OHM's LAW

Ohm's law states that the current through a conductor between two points is directly proportional to the potential difference across the two points. Introducing the constant of proportionality, the resistance,^[1] one arrives at the usual mathematical equation that describes this relationship:^[2]

where I is the current through the conductor in units of amperes, V is the potential difference measured across the conductor in units of volts, and R is the resistance of the conductor in units of ohms. More specifically, Ohm's law states that the R in this relation is constant, independent of the current.^[3]
The law was named after the German physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current through simple electrical circuits containing various lengths of wire. He presented a slightly more complex equation than the one above (see History section below) to explain his experimental results. The above equation is the modern form of Ohm's law.
In physics, the term Ohm's law is also used to refer to various generalizations of the law originally formulated by Ohm. The simplest example of this is:

where J is the current density at a given location in a resistive material, E is the electric field at that location, and σ is a material dependent parameter called the conductivity. This reformulation of Ohm's law is due to Gustav Kirchhoff



In circuit analysis, three equivalent expressions of Ohm's law are used interchangeably:

Each equation is quoted by some sources as the defining relationship of Ohm's law, or all three are quoted,or derived from a proportional form, or even just the two that do not correspond to Ohm's original statement may sometimes be given.
The interchangeability of the equation may be represented by a triangle, where V (voltage) is placed on the top section, the I (current) is placed to the left section, and the R (resistance) is placed to the right. The line that divides the left and right sections indicate multiplication, and the divider between the top and bottom sections indicates division (hence the division bar).

Resistive circuits

Resistors are circuit elements that impede the passage of electric charge in agreement with Ohm's law, and are designed to have a specific resistance value R. In a schematic diagram the resistor is shown as a zig-zag symbol. An element (resistor or conductor) that behaves according to Ohm's law over some operating range is referred to as an ohmic device (or an ohmic resistor) because Ohm's law and a single value for the resistance suffice to describe the behavior of the device over that range.
Ohm's law holds for circuits containing only resistive elements (no capacitances or inductances) for all forms of driving voltage or current, regardless of whether the driving voltage or current is constant (DC) or time-varying such as AC. At any instant of time Ohm's law is valid for such circuits.
Resistors which are in series or in parallel may be grouped together into a single "equivalent resistance" in order to apply Ohm's law in analyzing the circuit. This application of Ohm's law is illustrated with examples in "How To Analyze Resistive Circuits Using Ohm's Law" on wikiHow.

Reactive circuits with time-varying signals

When reactive elements such as capacitors, inductors, or transmission lines are involved in a circuit to which AC or time-varying voltage or current is applied, the relationship between voltage and current becomes the solution to a differential equation, so Ohm's law (as defined above) does not directly apply since that form contains only resistances having value R, not complex impedances which may contain capacitance ("C") or inductance ("L").
Equations for time-invariant AC circuits take the same form as Ohm's law, however, the variables are generalized to complex numbers and the current and voltage waveforms are complex exponentials.^[26]
In this approach, a voltage or current waveform takes the form , where t is time, s is a complex parameter, and A is a complex scalar. In any linear time-invariant system, all of the currents and voltages can be expressed with the same s parameter as the input to the system, allowing the time-varying complex exponential term to be canceled out and the system described algebraically in terms of the complex scalars in the current and voltage waveforms.
The complex generalization of resistance is impedance, usually denoted Z; it can be shown that for an inductor,

and for a capacitor,

We can now write,

where V and I are the complex scalars in the voltage and current respectively and Z is the complex impedance.
This form of Ohm's law, with Z taking the place of R, generalizes the simpler form. When Z is complex, only the real part is responsible for dissipating heat.
In the general AC circuit, Z varies strongly with the frequency parameter s, and so also will the relationship between voltage and current.
For the common case of a steady sinusoid, the s parameter is taken to be , corresponding to a complex sinusoid . The real parts of such complex current and voltage waveforms describe the actual sinusoidal currents and voltages in a circuit, which can be in different phases due to the different complex scalars.

Linear approximations

See also: Small-signal modeling and Network analysis (electrical circuits)#Small signal equivalent circuit

Ohm's law is one of the basic equations used in the analysis of electrical circuits. It applies to both metal conductors and circuit components (resistors) specifically made for this behaviour. Both are ubiquitous in electrical engineering. Materials and components that obey Ohm's law are described as "ohmic" which means they produce the same value for resistance (R = V/I) regardless of the value of V or I which is applied and whether the applied voltage or current is DC (direct current) of either positive or negative polarity or AC (alternating current).
In a true ohmic device, the same value of resistance will be calculated from R = V/I regardless of the value of the applied voltage V. That is, the ratio of V/I is constant, and when current is plotted as a function of voltage the curve is linear (a straight line). If voltage is forced to some value V, then that voltage V divided by measured current I will equal R. Or if the current is forced to some value I, then the measured voltage V divided by that current I is also R. Since the plot of I versus V is a straight line, then it is also true that for any set of two different voltages V₁ and V₂ applied across a given device of resistance R, producing currents I₁ = V₁/R and I₂ = V₂/R, that the ratio (V₁-V₂)/(I₁-I₂) is also a constant equal to R. The operator "delta" (Δ) is used to represent a difference in a quantity, so we can write ΔV = V₁-V₂ and ΔI = I₁-I₂. Summarizing, for any truly ohmic device having resistance R, V/I = ΔV/ΔI = R for any applied voltage or current or for the difference between any set of applied voltages or currents.

