively poor. For example, 1 tonne or 1000 kg of water that is elevated 10 m behind a damn contains E = mass x height x acceleration of gravity = 1000 x 10 x 10 = 10 5 J (1/15 th the energy of a candy bar) The group working in the flume will also be surprised when they calculate how much energy is stored temporarily in the moving water. Hydropower only works because such large amounts of water are involved. Energy Transmission Energy arrives at the earth via radiation of electromagnetic waves from the sun we will talk about this more next lecture. All waves provide a means to transport energy (e.g., ocean waves transport the energy from storms out at sea to our beaches) Heat Energy can be transmitted via conduction or by convection or the movement of fluid a process that you can look at with one of the experiments Humans transfer energy via electric transmission. Quite efficient although on average about 10% of the electricity we generate in the USA is lost is transmission and at times the fraction is even higher. Conversion of Energy As many of you are experiencing in the experiments you are doing, energy can be converted from one form to another. For example in the Sterling engine chemical energy is converted to heat energy which is converted to kinetic energy and the back to heat energy as the glass balls hit the end of the test tube. One very important concept for our use of energy is efficiency. 5

6 Efficiency = Amount of energy converted to a useful form / (amount converted to a useful form + the amount wasted in the form of heat). Usually expressed as an percentage. Cars + our bodies Hydrocarbon Power Plants Hydropower plants Electric motors 20% efficient 35% efficient 90% efficient ~90% efficient Our utilization of energy is often remarkably inefficient. For example when we drive to work. Engine 20% efficient Weight of person (75 kg) / weight of car (1500 kg) 1/20 th of total mass. 99% of energy is lost. What about energy used to make the car and supply the gasoline? Amory Loving estimates our efficiency (materials and energy) is 0.02% 6