You have studied the elements before. All of the known elements are organized in the periodic table.

Transcription

1 Building for Physics, Mr. Kent van de Graaff Reading You have studied the elements before. All of the known elements are organized in the periodic table. The smallest particle of an element is the atom which itself is comprised of Protons, Neutrons and Electrons. Protons and neutrons reside in the atom s nucleus while electrons travel in different orbits outside the nucleus. Bohr Model of an Atom This is a simplified view of an atom called the Bohr Model. Today, we know that protons, neutrons and electrons can be broken down into smaller pieces. We also know that electrons don t travel in neatly defined orbits. Rather, they move between orbits. But for the purposes of this discussion, the Bohr model (protons, neutrons and electrons) shown to the left will suffice. Two of the three components of an atom are charged: Positive or Negative Proton: Positive Neutron: No charge Electron: Negative Charged particles behave a lot like magnets when they are close together. Magnet: Unlike poles attract Charges: Unlike charges attract Magnet: Like poles repel Charges: Like charges repel 1

2 One interesting example of how charges work is the old bend a stream of water trick which you will try at the front of the room when you re ready. To bend a stream of water: Of course, you know the chemical formula for water is H2O. What you might not know is that water molecules are polarized, that is, one end of a water molecule has positive charge while the other has negative charge. The two hydrogen atoms each have a positive charge while the single oxygen atom has a single negative charge. 1. Use a paper clip to poke a small hole through the bottom of a styrofoam cup 2. Place enough water in the cup to cause a very small stream of water to flow out of the hole. Make this stream as small as possible. 3. Rub an inflated balloon on a blanket or piece of cloth until it develops static electricity 4. Hold the balloon close to the small stream of water coming out of the up. Get out of your seat and try this little demo on your own. What happens? Draw a picture. Please do not draw here. Rather, draw on a separate page. The interaction of charged particles can cause them to move. Consider the following example: Let s say that electron 1 and the proton are fixed in their locations: They cannot move. Electron 2 can move. How will electron 2 move and why (2 reasons)? Electron 2 will move to the right because: 1. It is repelled to the right by electron 1 2. It is attracted to the right by the proton 2

3 This movement creates electricity which is defined as: Moving electrons. Does that sound too simple for something as important as electricity? Nonetheless, that s what electricity is. Let s say that you have a circular piece of metal and that you put 4 electrons close together on its surface. What will these electrons do? They will move to be as far as possible from each other. If you took one of the electrons in the right circle and moved it, that arrangement would not be stable. The electrons would move again to be as far from each other as possible. Next, let s say that we had two circular pieces of metal, one with a bunch of electrons on it and one with a bunch of protons on it. For two reasons, it s hard to move protons. First, protons are much more massive than electrons and thus more difficult to move. Second, they re tightly packed in the nucleus of an atom with other protons and neutrons. Basically, electrons move much more than protons. What would the electrons on the left piece of metal loooooove to do if they could? They d love to move over to the piece of metal on the right and for the same 2 old reasons: They are repelled by their electron neighbors and they are attracted by the positive protons on the circle to the right. But can the electrons move from left to right? The answer to that question is usually No : Electrons need a piece of material to travel along they can t just jump through the air (note that I said usually : Stay tuned). Materials that electrons can flow through are called a conductors. Different types of materials have different capacities for conducting electrons. The most common materials used as conductors are metals. Other materials, such as wood, are very poor conductors. These materials are called insulators. Now, what would happen if we connected the two round pieces of metal with a length of conducing wire as below? Electrons would flow across the wire from left to right, of course, creating what? Electricity. 3

4 This is called Static Electricity because the number of electrons doesn t change: That number is static. Later, we ll see how a battery works. Since batteries generate more and more electrons, their electricity is not called static. Have you heard of the word current when electricity is discussed? Current is a measure of how many electrons travel through a conductor in a period of time. In which of the following pictures (A or B), do you think, there is more current? Figure 9 There is more current in the wire in B for the same old reasons: Since there are more electrons packed together on the left, the repellent force is greater. Also, since there are more protons packed together on the right, the attractive force is greater. More electricity will flow through the wire in B meaning that that wire has more current than the wire in A. What about this situation? Would there be current from left to right? Figure 10 The charge on the left is negative 5, correct? (5 electrons). What s the charge on the right? Zero (0) right? The 4 protons cancel out the 4 electrons. We say that the disk on the right is neutrally charged or just neutral. Would electrons move from left to right here? Yes they would. Since the disk on the right is neutrally charged, it cannot pull the electrons from the disk on the left. But isn t there still a push from the left cause by the electrons repelling each other? Yes there is. Current will still flow to the right. So depending on the situation, more or less current will flow in the wires between the disks in the above diagrams. Consider Figures 9 and 10. Here s a ranking of the amount of current that would flow in the wires: 1. Figure 9, to the right 2. Figure 9, to the left 3. Figure 10 4

