Chapter 20: Electricity Section 20.1 Electric Charge and Static Electricity Section 20.2 Electric Current and Ohm’s Law Section 20.3 Electric Currents.

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Chapter 20: ElectricitySection 20.1 Electric Charge and Static ElectricitySection 20.2 Electric Current and Ohms LawSection 20.3 Electric CurrentsSection 20.1 Electric Charge and Static ElectricityLightning and static cling result from the movement of electric charges.Electric ChargeDef.-a property that causes subatomic particles such as protons and electrons to attract or repel each otherTwo types of electric charge: positive and negativeProtons (+ charge) and electrons (- charge)Section 20.1 Electric Charge and Static ElectricityElectric ChargeExamples of movement: lightening boltsExamples of attraction: clothes taken out of the dryerAlmost everything in our day to day lives is affected some way by charges.Section 20.1 Electric Charge and Static ElectricityElectric ChargeCloud of () electrons surround (+) nucleusAtom is neutral b/c #protons=#electronsAtom gains 1 or more electrons becomes a negatively charged ionAtom loses electrons become a positively charged ionKey Concept: An excess or shortage or electrons produce a net electric charge.Section 20.1 Electric Charge and Static ElectricityElectric ChargesSI Unit of electric charge= C or coulombTakes 6.24 x 1018 electrons to produce a single coulombSection 20.1 Electric Charge and Static ElectricityElectric ForcesKey Concept: Like charges repel, and opposite charges attract.Electric force-the force of attraction or repulsion between electrically charged objectsCharles-Augustin de Coulomb (1736-1806)-discovered that electric forces obey laws similar to the law of universal gravitationSection 20.1 Electric Charge and Static ElectricityElectric ForcesElectric force b/t 2 objects directly proportional to net charge on each object and inversely proportional to the square of the distance between them.Doubling net charge on one object doubles the electric forceDoubling distance between objects: electric force is one fourth as strongElectric forces inside atoms are much stronger than gravitational forcesSection 20.1 Electric Charge and Static ElectricityElectric FieldsDef.-the effect an electric charge has on other charges in the space around itKey Concept: The strength of an electric field depends on the amount of charge that produces the field and on the distance from the charge.Figure 4Section 20.1 Electric Charge and Static ElectricityElectric FieldsExert forces on any charged object placed in the fieldForce depends on: net charge in the object, strength and direction of the field at the objects positionThe greater the net charge an object has, the greater the force on it.Section 20.1 Electric Charge and Static ElectricityStatic Electricity and ChargingStatic electricity-the study of the behavior of electric charges, including how charge is transferred between objectsKey Concept: Charge can be transferred by friction, by contact, and by induction.When charge transfer occurs, the total charge is the same before and after the transfer (law of conservation of charge) Section 20.1 Electric Charge and Static ElectricityCharging by FrictionEx. Balloon attracts hairElectrons move from hair to balloon making balloon negative and hair becomes positiveEx. Walking across a carpetSection 20.1 Electric Charge and Static ElectricityCharging by ContactHair standing on endPerson acquires a charge large enough where hairs have like chargesLike charges repelSection 20.1 Electric Charge and Static ElectricityCharging by InductionEx. Walk across carpet and reach for doorknobYou pick up electrons from the carpet: hand is negatively chargedNet negative charge of hand repels the electrons in the metal doorknob Electrons move to base of doorknob=doorknob is positively charged in partDoorknob is still neutral but charge moved into itInduction-a transfer of charge without contact between materialsSection 20.1 Electric Charge and Static ElectricityStatic DischargeShock from doorknobs or other objects=result of static dischargeKey Concept: Static discharge occurs when a pathway through which charges move forms suddenly.Air becomes charged suddenly when the gap b/t your finger and the doorknob is small. Air provide path for electrons to flow from your hand to the doorknob. Spark can be seen in the dark.Section 20.1 Electric Charge and Static ElectricityStatic DischargeLightning-more dramatic dischargeFriction between moving air masses causes charge to build up in storm cloudsNegative charge in lower part of cloud induces a positive charge in the ground below itAmt. of charge in cloud , attraction b/t charges in cloud and ground Air becomes charged (forms pathway for electrons to flow from cloud to the ground)Section 20.2 Electric Current and Ohms LawElectric CurrentDef.-the continuous flow of electric chargeSI Unit of electric current-ampere (A); equals 1 coulomb/secondKey Concept: The two types of current are direct current and alternating current.Direct current (DC)-charge flows in one directionFlashlight; most battery-operated devicesSection 20.2 Electric Current and Ohms LawElectric CurrentAlternating current (AC)-a flow of electric charge that regularly reverses its directionHouses and schoolsA FlashlightFigure 8Section 20.2 Electric Current and Ohms LawConductors and InsulatorsElectrical conductor-a material through which charge can flow easilyElectrical insulators-a material through which charge cannot flow easilyMetals-have ions in a lattice that dont move, but have some electrons that a not bound to the lattice =they can conduct charge**Most materials dont conduct charge b/c they have no free electronsMetal such as copper and silver are good electrical conductors. Wood, plastics, rubber and air are good electrical insulators.Section 20. 