5.1 - Potential Difference, Current and Resistance 1
How can we model electric circuits?A model can helpus to understandhow current worksin an electric circuit.In this model, themoped ridersrepresent the flowof charge and thepizzas representthe electricalenergy carriedaround the circuit.What do the pizza shop and thehouse of party-goers represent?
Potential DifferenceThe easiest way to thinkabout what batteries do isto use a water analogy.Batteries ‘lift’ charges (Q)to a higher Potential (V).There is a PotentialDifference (V) betweenone end of the battery andthe other.Batteries store PotentialEnergy as ChemicalEnergy. 3
Battery Voltage Simulator on PhET website This simulator shows another way of imaging what batteries do.http://phet.colorado.edu/en/simulation/battery-voltage or click the pictures
Water model of a circuitThis page has embedded flash which only works in Powerpoint.
What are Coulombs?Because charge is made out of electrons which are verysmall, it seems silly to measure charge in electrons becausethe numbers of charges that go round a circuit would bebillions and billions.Instead Charge (Q) is measured in Coulombs (C)Using this scale 1 electron is only: 1.6x10-19 C1 Coulomb is: 6,250,000,000,000,000,000 electrons Remember this number 6
Current (I)Batteries ‘lift’ charges toa higher potential.The charges then flowaround the circuit.The flow of charges persecond is called:current. ChargeCurrent Time 7
What is conventional current?Before the discovery of the electron, scientists assumed that current was due topositively-charged particles moving from the positive terminal around a circuit tothe negative terminal.This way of representing thedirection of current is calledConventional Current.It is now know that charge iscarried by electrons, flowingfrom the negative terminalto the positive terminal.This is called electron flow.Today, both conventional current and electron flow can beused to represent the direction of current.
Potential Difference (V) ..sometimes known as VoltageBatteries ‘lift’ charges to ahigher potential.There is a PotentialDifference because eachcoulomb of charge has adifferent potential energy ateither end of the battery. EnergyPotentialDifference Charge 9
Electromotive Force(EMF) and Potential Difference:Potential Difference (V) Electromotive Force The total amountThe total amount of Chemical Energyof Electrical Energy in the batterytransferred to Heat transferred toby each Coulomb Electrical Energyof charge by each Coulomb of charge 10
How do metals conduct electricity? It is the delocalized electrons involved in metallic bonding that allow metals to conduct electricity. The delocalized electrons are free to flow through the metal and so carry a current. Insulating materials do not contain free electrons and so current is unable to flow. Ionic solutions are also able to conduct electricity because they have mobile charge-carrying particles. delocalized electrons
Resistance (R)Some materials are better than others at allowing current toflow.A material that doesn’t let much current flow for a givenPotential Difference is said to have a high Resistance.This resistance depends on the material and the dimensions ofthe conductor 13
Factors affecting Resistance on PhET website http://phet.colorado.edu/sims/resistance-in-a-wire/resistance-in-a-wire_en.html or click the picture. 14
Why does Length affect resistance?The affect of length of a wire on resistance can beunderstood by looking at the atomic structure.Resistance is caused by electrons colliding with metal ions.When the length of the wire is increased, the electronshave to travel further. So the chance of collisions willincrease, causing the resistance to increase.
Why does Cross Sectional Area affect resistance? Increasing the thickness of a wire increases the cross sectional surface area that the electrons can flow through. This decreases the chance of collisions with metal ions. In thick materials the charge carrying particles are able to move through the conductor more easily, reducing resistance.
What is Resistivity?Resistivity is just a property of the conductor.Every material has a resistivity.It is actually the resistance of a 1m long piece of wire with across-sectional area of 1m2.As you can imagine this is always a very low number.For Copper ρ = 1.72 x 10-8 Ωm Units = Ωm
Battery Resistance Simulator on PhET website http://phet.colorado.edu/en/simulation/battery-resistor-circuit 18
Ohms LawOhms law relates the current flowing through a conductorwith the potential difference across it. V∝I V=IR R is the constant of proportionality between I and V 19
Both these circuits have 2 Volts per Amp or a resistance of 2Ω 20
Ohms Law simulator on the PhET websitehttp://phet.colorado.edu/sims/ohms-law/ohms-law_en.html or click on the picture 21