Electric Charge, Fields, Potentials, and Current

1. Electric Charge, Fields, Potentials, and Current

2. Electrostatics Electric Forces and Charges Much stronger than gravity Both attractive and repulsive�balances out in nature Balance due to equal numbers of protons and electrons in atoms

3. Electrostatics Forces due to charges remaining in place Charge is due to electrons being moved from one object to another Charges move easily in conductors, poorly in insulators Law of electric forces: Like charges repel, unlike attract Law of conservation of charge: Total amount of electric charge remains constant in the universe, not created or destroyed.

4. Coulomb�s Law Charles Coulomb measured the forces due to electric charges; and mathematically quantified them Force is directly proportional to amount of charge and inversely proportional distance between charges squared F = kq1q2 r2

5. Conductors and Insulators Conductors allow electrons to pass thru easily�metals, since electrons are loose Insulators keep charges from passing thru�wood, glass, plastic�with tightly held electrons Semiconductors like silicon allow electrons thru under certain conditions Not when pure, only when �doping� materials are used Transistors and integrated circuits made this way

6. Electrostatics Causing charges to be moved By friction � electrons pass from one material to another in direct contact.

7. Charging Materials By induction: 1. Charges object brought near an uncharged object causing repulsion of like charges 2. Like charges are removed from new object by grounding 3. Original charged object is removed, leaving second object with an opposite charge. Induction happens in thunderstorms�negative cloud causes ground to become positive�lightning rods help protect buildings by dissipating the charge

8. Polarization Nonconductors become �charged� by their electrons being displaced to the opposite side of the object

9. Homework #1 Chapter 32 RQ 7, 10, 11, 13, 14, 16, 17 T&E 2, 7, 8

10. Electric Fields Just like gravity field, charges have a force field as well, measured in force per unit charge E = F = kQ q r2 where Q is a positive test charge Direction of fields � away from a positive charge, toward a negative charge

11. Force Fields Fields have strength and direction Field is strongest where the force is the strongest�determined by the force and direction of motion of a positive test charge

12. Electric Shielding Electrons repel toward the outside of any conducting surface Net charge inside is zero Electrons flow outward evenly, but pile up on sharp corners

13. Electrical Potential Just like gravity�the potential (possibility) of falling to earth, charges have the potential to move toward or away from each other

14. Storing Charges Capacitors can store charges on plates which are separated�as in Franklin�s Leyden jars Large amounts of energy can be stored Any system of two conductors separated by an insulator can store charge Capacitors can act as �electrical springs� where electrical charge energy is kept. Capacitors also can isolate circuit parts from unwanted potentials

15. Van de Graaf Generator This machine is capable of producing very high electrostatic potential differences in the order of millions of volts It works by friction of the belt with the rollers and separates charges at combs which take the charges to the dome and picks them up from the ground at the base

16. Van de Graff Generator

17. Homework #2 Chapter 33 RQ 3, 4, 6, 8, 11, 14, 15, 18 T&E 4, 5, 7, 10

18. Current Electricity Current is defined as how much charge moves in a unit of time I = Dq Dt Units are amperes, 1 coulomb per second Current can only flow in a closed path called an electric circuit Current direction is defined as the opposite direction to the flow of electrons Potential difference in a circuit causes current and the difference across a battery is called electromotive force, emf, symbolized E

19. Voltage Sources For current to flow there must be an electron pump to keep them moving The source must be able to provide a steady flow Common sources are batteries and generators

20. Resistance and Ohm�s Law Resistance is the ratio of applied voltage to the current flow; it is the measure of how hard it is to get current flowing, measured in ohms. Resistance of a wire is proportional to its length and inversely proportional to its cross sectional area (thickness). Additional factors are temperature and conductivity of the material. Potential difference across a resistance equals the current times the resistance, Ohm�s Law V = IR

21. Resistance and Electric Safety If you are to be hurt by electric current, you must have a lot of current pass thru your body Current goes thru the path of least resistance, so if it has the possibilities of your skin or your blood, the blood with resistance of 100 ? versus the skin with 1000 times as much would be the path. As little voltage as 1.5 volts could create enough current to stop your heart if it goes thru the blood A bird on a wire has no problem since the potential difference between his feet is zero�but if he touches another wire�fried bird!!

22. AC / DC!! Direct current has flow of energy in one direction only�from negative terminal to the positive Alternating current fluctuates between positive and negative at a frequency of 60 Hz in the U.S. AC is the preferred form for stationary applications since it can be transmitted long distances more easily AC can be converted to DC by use of diode rectifiers, which allow current only in the positive direction�with added capacitors, this smoothes the current.

23. Electrons in Circuits Electrons themselves do not move at great speeds in a circuit�what moves is the field�at the speed of light. The electrons also do not come from the source, but are already in the wires�the energy is what moves�like waves in water

24. Power and Energy in Electric Circuits Power, from earlier chapters, is the rate of doing work or using energy This equation relates power to the resistance and voltage across an electric device. Power is measured in Joules/second = watts Power is proportional to the current, I, and voltage, V Another equation which relates power to the resistance and voltage across an electric device: P = IV =V2/R=I2R V = voltage, R = resistance

25. Power Units The power you receive form the power companies is measured in kilowatts You are not charged for power, but for the energy you use�the energy companies charge you by how many hours you use the kilowatts�units are kilowatt-hours

26. Homework #3 Chapter 34 RQ 2, 4, 8, 9, 12, 13, 14, 16, 17, 18, 20, 25 T&E 2, 3, 6, 8, 9, 11, 12