Static and Current Electricity

Static and Current ElectricityESCI 215Chapter 7Historical Background600 BC Greeks found that amber (tree resin) attracted dust, lint and paper when rubbed together1672 AD a German physicist, Otto Van Guericke, built a machine that would charge a rod by rubbing itIn 1745 E.G Kleist and in 1746 Pieter von Musschenbroek built devices to store electricityThe Leyden jar or electrical condenserToday a devise that stores electricity is called a capacitorWhat is Lightning?Benjamin Franklins lightning experimentHe stood on dry ground holding a piece of dry string, attached to thin wire that was attached to a kiteHe touched an empty Leyden jar to a key that hung from the wireHe was finding out if the energy from the clouds that would fill the jar was the same as the energy in his lab (where he rubbed amber with wool)They were the sameLightning is a powerful form of static electricityCaution this experiment is too dangerous to try

Lightning RodFranklins discovery lead to the invention of the lightning rodAn object with a pointed tip loses its charge quicklyA rod mounted on a roof and connected to a cable that is anchored deep in the groundHow Lightning Rods Protect:Electrons escape from tip of rod so the building does not get a charge, making a lightning strike less likelyIf lightning strikes, it follows the cable to the ground and does not hit the building

Static ElectricityA charge that can be released only in a single instant (not controlled)Causes of Static Electricity:Substances gain and lose electrons differentlyElectrons move from one substance to another when these substances are close together (i.e. Rubbed)When an object has an equal # of electrons, it has no static charge (neutral)If it gains electrons, it has a negative chargeIf it loses electrons, it has a positive chargeLaws of Static ElectricityCharged objects attract uncharged objects

Like charges repel

Unlike charges attract each otherCharged Objects Attract Uncharged ObjectsAlmost anything (even liquids and gases) can get a charge when rubbed and almost anything is attracted by static electricityEvent 7-A The Balloon and its Invisible Shelf Rubbing balloon with wool gives it a negative charge or with silk for a positive charge (either works) Attracted to uncharged wall for hours because the electrons do not move well on the balloon (poor conductor)Slowly the electrons move to the wall until the charges are neutralized

Charged Objects Attract Uncharged ObjectsEvent 7-B Make the Water Bend Negatively charged comb attracts uncharged water streamEvent 7-C Pepper and SaltSalt and pepper are both attracted to the comb, but can be separatedLighter pepper grains are attracted firstSalt tends to drop off the comb first with tappingEvent 7-D Twist the StrawThe hanging straw is attracted to the charged combLike Charges Repel2 objects with negative charges will repel2 objects with positive charges will repelEvent 7-E Attraction or Repulsion demonstrates thisCharged comb attracts uncharged ballCharge moves from comb to ball when they touch giving the ball the same chargeBall repels the combCharged ball is attracted to your uncharged handBall is discharged (neutralized) when it touches hand (electrons from ball are absorbed by large body)Sequence repeatsBall can be: ping pong ball sprayed with aluminum paint, carbon end of burnt match, puffed wheat, bits of styrofoam Like Charges RepelEvent 7-F Fill the Stocking Nylon stocking is charged by rubbing a piece of polyethylene (stretchy plastic bag) along itStocking fills out because all the fibers in it have the same charge and repel each otherStocking is neutralized when pulled through your hand Like Charges Repel - ElectroscopeElectroscope a device that can detect static charges

Event 7-G Leaping LeavesTouching an electroscope with a charged comb causes the leaves to repel because the charge is transferred to themEvent 7-H Newspaper ElectroscopeMake an electroscope by rubbing newspaper with woolNewspaper sticks together then jumps apart

Unlike Charges Attract2 objects with opposite charges will attract each other

Event 7-I Unlike Charges Attract #11st pith ball gets a negative charge when touched by a glass rod and 2nd pith ball gets a positive charge when touched by a plastic rodThese balls are attracted to each other more than they are attracted to uncharged objectsEvent 7-J Unlike Charges Attract #22 balloons with opposite charges attract

How to Build an ElectroscopeMaterials: bottle, cork or rubber stopper, wire coat hanger, foilMethods: Heat bottle in oven on low heat to dryInsert wire through corkSeal where the wire goes through cork with waxPut foil over the end of wire to make the leavesPut a ball of tin foil on the top of wireUse a charged object to repel the leaves