2 Electrical current and Ohms LawResistanceDef.-opposition to the flow of charges in a material (SI unit=ohm)Key Concept: A materials thickness, length, and temperature affect its resistance.Resistance > in long wire b/c charges have to travel farther; Temp. >=more resistance b/c electrons collide more oftenSection 20.2 Electric Current and Ohms LawResistanceCan resistance be 0?Superconductor-material that has almost zero resistance when it is cooled to low temperature.Section 20.2 Electric Current and Ohms LawVoltageKey Concept: In order for charge to flow in a conducting wire, the wire must be connected in a complete loop that includes a source of electrical energy.Potential DifferenceObjects at greater height have more potential than those at a lower height. So those objects fall from higher to a lower potential energy. This is true of charges. Charges flow spontaneously from a higher to lower potential energy.Section 20.2 Electric Current and Ohms LawVoltage (Potential Difference)Potential difference of a charge depends on its position in an electric field.Def.-the difference in electrical potential energy between two places in an electric fieldMeasured in Joules/Coulombs (Volts) aka VoltageVoltage SourcesBatteries, solar cells, and generators.Have terminals (positive and negative) that connect to wires in a circuitBattery-a device that converts chemical energy to electrical energy.Section 20.2 Electric Current and Ohms LawOhms LawOhm unit-German scientist-Georg Ohm (1789-1854)1st determined how resistance and current affect voltage.Discovered voltage is not the same everywhere in a circuit; Hypothesized that resistance reduces voltage.Found a relationship b/t voltage, current, and resistanceSection 20.2 Electric Current and Ohms LawOhms LawVoltage (V) in a circuit equals the product of the current (I) and the resistance (R).V=I x R or I=V/RIf current in amps and resistance in ohms, voltage in voltsKey Concept: Increasing the voltage increases the current. Keeping the same voltage and increasing the resistance decreases the current.Section 20.3 Electric CurrentsCircuit DiagramsElectric current-a complete path through which charge can flow; wires in houses, etc. have complex networks of circuitsElectricians use circuit diagrams to monitor how elements in a circuit are connected.Key Concept: Circuit diagrams use symbols to represent parts of a circuit, including a source of electrical energy and devices that run by the electrical energySection 20.3 Electric CircuitsCircuit DiagramsShow one or more complete paths where charge can flowSwitches show where circuit can open; if open circuit not in a complete loop so current stops=open circuitIf switch closed, the circuit is complete and charge can flow=closed circuitSection 20.3 Electric CircuitsElectrons in a wire flow in the opposite direction.Series Circuits**Charge has only one path through which it can flowKey Concept: If one element stops functioning in a series circuit, none of the elements can operate.One light bulb blows it becomes an open circuit.Bulbs are a source of resistance; the more present the more resistance there is.>resistance=Section 20.3 Electric CurrentsParallel CircuitsCircuits in homes are mostly parallel.Def.-an electric circuit with two or more paths through which charges can flowKey Concept: If one element stops functioning in a parallel circuit, the rest of the elements still can operate.Allows independent operation of devices.Series and Parallel CircuitsFigure 12Section 20.3 Electric CircuitsPower and Energy CalculationsElectric power-the rate at which electrical energy is converted to another form of energyUnits of joules per second (watt, W). Power measured in watts or kilowatts (kW)Key Concept: Electric power can be calculated by multiplying voltage by current.P (watts)=I (amps) x V (volts)**Appliances vary in the amount of power they use.Section 20.3Section 20.3 Electric CurrentsElectrical SafetyInspectors check all new homes to make sure electrical wiring is installed safely.Key Concept: Correct wiring, fuses, circuit breakers, insulation, and grounded plugs help make electrical energy safe to use.The amount of current in a circuit depends on how many devices in the circuit. > # of devices turned on the > the currentIf current exceeds the circuits safety limit, wire may overheat and start a fire.Section 20.3 Electric CurrentsHome SafetyFuses prevent current overload in a circuit; wire in the center of a fuse melts if there is too much current passing through it blowing a fuse. Has to be replaced with a new one to use circuitMost homes use circuit breakers (a switch that opens when current in a circuit is too high); has to be reset for circuit to be used againSection 20.3 Electric CurrentsPersonal SafetyElectrical wiring is insulated to protect from shockDont touch electrical devices with wet handsInsulation prevents short circuits; three pronged plugs help prevent shocks from short circuitsCircular prong connects to ground; current takes easier path to ground instead of entering your bodyGrounding-the transfer of excess charge through a conductor to Earth


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