The mathematical symbol of each quantity is also significant. The “R” for resistance and the “V” for voltage are both self-explanatory, while “I” for current seems a bit odd. It is thought that the “I” should represent “intensity” (of the charge flow), and the other voltage symbol, “E”, stands for “electromotive force”. From what I have been able to do, there seems to be a debate about the meaning of the “I.” However, there are components of electrical circuits that do not obey Ohm`s law; That is, their relationship between current and voltage (their I-V curve) is nonlinear (or non-ohmic). An example is the transition diode p-n (curve to the right). As can be seen in the figure, the current does not increase linearly with the voltage applied for a diode. A value of current (I) can be determined for a given value of the applied voltage (V) from the curve, but not from Ohm`s law, since the value of the “resistance” as a function of the applied voltage is not constant. In addition, the current increases significantly only if the applied voltage is positive and not negative. The ratio V/I for a point along the nonlinear curve is sometimes called static, cordal or DC resistance,[31][32] but as shown in the figure, the value varies from total V to total I depending on the particular point along the chosen nonlinear curve. This means that the “DC resistance” V/I at a certain point on the curve is not the same as that which would be determined by applying an AC signal with peak amplitudes ΔV volts or amps ΔI centered at the same point along the curve and measuring ΔV / ΔI. However, in some diode applications, the AC signal applied to the device is small and it is possible to analyze the circuit in terms of dynamic, small signal or incremental resistance, defined as the slope of the V-I curve at the mean (DC operating point) of the voltage (i.e.

above the current dissipation relative to the voltage). For sufficiently small signals, the dynamic resistance makes it possible to calculate the low signal resistance of Ohm`s law as approximately one on the slope of a line drawn tangentially to the V-I curve at the DC operating point. [33] Technicians refer to component nameplates for standard voltage and current values. If technicians determine during the test that current readings are not being recorded on their digital multimeters or tweezers, they can use Ohm`s Law to detect which part of a circuit is shaking and determine where a problem might arise. The development of quantum mechanics in the 1920s changed this picture somewhat, but in modern theories it can still be shown that the average drift velocity of electrons is proportional to the electric field, thus deriving Ohm`s law. In 1927, Arnold Sommerfeld applied the Fermi-Dirac quantum distribution of electronic energies to the Drude model, which led to the free-electron model. A year later, Felix Bloch showed that electrons move by waves (Bloch electrons) through a solid crystal lattice, so that scattering of lattice atoms, as postulated in Drude`s model, is not an important process; Electrons disperse impurity atoms and defects in the material. The latest successor, the modern theory of quantum bands of solids, has shown that electrons in a solid cannot absorb energy, as assumed in Drude`s model, but are limited to bands of energy, with spaces between them of energies that electrons are not allowed to have. The size of the band gap is a characteristic of a particular substance that has a lot to do with its electrical resistance, which is why some substances are electrical conductors, some semiconductors, and some insulators. If you know E and I and want to determine R, just remove R from the picture and see what`s left: It was only after considerable effort and in the second attempt that he managed to develop what we know today as Ohm`s law.

To get a first idea of what is happening, it is possible to compare the electrical situation with that of the water flow in a pipe. The voltage is represented by the water pressure of the pipe, the current is represented by the amount of water flowing through the pipe, and finally the resistance is the equivalent of the size of the pipe. As mentioned earlier, voltage is the measure of potential energy per unit charge available to motivate the flow of current from one point to another. Before we can define exactly what a “volt” is, we need to understand how to measure this amount, which we call “potential energy.” The general metric unit for energy of any kind is the joule, equal to the amount of work done by a force of 1 newton exerted by a movement of 1 meter (in the same direction). Each equation is cited by some sources as the determining relation of Ohm`s law,[2][23][24] or all three are cited,[25] or derived from a proportional form,[26] or even only the two that do not correspond to Ohm`s original statement can sometimes be given. [27] [28] Example 1: The voltage (E) and resistance (R) are known.