Discovery of Current Electricity1700s an Italian doctor, Luigi Galvani, noticed a frogs leg twitched during a dissection when a nearby static electricity machine sparkedGalvani believed the twitch came from in the frog and was related to electricityGalvani called it animal electricityItalian physicist Alessandro Volta believed the source of the twitch was outside the frogVolta proved that muscular twitches could only happen when the dissecting knife was metal the twitches were caused by the metal not the legVolta called it contact electricity

Animal versus Contact Electricity DebateGalvani made the frog twitch by touching one nerve end to another (no metal) it must be the animalVolta touched two oppositely charged metals together in liquid and produced even more energyVolta discovered current electricityGalvani was right tooCells in the body have an electrical currentElectricity regulates the beating of our heartWhat is Current Electricity?A continuous controlled flow of electrons that move due to an electrical pressure

Some uses: heatlight motors electronics

How do Electrons Flow?Electrical current moves along a conductor quickly, but the individual electrons do not move much

Event 7-L The Mysterious MarbleThe 1st marble hits the row and the end marble jumps ahead, but the others barely move

Electron FlowElectrical currents can: be large or small have high pressure or low pressureFlow easily through some substances and not through others

Caution: Do not use the current from wall plugs to demonstrate electricity current is too strong

Terms to Describe Flow of CurrentTermDefinedCompared to waterVoltPotential difference or electrical pressure(V = amp x ohm)Similar to water pressure in pounds per square inchAmpere (Amp)Unit of current flow(1 amp lights a 100-watt bulb)Similar to liters per secondOhmResistance depends on the composition, length, thickness, and temperature of a substanceSimilar to the resistance of water moving through a pipeCoulombA specific quantity of charge Similar to a quantity of water (5 liters)What is a Conductor?Electrical current does not travel through all materialsConductors substances that are good carriers of electricityAllows current to flowAll metals, but copper and aluminum are bestGold and silver are better than metal used in space satellites and some computersNonconductors substances that are very poor carriers of electricity (also called insulators)Paper, rubber, plastic, glass, woodEvent 7-M Conductor Tester allows students to determine what materials are good conductorsAtoms and ElectronsMaterials that are good conductors are made of atoms that have a number of free or loosely held electronsCopper (Cu) atom has 29 electronsOnly 1 electron in the outer shell so it is not strongly attached to the atom and is easily pulled awayThis electron leaves and joins another atom, transmitting electrical energy

Sources of ElectricityThere are many different sources of electricity:Chemical (or electrochemical)Dry cell, potato battery, wet cell, silver dime, pennyMechanical (electromotive)Coil and magnetHeat (electrothermal)ThermocoupleLight (photoelectric)Electric eye, photo cell, solar cell, light meterPressure (piezoelectric)Crystal detectors fro radios, microphones, headsetsLight and pressure sources of electricity are too difficult for elementary classroomsChemical Sources of ElectricityThe first kind of current electricity discoveredChemical energy comes from differences in the atoms of substances (atomic differences)Some substances lose electrons easily and some substances gain electrons easilyWhen electrons move from the atom of 1 substance to the atom of another substance there is an electrical currentEvent 7-N Current from Coins Silver dimes or metals like zinc, aluminum, brass, tin produce the best currentsChemical Sources of ElectricityEvent 7-O Potato Power shows electricity from a chemical source Moisture in the potato conducts the current produced by dissimilar metals Dissimilar metals 1 type of metal attracts electrons and the other type loses electronsNote this requires a voltage sensor Mechanical Sources of ElectricityThe most common source of electricity todayGenerated in power stations and sent to our houses Event 7-P Current from a Coil shows electricity from a mechanical sourceA magnet is pushed through a coil and pulled out creating a currentWhen the magnet is reversed the reading on the voltage sensor is reversedThis creates a alternating current (AC)All other sources of electricity produce a direct current (DC)CircuitsCircuit a complete path that current travels along; from the energy source through a conductor, back to the energy sourceEvents 7-Q Nervousness Tester and 7-R Light the Bulb are simple circuits that do something so students have proof that something is happening (ie. Electrical current)

CircuitsOpen circuit the path of the electrical current is interruptedWhen a light switch is turned off, the circuit opens and cuts off the flow of electrical current turns light offClosed circuit the path of the electrical current is completeWhen a light switch is turned on, the circuit closes and allows the electrical current to flow turns light onEvent 7-S Stop and Go Signals for Electricity Students have to make a switch that opens and closes the circuitInserting a piece of foil between the wires acts as a switch

