chapter 9 static and current electricity

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Chapter 9 Static and Current Electricity. Electricity and Magnetism. The electromagnetic force is what binds electrons to the nuclei of atoms Responsible for all of chemistry When we liberate electrons from individual atoms, we can make them do work for us - PowerPoint PPT Presentation

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• Chapter 9Static and Current Electricity

• Electricity and MagnetismThe electromagnetic force is what binds electrons to the nuclei of atomsResponsible for all of chemistryWhen we liberate electrons from individual atoms, we can make them do work for usSo, the topic is extremely important if we are to understand the world around us

• Electricity and MagnetismMany of the fundamental particles have something we call electric chargeWe dont know what this is, we can only describe the results when particles have the propertyThere are two kinds of charges which we call Positive and Negative

• Electricity and MagnetismWe observe that like charges repelWe observe that unlike charges attractThe forces are inverse square law forces, just like gravity, but very much more powerfulAtoms have a positive nucleus with surrounding negatively charged electrons

• Electricity and MagnetismEach electron in an atom is identical to every other electron so they all have the same mass and the same negative chargeThe nucleus is composed of positively charged protons and uncharged neutronsAll protons are identical and the charge of the proton is exactly the same size as the charge of the electron, but it is opposite

• Electricity and MagnetismNormal atoms have identical numbers of protons and electronsAtoms with a missing electron are called positive ionsAtoms with an extra electron are called negative ions

• Coulombs LawRelationship describing the force between two charged particles

Charge is measured in coulombsForce in newtonsDistance in meters

• Coulombs Law

• Charge PolarizationMake a tiny difference in the average positions of electrons in an atomDistorts electrons like tides on the earth and pulls the nucleus to the right

• Electric FieldHow do things interact with each other when they dont physically touch?When we talked about gravity, we said that objects appear to experience a force attracting them to each other.Place a charged particle in space and it creates an electric fieldUsing Coulombs Law, we can calculate the force on a test charge at every point in space

• Electric Field

• Electric PotentialLift an object into the air and we give it gravitational potential energy

• Electric PotentialSeparate a positive charge from a negative charge and give it electric potential energy

• Electric PotentialDefine electric potential as the electric potential energy per unit charge

• Voltage SourcesIf we want to move charge from one place to another, we must apply a force to make it moveAnother way of thinking about this is to say that we must give the charges some potential energy

• Voltage SourcesWe can give an object gravitational potential energy by lifting it into the airWe have to do work on the object to lift it into the airDoing work uses energyConservation of Energy

• Voltage Sources

• Voltage SourcesWe need an pump that pumps charges!The simplest charge pump is a batteryIt uses chemical reactions to separate charges and thus create electrical potential energyMore convenient to talk about PE/charge or electric potential measured in voltsSo, a battery is a kind of electric pump

• Voltage SourcesAnother kind of source is a generatorA generator converts mechanical energy into electrical energyIn any case, what we need is a device to separate charges!!!

• Electric CurrentThe movement of charge is called a currentMetals have some electrons that are not needed to bond the atoms together in the solidThey are pretty free to just roam about the material and are not associated with any individual metal atom

• Electric CurrentCompare to the flow of water in a hose

• Electric CurrentWe measure the flow of water in gallons/minuteWe measure the flow of charges in coulombs/sec1 coulomb/sec = 1 ampereRemember this is 6.25 x 1018 electrons moving past a point in a wire per second

• Electric CurrentIn a wire, the electrons actually move quite slowly, less than 0.01 meters/secHowever, their electric field moves at the speed of light!!!So, we can send signals down a wire very quickly, because the information moves at the speed of the changing electric field which is at the speed of light

• Electric Current+-Electron flow

• Electric CurrentIn the 1700s people figured out that charges could moveThey had two choicesPositive charge movesNegative charge movesThey guessed WRONG!!!!!We know electrons move

• Electric CurrentHook up a battery and electrons flow from minus terminal to plus terminalAncients defined current as positive charge flowDirection of current is from plus to minus

• DC and ACHook up a battery and we have direct current, i.e. current flows ALWAYS in one directionModern generators in power plants reverse the polarity of the output terminals 60 times per secondThis is called alternating currentIt is what comes out of the wall plug

• DC and AC

• Electrical ResistanceSo, we hook up a wire between the terminals on a batteryThe question is, how much current flows?We have lots of free electrons able to move in the metal composing the wireSo, how much charge moves past any point in the wire per second?

