About the course: We will try to use various forms of interactive classroom instruction. Your help and feedback will be most appreciated!

Transcription

1 About the course: We will use the ANP rule (Absolutely No Propaganda!). Dr. Tom will do his best to act as a scientist i.e., to give you high-quality unbiased information. He will encourage you to use your intellect to reach your own conclusions concerning issues that are now the matter of heated debates (such as, e.g., global warming). We will try to use various forms of interactive classroom instruction. Your help and feedback will be most appreciated!

2 Material: the first thing we need to talk about is the definition of energy. Energy is the capacity of doing (providing, delivering) work -- this is the textbook definition. So, one has to know first what work is, right? Supposethat a force F exerted to a body resultsin its displacement from x 1 to x 2. Then the work W done by theforce is : W x x F x F 2 1

3 However, the path may be not a straight line, and the force may change along the path. What then? Then, we use a more sophisticated definition: W r 2 F( r) dr r 1 But no matter which definition we use, the units are always the same: (the unit of work) = (the unit of force) (the unit of length) In this course, we will use primarily the SI units.

4 TheSI 2nd Law : times SI of So, unit of acceleration is The abreviated symbolis TheSI thesi unit of F mass is unit of unit of 1 Newton 1meter force is m a acceleration. a m/s force is : work is based on Newton's - force is kilogram (kg), 2. N. thus: kg m kg m/s 2 /s equal 2 to mass and 2 the unit 1 Newton 1Joule (1J) So, since energy is equivalent to work, the SI unit of energy is also 1 J.

5 One of the most important laws of physics is the Energy Conservation Law. It says: energy can neither disappear, nor spontaneously emerge from Nowhere. The amount of energy in an isolated system is constant! ( isolated system - a one from which energy cannot escape, and cannot enter it from outside). However, there are many different forms of energy. Kinetic energy and potential mechanical energy are just two such forms. What other forms of energy do we know let s try to make a list on the next slide:

6 I propose that we order our list according to the time in history when humans started using a given form of energy at a meaningful scale: Biological energy the energy stored in our bodies. Humans started using this form of energy long before they became humans (for moving around, climbing trees, hunting, fighting ). Solar energy same as above. E.g., to get warm after a chilly night. Chemical energy that stored in the food. Again, humans started taking advantage of this form long before they became humans. Thermal energy that from fire. At the onset of civilization! It was the first form the use of which required human intellect! Animal energy : here we mean biological energy, but that stored in the bodies of domesticated animals, not our own bodies. Wind energy first ever hi-tech devices built by humans used it! In sailing boats, wind mills, simple water pumps for irrigating fields. Hydro-energy kinetic/potential energy of water in streams and artificial ponds probably, started being used about the same time as wind energy. Ancient civilizations, e.g., Egyptians, used both! Then, there was a long period with no significant progress in using new energy forms

7 Things started moving in fast pace only in the Second Millennium. The new forms that people employed then were: Chemical energy from synthetic, i.e. man-made carriers: gunpowder (a.k.a. black powder ), first invented in China about 1000 years ago, was perhaps the first? Chemical energy from mined sources (e.g., coal): widespread use begun in XVIII Century; then (XIX Cent.) came oil, and natural gas. Energy from heat engines when coal became available; the first heat engine was built in England by Newcomen in early 1700-s then a far better machine was built about 50 years later by James Watt. When that happened, the Industrial Revolution began! Electrical energy XIX Century; the proud XIX-century people called their times The Century of Steam and Electricity. Geothermal energy water from hot sources had been used for millennia by people who dwelled nearby, but a utilization on industrial scale began only in the early years of the XX Century. Atomic (nuclear) energy first used for not-so-nice purposes in 1945, for peaceful purposes a few years later. Mass, as discovered by A. Einstein, is also an energy form (E=mc 2 ). Future, not yet known usable forms????. Is this list complete? Can you think of any other items?

8 Now, let s return to the Energy Conservation Law. Energy cannot disappear, but one energy form can be converted into another. Sometimes Form X can be converted directly into Form Y. Sometimes both ways, sometimes only one way. Sometimes such conversion has to be a multi-stage process, e.g., Form X Form A Form B Form Y. Now, I want to make the first test of how the envelope system I talked about last time would work in this class. Please form groups of about 10 people in each. Fill the names in the form provided. On the form, there is a number. It refers to the item on the list that you will see in a moment it s a shortened list of the energy form we talked about minutes ago. I want you to spend a few minutes on discussing in the group the following question: How can this form of energy be converted into mechanical energy? (either kinetic, or potential). After a few minutes spent on the discussion, let one of you write the answer, put it in the envelope, and return it to me. Use your sense of humor! The answer not necessarily has to be in words it may also be in a graphic form! Examples of what I mean by a graphic form are on the next three slides.

9 Example: graphic illustration of magic energy conversion to mechanical (potential) energy Harry Potter Hermione

10 3. 4.

11 Conversion of chemical energy (in gunpowder) to mechanical (potential) energy

12 1. Biological energy (in our own bodies) 2. Solar energy 3. Chemical energy (in food) 4. Thermal energy (from fire) 5. Animal energy 6. Wind energy 7. Hydro-energy 8. Mined chemical energy 9. Energy from synthetic fuels (e.g., black powder) 10. Electrical energy 11. Geothermal energy 12. Atomic (nuclear) energy 13. Energy contained in mass (Einstein s formula).