Slide 1 Slide 6 Slide 2 Slide 7 Slide 3 Slide 8 Slide 4 Slide 9 Slide 5 Slide 10 Physics 10 Electrostatics Slide 11 Slide 12 Slide 13 Slide 14 Slide 15 Slide 16 Slide 17 Slide 18 Slide 19 Slide 20 Slide 21 Slide 22 Slide 23 Slide 24 Slide 25 Slide 26 Slide 27 Slide 28 Slide 29 Slide 30 Slide 31 Slide 32 Slide 33 Atomic Structure nucleus: consists of protons (+ charge) and neutrons (no charge) surrounding the nucleus are electrons (- charge) electric charge that property to which the attractive and repulsive behavior of protons and electrons is attributed We do not know what charge is, we only know that there are two types of charge. The two types of charge were called positive (+) and negative (-) by Benjamin Franklin in 1747. Important Facts About Atoms Every atom is composed of a positively charged nucleus surrounded by negatively charged electrons. The electrons, of all atoms are identical; they have the same mass and the same charge (also true of protons and neutrons) Protons have the same charge as electron (but opposite signs) but have about 1800 times more mass. Neutrons have a little more mass than protons but they have no charge. Most atoms are neutral because they have the same amount of protons as electrons. 1
opposite charges attract like charges repel Exercise 15 It is relatively easy to strip the outer electrons from a heavy atom like that of uranium but very difficult to remove inner electrons. Why do you suppose this is so? For the outer electrons, the attractive force of the nucleus is largely canceled by the repulsive force of the inner electrons. The inner electrons fell the full force of the nucleus, and a large force is required to remove them. Conductors Insulators One or more of the outer electrons in a conductor are free to wander within the metal. ex: copper, aluminum, silver, gold Electrons are tightly bound to the atoms in an insulator. ex: plastic, rubber, wood, pure water 2
Exercise 22 Two equal charges exert equal forces on each other. What if one of the charges has twice the magnitude of the other. How do the forces they exert on each other compare? The forces they exert on each other are still the same! Newton s third law applies to electrical forces as well as all forces. The three ways to charge an object charging by friction electrons are transferred by friction when one material rubs against another charging by contact electrons are transferred from one material to another by simply touching them together (no rubbing) charging by induction the process of charging one object without touching it with another charged object Charging by Friction Example of charging by friction Objects can gain or lose electrons by being rubbed together. electron are rubbed off of the man s shoes and onto the rug (charge is conserved) 3
Another example of charging by friction Charging by contact electrons are rubbed off the man s head onto the balloon (charge is conserved) Electrons are transferred from the rod to the metal sphere by simply touching them together. Charging by induction Lightning Charging by induction occurs during thunderstorms. The negatively charged bottom of clouds induce a positive charge on the Earth s surface. No contact was ever made with the charged object! Lightning is the electrical discharge between a cloud and the ground or between oppositely charged parts of clouds. 4
Lightning (cloud to ground lightning) Lightning (cloud to ground lightning) If the negatively charged stepped leader meets a positively charged upward streamer, a path is formed and electrons rush from the cloud to the ground. This is a lightning strike! Before lightning strikes, negatively charged stepped leaders come down from the cloud and positively charged upward streamers rush upwards from objects on the ground. Lightning (cloud to ground) Lightning (cloud to cloud lightning) In both of these picture, you can see a upward streamer rushing up from the ground. Most lightning actually occurs between oppositely charged parts of clouds. 5
Lightning (ground to cloud lightning) Lightning (ball lightning) Lightning can also strike from the ground up to a cloud. There have also been reports of ball lightning, which usually appears as a mysterious glowing sphere which drifts horizontally through the air. Why objects lose electric charge Induced charge water vapor and other molecules running into a charged object can eventually remove all excess charge from the object induced charge: positive and negative charges in the molecules of an insulating material become slightly separated 6
Since the negative charges are closer to the (positively) charged comb than the positive charges, the paper feels a net attractive force (closeness wins). The balloon sticks to the wall because the positive charges in the wall (which exert an attractive force) are closer to the balloon the negative charges in the wall (which exert a repulsive force). Closeness wins! Electric Field Electric fields are represented by field lines. The field lines point in the direction of the electric field. The closer together the field lines, the stronger the electric field. 7
Some Electric Fields Excess charge placed on a conductor will move to the surface of the conductor because the electrons mutually repel each other. Excess charge will arrange itself on the surface of the conductor such that the electric field inside the conductor is zero. It will only be evenly distributed if the conductor is spherical. 8
The Milagro Gamma-Ray Detector The Milagro Gamma-Ray Detector 9