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Electricity A. Electric Charge 1. Static electricity is the accumulation of excess electric charges on an object. a. More e¯ = negative charge b. More protons = + charge 2. Charge is conserved (e¯ move from one object to another). 3. Law of Charges a. Opposite charges attract. b. Like charges repel. 4. Electric fields a. Electric fields exert force on objects within the field. b. Weaker with distance. 5. Transferring electric charge a. Conductors: e¯ move easily. b. Insulators: hold e¯ tightly. c. Contact charging is done when two materials are rubbed together (best with insulators). d. Charging by induction is done when one charged object induces a charge on another. 6. Lightning a. Large static discharge between the earth and clouds. b. Lightning was found to be static electricity by Ben Franklin. 7. Grounding a. Conductive path to Earth. b. Lightning rods & plumbing. 8. The electroscope can be used to detect electric charge B. Electric Current 1. The reason electric charge flows from one place to another is voltage. HIGH LOW a. Voltage is the difference in electrical potential between two places where e¯ are flowing. b. Voltage is the “push” that makes electric charges move. c. Measured in volts (V). 2. The flow of electric charge is called current. a. Current is measured in amperes, or amps (A). b. Voltage causes current. 3. The amount of electric charge is measured in coulombs. a. 1 coulomb is the charge carried by 6.24 x 10^18 e¯. b. 1 amp is 1 coulomb per sec. 4. Batteries are e¯ pumps. a. They provide a voltage difference to a circuit. b. Types: wet-cells & dry-cells 5. Resistance a. Opposition to the flow of e¯. b. It changes electrical energy into thermal energy and/or light. c. Measured in ohms. d. Conductors have less resistance than insulators. e. Wire resistance greater for: 1) Longer wires 2) Thinner wires 3) Higher temperatures 6. Ohm’s law Voltage Difference Current = Resistance V I R C. Electrical Circuits 1. A circuit is a conducting path. 2. Series circuit a. One path b. Any break & all devices go out c. Current is the same throughout the circuit Series Circuit Concepts (assuming 3 resistors) RTOTAL = R1 + R2 + R3 V drop for each R is VX = I • RX so, VTOTAL = I • R1 + I • R2 + I • R3 or, VTOTAL = I • (R1 + R2 + R3) since I=V÷R then… ICIRCUIT = V ÷ (R1 + R2 + R3) 3. Parallel circuit a. Multiple paths b. A break in one branch & the other branches stay on c. Voltage is the same in each branch, but current and resistance may be different Parallel Circuit Concepts (assuming 3 paths) V is the same everywhere in the circuit ITOTAL = I1 + I2 + I3 since ITOTAL = V ÷ R and I1 = V ÷ R1 then V ÷ R = V ÷ R1 + V ÷ R2 + V ÷ R3 1 1 1 1 RTOTAL R1 R2 R3 Comparison of series and parallel circuits The same voltage battery Notice the brightness of the bulbs 4. Household circuits a. Mostly parallel. b. 120 V in the U.S. c. More devices plugged in a circuit mean more current. d. More current means more heat in the wiring. 5. Electrical safety devices a. Fuses - one time use. b. Circuit breakers - can be reset and reused. D. Electric Power and Energy 1. Electrical power is the rate at which electricity is converted into another form of energy. a. Power = current x voltage b. Unit is the watt or kilowatt. 2. Electrical energy a. Energy = power x time b. Unit is the kilowatt-hour. (1000 watts for 1 hour) c. This is what we buy from the electric company. E. Light Bulbs 1. Incandescent a. Have a tiny filament that resists the flow of electrons. b. The filament gets hot and glows to produce light. c. Very hot and inefficient. 2. Fluorescent a. Filled with gas and coated with phosphor. b. Electricity excites the gas making it give off U-V rays. c. Phosphor absorbs U-V rays and glows to make light. d. Very cool, efficient, and last a long time.