• Electrical ResistanceThe amount of current that flows is determined by how much resistance there is to the flow of the chargesWhat makes this resistance?Collisions of the electrons with each other and with the stationary atoms of the metalAtoms are actually vibrating in the lattice

• Electrical ResistanceResistance depends on the structure of the material and the temperatureThe higher the temperature, the more the atoms vibrate and the more electrons make collisions with the atomsHow much current flows was discovered by George Ohm, and we honor him by naming the unit of resistance an ohm

• Ohms LawOhms discovery was that current is proportional to voltage and inversely proportional to resistance

• Ohms LawThree forms are convenient

• Electric ShockWe all know not to stick our finger into a light bulb socket or to drop an electrical appliance into our bath tubWhat causes the damage to our bodies?It is the amount of current that flows through the body that can cause problemsHuman skin has resistance ranging from 100 ohms to 500,000 ohms (wet to dry)

• Electric ShockWe can use Ohms Law to calculate current based on the size of the applied voltageYou can feel 0.001 ampere (1 milliampere)0.005 amperes hurts0.010 amperes causes muscle spasms0.015 amperes loss of muscle control0.070 amperes disrupts heart rythyms (fatal)

• Electric ShockThere must be a potential difference between one part of your body and anotherYou must become a conductor of electricityOK for birds to sit on a 5000 V transmission line as long as no part of the bird touches something elseSquirrels get across the transformers

• Electric CircuitsA circuit is a path where a current can flowIf the flow is to be continuous, the can be no gaps in the pathIntoduce gaps in the form of switches, so we can control completing a circuit Most circuits have more than one device that we want to provide with electrical energy

• Electric CircuitsThere are two ways to connect multiple devices to a voltage sourceOne is called seriesThe other is called parallelEach has unique properties which we now examine

• Series Circuits

• Series CircuitsA single pathway through the circuitThe current is the same everywhere in the circuitEach device provides resistance and total resistance is the sum of the devicesVoltage divides among the devicesVoltage drop across each device is Irdevice

• Parallel Circuits

• Parallel CircuitsEach device connects to the voltage sourceVoltage is the same across each deviceCurrent from source divides into devicesTotal current is the sum of device currentsCurrent in each device is just V/RAdd devices, lower total resistance

• CircuitsIgnore resistance of wires for calculationsJust as we ignored air resistanceOhms Law answers all questionsLets do some sample calculations

• Series Circuit Calculation12 Volt10 ohm20 ohm30 ohm

• Parallel Circuit Calculation12 Volt10 ohm20 ohm30 ohm

• Electric PowerMoving charges do workWe can heat the filament in a light bulbWe can turn the rotor in a motorThe rate at which work is done is powerElectric Power = current x voltageUnits are watts = joules/sec = amps x volts

• Electric Power

• Power Calculation12 Volt10 ohm20 ohm30 ohm

• Power Calculation12 Volt10 ohm20 ohm30 ohm

• Energy Loss in Power LinesWires have some resistanceTo transport energy from Point A to Point B, we connect wiresEach wire has resistanceEnergy Loss in one second is I2RMake I small to minimizeTransformers convert AC Voltages

• FusesLimit the current that runs through wires in your houseThese wires have some resistanceEnergy loss by I2R converts to heatHot wires can start a fireLimit the current with a fuse or circuit breaker

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