The I–V curves of four devices: Two resistors, a diode, and a battery. The two resistors follow Ohm's law: The plot is a straight line through the origin. The other two devices do not follow Ohm's law.

There are, however, components of electrical circuits which 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 p-n junction diode (curve at right). As seen in the figure, the current does not increase linearly with applied voltage for a diode. One can determine a value of current (I) for a given value of applied voltage (V) from the curve, but not from Ohm's law, since the value of "resistance" is not constant as a function of applied voltage. Further, the current only increases significantly if the applied voltage is positive, not negative. The ratio V/I for some point along the nonlinear curve is sometimes called the static, or chordal, or DC, resistance,but as seen in the figure the value of total V over total I varies depending on the particular point along the nonlinear curve which is chosen. This means the "DC resistance" V/I at some point on the curve is not the same as what would be determined by applying an AC signal having peak amplitude ΔV volts or ΔI amps centered at that 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 the dynamic, small-signal, or incremental resistance, defined as the one over the slope of the V–I curve at the average value (DC operating point) of the voltage (that is, one over the derivative of current with respect to voltage). For sufficiently small signals, the dynamic resistance allows the Ohm's law small signal resistance to be calculated as approximately one over the slope of a line drawn tangentially to the V-I curve at the DC operating point.

Insulation Materials

Materials called insulators or insulation material is often an ingredient that is used in order to separate the parts - parts that voltage or part - the part that is active . So for this insulator material to note about the properties of these materials , such as electrical properties , mechanical properties , thermal properties , resistance to chemicals and others.

1 . Electrical Propertieswhich is a material that has a large electrical resistivity in order to prevent the propagation or conducting electrical current leakage between the different voltage or the ground .

2 . Mechanical properties
Given the extent of the use of insulating materials , it is necessary to consider its power can be limited so that these things cause damage as a result of misuse . Example requires materials that are resistant to the pull , the selected materials of cloth instead of paper because it is more powerful than the other papers .

3 . Thermal propertiesThe heat caused by the material due to the flow of electric current or a magnetic force to the insulating effect including the effect of heat from the outside surroundings. Occurs when the heat is high enough , it is necessary to use proper insulation so that the heat does not damage the insulation must .FORM insulatorInsulator In Electric Materials are divided into 3 :a. Solid insulatorb . Liquid insulatorc . Gaseous insulators

SUPERCONDUCTOR MATERIALS

A. History of SuperconductorsSuperconductors were first discovered by a Dutch physicist , Heike Kamerlingh Onnes , of the University of Leiden in 1911 . On July 10 , 1908, Onnes succeeded in liquefying helium by cooling to 4 K or 269oC . Then in 1911 , Onnes began studying the electrical properties of metals at extremely cold temperatures . At the time it was known that the resistance of a metal will fall when cooled below room temperature , but no one can know how the lower bound constraints is achieved when the metal temperature approaches 0 K or absolute zero . Some scientists at that time as William Kelvin predicted that electrons flowing in the conductor will stop when the temperature reaches absolute zero . On the other hand , others including Onnes scientists estimate that the barriers will disappear in these circumstances . To find out what actually happened, Onnes then drain current in a very pure mercury wire and then measure the resistance while lowering the temperature . At a temperature of 4.2 K , Onnes get resistance suddenly disappeared . The current flowing through the wire mercury constantly.In the absence of obstacles , the current can flow without energy loss . Onnes experiment with a current on a superconducting coil in a closed circuit and then revoke the current source and measure current flow turns one year later still flowing . This phenomenon was named later by Onnes superkondutivitas . The above findings , Onnes was awarded the Nobel Prize in Physics in 1913 .
B. Understanding Superconductors
           Superconductors are materials that have zero electrical resistance at very low temperatures . It means that superconductors can conduct current even in the absence of a voltage source . Characteristics of Superconductors are materials medanmagnet in superconductors is zero and Meissner effect experienced . The resistivity of a material is zero if below its critical temperature .

C. Superconductors group
          Based on the value of the critical temperature , superconducting divided into two groups:

    1 . Low critical temperature superconductors
          This type of superconducting critical temperature less than 23 K. This type of superconducting abandoned because of the cost of expensive to cool the material .

    2 . High critical temperature superconductors
         This type of superconducting critical temperature greater than 78 K. This is a type of superconducting materials are being developed that are expected to obtain the superconducting at room temperature making it more economical .

D. Superconductor applications
           Superconductor applications in life include :a. Power Cord .By using superconducting materials , the electrical energy will not experience dissipation due to resistance in the superconducting material is zero . Then the use of electrical energy will be more efficient .
b . Transport EquipmentThe use of superconductors in the field of transport is super fast Electric Railway , known as the Magnetic Levitation ( Maglev ) .