Transcript Document
Lecture series for Conceptual Physics, 8th Ed. Electrical forces p373 Electrical forces are a gazillion times stronger than gravity. They should crush together or tear everything apart. BUT THEY DON’T. Because there are two (2) electrical forces that bunch together in atoms and are basically neutral. Thank you atoms! Electrical Charges p373 1. Positive nucleus…negative electrons. 2. Electrons are all the same. 3. Nucleus: protons and neutrons…blah, blah. 4. Atoms are neutral until they become ions…….. Electrons don’t orbit in orbitals. They exist in an area. Think of electrons as waves that need a certain amount of space. The areas are called shells…which are divided into subshells…which are divided into orbitals…which are divided into spins. The whole periodic table is designed along these lines. Actually, these lines: Shells Subshells s p d f spin Why don’t positive protons fly apart? Because there are even stronger forces at short range. Conservation of Charge p374 The hair and the plastic start out neutral. Rubbing the electrons onto the plastic makes the plastic rod negative and the hair is now positive. + Charges aren’t created or destroyed, but they can be moved around. Please remember that it takes energy to add/remove electrons. Coulomb’s Law p376 K q 1 q2 F= r2 Where k = 9.0x109 N-m2/C2 Notice the similarity to Newton’s Law of Gravitation: G m1 m2 F= r2 Conductors and Insulators p377 Conductors have a “sea” of electrons. When a positive charge is put on one side of the wire and a negative charge on the other, then, electrons migrate toward the positive side. Insulators have tightly bound electrons which don’t move. Actually, there’s a rainbow of conduction/insulation. Semiconductors p378 They’re in the middle of the strong/weak electron bond thing. By adding impurities they can be made to conduct or to insulate. A whole bunch of these semiconducting thingies make a transistor. Superconductors p378 A material with infinite conductivity. 1987- a material that would superconduct over 100K was discovered. Charging p379 Charging by friction Rubbing on/off electrons. Then by contact… Charging by 3 and 4 3 Charging by Induction 4 Charging by Grounding p379 p380 Friction of ice chunk vs ice chunk may be the method of charging the cloud. The negative bottom of the cloud drives electrons away on the ground below. Charge Polarization p381 In an insulator electrons can only shift a little. Fig. 21.11 shows a shift within a molecule. A charged rod brought near an insulator A negatively charged balloon induces polarization in a wall. A positive comb induces separation of charge in a piece of paper. OR, molecules may be polar by themselves. Electric Field A gravity field is a kind of force field g= F m p383 An electrical field is another kind of force field. E= Fig 21-18 Shows some electric field lines. F q More on electric field The lines used to represent electric fields have direction: away from positive and toward negative. + - Bits of cotton thread act like the piece of paper in fig.21.13 and twist in place to line up with the electric field. When charges move, the disturbance in the E-field affects other charges. An E-field can store charge. An E-field can be channeled An E-field teamed with a magnetic field can move through empty space. through metal wires. Electric Shielding p386 A and B push equally on the charged point. “A” is 4x the charge but 2x the distance. The NIFTY thing is that the E-field inside a conductor is always zero. 0 0 0 Electric Energy Storage p390 Electric energy is stored in a capacitor. The 12V battery puts 12V of potential (V) between the plates. C is proportional to 12V Area of capacitor Distance between them Televisions use capacitors to store energy. Keys on a keyboard push capacitor plates closer together. Van de Graaff Generator p392 Scary, but safe. Large V…small # charges. The end