Short CircuitsAn electrical circuit that allows a current to travel along an unintended path where there is usually low resistance to the currentDecreased resistance = increased current It can cause circuit damage, overheating, fire, explosionExample: a damaged toaster cord causes a short circuitCurrent cannot complete the full circuit and returns to plug with less resistanceThe cord becomes hot

Fuses and Circuit BreakersAutomatic switches that turn off the current when it is too high; protecting against short circuits

FuseCurrent has to travel through it, along a metal filamentHigh current melts the metal so current no longer passes Must be replaced Circuit Breaker Current passes across an electromagnetHigh current increases magnetic force and causes a magnetic switch to be turned off, breaking the currentFlip switch back on

FusesEvent 7-T Pop the Fuse demonstrates how a fuse worksGum wrapper of foil fuse melts with high current

Fig 7.17 page 120Series Wiring in BulbsWhen we turn on a 2nd lamp in our house, the light from each lamp becomes dimmer. Why?Series wiring current goes through each bulb in the series; causing dimmer lightCurrent enters a bulb and encounters resistance while traveling through fine wire inside the lightIf 2 bulbs are connected to same current, current has 2x more resistance as it travels through both bulbsLess current gets through Less light is producedOld Christmas lights would not work if 1 bulb was out

Parallel Wiring in BulbsParallel wiring current has alternate paths and the current that goes through 1 bulb does not go through the others; causes brighter light Current moves from (-) to (+)Different current enters each branch of circuit Current travels through its bulb and back to cellCurrent continues through the circuitNo electron goes through more than 1 bulb, so resistance is not increasedMore light is produced than if these lights were wired in a series

Series and Parallel Wiring in CellsThe wiring of batteries (cells) works opposite of bulbsSeries wiring in cells produces more light than parallel wiring in cells. Why?The energy of the 2 cells is combined in series wiringTwo 1.5 volt cells produce 3 volts when wired in a series The energy of the 2 cells remains the same in parallel wiring, but they will last 2 times longerTo produce the brightest light, what type of cell wiring and bulb wiring should you use?Series wiring for cells and parallel wiring for bulbs

Series and Parallel Circuit EventsEvent 7-U Series and Parallel Circuits Students compare the effects of series and parallel circuits on the brightness of the light bulbAppropriate for older elementary students Event 7-V Quiz Board Students answer the math questions on the board and the light goes on if they are correctStudents figure out the wiring for the board and draw it in a diagramThe board only uses series wiring When answered correctly, the circuit is closed and light goes onUses of ElectricityElectricity has many uses:Produce heat in toasters, ovens, ironsProduce light in light bulbsTo do work in electromagnets and motorsTo do work in sewing machinesEvent 7-W Power-Packed Performer shows that electricity can be used to produce heat and lightElectrical energy from a battery heats the fine wires in steel wool and it burnsCaution: Only teacher should do thisUses of ElectricityElectricity and magnetism are relatedElectric current produces a magnetic field around the conductorCompass needle will jump when put close to an electric wire when current is turned on and offEvent 7-X Stop and Go Magnets shows this magnetic fieldMake an electromagnet by wrapping insulated wire around a nail and attaching the ends to a cell (battery)Wrapping the wire around the nail adds more magnetism up to a point, then more wrapping makes it weaker Electromagnet will collect the paper clips when the electric current is turned on Assessment and InstructionStudents select and build a model of one of the following devices: 3-way switch house, dimmer switch, automatic traffic light, model telegraph, electroscope (see information on page 127-128)Student prepare a report for their project that includes:Written description of the device (model)A schematicAdvice on how to build it for another student to useList of help they got from other people/resources during projectList of materials and designs they tried that did not work and whyList of everyday uses for this deviceAssessment and InstructionEvaluation:How well does the device work?Neatness of construction, durabilityNeatness of reportQuality of reportDoes it logically describe the requirements listed? Websiteswww.fi.eduwww.madsci.org/experimentswww.howstuffworks.com/vdg.htmwww.howstuffworks.com/wildfire.htmwww.howstuffworks.com/circuit-breaker.htmwww.iii.edu/~smile/physinde.html#p4www.howstuffworks.com/electromagnet.com

DiscussionWhere does this topic fit into the Science curriculum?Which grades and strands?Which curriculum objectives relate to the discrepant events?