5 The word voltage is used to describe this. If you have more voltage and you ll get more current, less voltage and you ll get less current. A good way to examine this is to compare 2 batteries: A 1.5 volt battery and a 9 volt battery. Batteries: Contain chemicals that react to free up electrons. Have two terminals When the chemicals react, they remove electrons from one terminal and place them on the other terminal. This creates one terminal with a bunch of electrons and another terminal with a bunch of protons. Of course, the electrons on one terminal desperately want to move to the other terminal but they cannot do this unless the terminals are connected by a conductor, almost always a wire. If the terminals are connected by a wire then electrons can flow. Of course this creates electricity otherwise known as current. In the third picture to the left this current is used to light a bulb. The chemicals in a 9 volt battery are more powerful than those in a 1.5 volt battery. That is, they are able to place more electrons on a terminal than a 1.5 volt battery can. A simpler way of saying this: It has more voltage. As we ve seen, higher voltage means that more electrons will flow over a wire between the terminals. Result: More electricity and more current. So, the voltage of a battery is a measure of how many electrons it can place on one of the terminals. The higher the voltage, the more electrons and thus the more current that the battery can produce in a wire. To the left, the 9 volt battery is able to place more electrons on one terminal than the 1.5 volt battery. Therefore, when its terminals are connected by a wire, more current flows than flows in a 1.5 volt battery. 5

6 Let s finally shift to the subject of this project: A van de Graaff generator. You ve seen one of these in Mr. Kent s room. If you haven t noticed it, it s probably on top of the bookshelf next to the classroom door. van de Graaff is the last name of the physicist who invented this type of generator. See the plastic tube running vertically? Inside of that resides a rubber belt. When the generator is turned on, this belt moves. At the top and bottom of the belt there are two metal brushes. It s too complex to explain here exactly why, but these metal brushes end up removing electrons from the metal globe at the top. To explain a bit further: Before the generator is turned on, the globe has no charge (is neutral) However, after the generator is turned on having lost a bunch of electrons - the globe is positively charged as in the diagram to the right. Now, let s say that you move a neutrally charged object such as your hand closer and closer to the globe. What will the electrons on your hand want to do? They ll want to move from your hand to the positively charged globe. But can they do this? Well, it turns out that a van de Graaff generator can generate enough voltage to allow the electrons on your hand to jump through the air. As they do this, they sometimes break the molecules in the air down and you can see this as a spark. One final thing about van de Graaff generators: In the proper conditions they can make your hair stand up! This happens if you put your hand on the charged globe when you are insulated from the ground. Electrons from your skin and hair flow from your body to the globe leaving you positively charged. Think of 2 hairs next to each other, both positively charged. What do they want to do? Move away from each other, of course. 6

7 Building for Physics, Mr. Kent van de Graaff Reading Worksheet Name: 1. What is the smallest particle of an element? 2. What model will be use of this smallest particle? 3. Name one thing about the Bohr Model that we now know is a simplification of reality: 4. List the 3 components of an atom, each with its charge: 5. How do like charges behave when they are close to each other? 6. How do unlike charges behave when they are close to each other? 7. What is the definition of polarized? 8. Draw a picture of a water molecule, including the charges of the atoms, to demonstrate the polarity of a water molecule: 9. Explain why a small stream of falling water bends when it passes near a balloon that has been rubbed on a piece of cloth. Questions 10 & 11: Three charged particles reside close to each other. Particle E and particle P2 are fixed: They cannot move. Particle P1 can move. 10. In what direction will particle P1 move? 11. Provide 2 reasons for this movement: a. b. 7

8 12. Define Electricity : 13. Why is it harder to move a proton than it is to move an electron? a. b. 14. Can electrons generally jump through the air? If not, what is normally required for electrons to flow from one place to another (do not answer A wire ). 15. What is the most common type of material used for a conductor? 16. What types of materials do not conduct electrons? Questions Consider this situation: 17. What will happen in this situation? What is this called? 18. What type of electricity is generated? 19. Definition of current : Questions Consider this situation: 20. What is the total charge on the right? 21. Will current flow? 22. If so, why? 23. What does voltage mean? (this is kinda tough: Form your own definition from the reading) 8

9 24. What do the chemicals in batteries do? 25. Why does a 9 volt battery have more voltage than a 1.5 volt battery? That is, what do the chemicals in a 9 volt do that the chemicals in a 1.5 volt can t? 26. What do the brushes in a van de Graaf generator do? 27. Before it s turned on, what is the charge of the globe of a van de Graaff generator (Positive, Negative or Neutral)? 28. While it s turned on, what is the charge of the globe of a van de Graaff generator (Positive, Negative or Neutral)? Questions 29 30: Pertain to a van de Graaff generator that is turned on full blast. 29. Can electrons jump through the air from your hand to the globe? 30. If so, explain why. 31. Explain how a van de Graaf Generator can make your hair stand up. 9

10 Building for Physics, Mr. Kent van de Graaff Quiz Name: 1. What is the definition of Electricity? Questions 2 & Will electricity flow? 3. If so, provide one or two reasons (if only one reason, leave b blank): a. b. Questions 4 & Will electricity flow? 5. If so, provide one or two reasons (if only one reason, leave b blank): a. b. Questions 6 & 7 6. In which situation (A or B) is there more voltage? 7. In which situation (A or B) is there more current? 8. On the back of this page, draw a picture containing: a. The globe of a van de Graaff generator that is turned on (show the proper charge). b. A neutrally charged hand near the globe (Include + and signs to show that the hand is neutral) c. An arrow representing a spark between the globe and the hand. Be sure to point the arrow in the direction that the electrons move. 10