Current and ResistanceNotesSlide 3Slide 4Slide 5Physical ResistorsWhat Happens?DEFINITIONSlide 9UNITSSlide 11ANOTHER DEFINITIONSlide 13OhmGraphA DIODELet’s look at the atomic level ..Slide 18Consider a metal conductorSlide 20The CurrentNotationSlide 23ReferenceConductivityGoing to the usual limit …ExampleSlide 28Range of r and sYe old RESISTANCEAdditional TopicsCurrent and ResistanceCurrent and ResistanceFebruary 22, 2006February 22, 2006Notes Notes New topic today – Current and New topic today – Current and ResistanceResistanceExam – Friday – March 3 Through Exam – Friday – March 3 Through Chapter 27.Chapter 27.No quiz on Friday.No quiz on Friday.Watch for new WebAssign.Watch for new WebAssign.A capacitor is constructed from two square plates of sides ℓ and separation d. A material of dielectric constant κ is inserted a distance x into the capacitor, as shown in the figure. Assume that d is much smaller than x. (a) Find the equivalent capacitance of the device. (b) Calculate the energy stored in the capacitor, letting ΔV represent the potential difference. (c) Find the direction and magnitude of the force exerted on the dielectric, assuming a constant potential difference ΔV. Ignore friction. (d) Obtain a numerical value for the force assuming that ℓ = 5.00 cm, ΔV = 2 000 V, d = 2.00 mm, and the dielectric is glass (κ = 4.50).Each capacitor in the combination shown in Figure P26.49 has a breakdown voltage of 15.0 V. What is the breakdown voltage of the combination?Current and ResistancePhysical ResistorsWhat Happens?“+”“+”“+”“+”IN TRUTH, THE ELECTRONSARE ACTUALLY MOVING THEOTHER WAY!--DEFINITIONCurrent is the motion of CHARGE through a circuit. Physically, it is electrons that move but …We define current as the motion of POSITIVE charge! (Blame you know who!)Conducting materialQ,tConducting materialQ,tdtdqiortQiCURRENTUNITSA current of one coulomb per second is defined as ONE AMPERE.A small sphere that carries a charge q is whirled in a circle at the end of an insulating string. The angular frequency of rotation is ω. What average current does this rotating charge represent?ANOTHER DEFINITIONAIareacurrentJ Figure P27.8 represents a section of a circular conductor of non-uniform diameter carrying a current of 5.00 A. The radius of cross section A1 is 0.400 cm. (a) What is the magnitude of the current density across A1? (b) If the current density across A2 is one-fourth the value across A1, what is the radius of the conductor at A2?OhmA particular object will resist the flow of current.It is found that for any conducting object, the current is proportional to the applied voltage.STATEMENT: V=IRR is called the resistance of the object.An object that allows a current flow of one ampere when one volt is applied to it has a resistance of one OHM.GraphA DIODEResistance Varies with Applied Voltage (actually with current)Let’s look at the atomic level ..Conduction is via electrons.They are weak and small and don’t exercise much.Positive charge is big and strong and doesn’t intimidate easily.It’s an ugly situation … something like ……-+Consider a metal conductorSo far, we have said that a metal is an equipotential because no charges were moving.Hence, no electric field in the metalYou can move a charge freely in the metal BECAUSE there is no electric field.NOW we have a current.This can only happen if we allow an electric field to push the charges.Thus, the metal is NO LONGER A TRUE EQUIPOTENTIAL.But almost …. as we shall see in the next chapter.lVVabEThe CurrentElectrons are going the other way. They probably follow a path like …Average “drift”speed - vdNotationvdaverage drift velocity of the electronn number of electrons (mobile) per unit volume.t interval of timex average distance the electron moves in time t.Q total amount of CHARGE that goes through a surface of the conductor in time t.dvJ nenevAIJenAvtQIetnAvQdavgdavgd )(Reference The average drift velocity of an electron is about 10-4 m/sConductivityIn metals, the bigger the electric field at a point, the bigger the current density.EJ is the conductivity of the material.=(1/) is the resistivity of the materialGoing to the usual limit …JdAIanddAdIJExampleA cylindrical conductor of radius R has a current density given by(a) J0 (constant)(b) rFind the total current in each case. )(100TT Range of and Ye old RESISTANCEIRVVAlRIAlVAIlVEElVEJElV11Additional TopicsPowerMicro basis of
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