1.1.2 Potential Difference

Chapter 1.1.2 Potential Difference

Physics Lecture Notes – Phys 395 Electronics Book
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Physics Lecture Notes – Phys 395 Electronics Book

  • CHAPTER 1. DIRECT CURRENT CIRCUITS71.1.2Potential DifferenceIt is often more convenient to consider the electrostatic potential V rather than electricfield E as the motivating influence for the flow of electric charge. The generalized vectorproperties of E are usually unimportant. The change in potential dV across a distance drin an electric field isdV =−E · dr.(1.2)A positive charge will move from a higher to a lower potential. The potential is alsoreferred to as the potential difference or, incorrectly, as just voltage:V = V21 = V2− V1 =V2V1dV.(1.3)Remember that current flowing in a conductor is due to a potential difference betweenits ends. Electrons move from a point of less positive potential to more positive potentialand the current flows in the opposite direction.The SI unit of potential difference is the volt (V).1.1.3Resistance and Ohm’s LawFor most materialsV∝ I;V = RI,(1.4)where V = V2−V1 is the voltage across the object, I is the current through the object, andR is a proportionality constant called the resistance of the object. Resistance is a functionof the material and shape of the object, and has SI units of ohms (Ω). It is more commonto find units of kΩ and MΩ. The inverse of resistivity is conductivity.Resistor tolerances can be as bad as±20% for general-purpose resistors to±0.1%for ultra-precision resistors.Only wire-wound resistors are capable of ultra-precision applications.The concept of current through and potential across are key to the understanding of andsounding intelligent about electronics.Now comes the most useful visual tool of this course.1.2The Schematic DiagramThe schematic diagram consists of idealized circuit elements each of which represents someproperty of the actual circuit. Figure 1.1 shows some common circuit elements encounteredin DC circuits. A two-terminal network is a circuit that has only two points of interest, sayA and B.