2The Origins of Electricity In the mid 18th century Ben Franklin created the idea of positive and negative electric charge.It wasn’t until 150 years later the electron was discovered.Franklin described an electric “fluid” that would flow depending on electric pressure.
3Electric ChargeIn 1909 Robert Millikan discovered charge was “quantized.”This means there is a smallest amount.Thing about it like this, In order to have a car, all the pieces must be there. If you try to take the engine out, it’s no longer a car.Electric charge is much the same.The electron has a set charge, take some away and it’s no longer an electron.
4The Millikan Experiment Click here to recreate the Millikan oil-drop experiment
5Charged ParticlesProtons (+e): Mass = x kg, Charge = 1.60 x CNeutron: Mass = x kg, Charge = 0Electron (-e): Mass = 9.11 x kg, Charge = x Ce = 1.60 x C
6Neutral ObjectsIf the number of electrons equals the number of protons the object is said to be electrically neutral.In general q (charge) = Ne, where N is an integer.Since protons are much more difficult to remove, most objects are charged by removing or adding electrons.
7Charged ObjectsWhen two dissimilar materials are rubbed together, electrons usually go from one to the other.Look on the triboelectric scale to see which way they go, positive or negativeAlso, charge is conserved.The net charge of an isolated system is constant.
8Like and Unlike charges Like charges repel each otherUnlike (opposite) charges attract
9Conductors & Insulators Materials that have loose valence electrons are conductors.Materials with tightly held valence electrons are insulators.Can you think of some?
10Charging by Induction (Conductors) Click on the picture to open an applet
11Example QuestionTwo separated, identical conducting spheres are charged with 4 mC and -12 mC, respectively. If the spheres are allowed to touch and then separated again, what will be the charge on each sphere?Answer:The net charge is -8 mC. So each sphere will have -4 mC of charge.
15Coulomb’s Law F = 1/(4peo) q1q2 / r2 1/(4peo) = k F = Force (N) eo = 8.85 x (electric permittivity of a vacuum)q = charge (Coulombs)r = distance between charges1/(4peo) = k
16Point ChargesWhen more than two charges are acting on each other we sum the forces.Treat each pair independently, then add the forces.r1r2q1q2q3
17Point Charges in 2DWhen more than two charges are acting on each other in 2D, sum the forces for x and y dimensions.Again, treat each pair independently, then add the forces.q3r2q1q2r1
18Electric FieldJust like mass creates gravitational fields, charges create electric fieldsWith gravity the field strength is measure as Newton per kilogramWhat do you think Electric fields are measured in?Newton's per Coulomb
19Measuring the Electric Field If the unit is Newton's per Coulomb, what is the equation?E = F / qo = k q/ r2Simple enough, right.E = Electric FieldF = Forceqo = charge producing field
20Summing electric Fields It is the surrounding charges that create an electric field at a given point in space.Look at Example 8 – Figure 18.18Two charged objects contribute as follows to the net electric at point P: Ea = 3.00 N/C directed to the right, and Eb = 2.00 N/C directed downward. What is the net electric field at P?
21Solution We use the electric field vectors to determine the resultant. Add the vectors with the Pythagorean theorem.And find the angle with arctan.E = o+EA++EBE
22Electric Field RulesFields start at positive and end at negative, or start or end at infinity.This is by convention. The field is said to predict the movement of a positive charge.The density of lines should represent the strength of the field.A positive charge will have a velocity tangent to a field line.Field lines do not actually exist since the are an infinite number of paths a test charge can take.
23Picturing the Electric Field Click on the picture to open an applet
24Electric Fields Inside Conductors Excess electric charge moves to the surface of a conductorAt equilibrium the electric field inside a conductor is zeroThis comes from the fact that free electrons will not move inside the conductor.So the electric field lines don’t penetrate the conductorThe electric field outside a conductor is perpendicular to the surfaceClick here for more info