Ohm's Law Website Links For
Ohms 		 

Information About

Ohm's Law
APPAREL
BABY
BEAUTY
BOOKS
CAR TOYS
CELL PHONES
DVD'S
ELECTRONICS
GOURMET FOOD
GROCERIES
HEALTH & PERSONAL
HOME & GARDEN
JEWELRY
MUSIC
MUSIC INSTRUMENTS
OFFICE PRODUCTS
SOFTWARE
SPORTING GOODS
TOOLS & HARDWARE
TOYS
VIDEO GAMES
SHOPPING HOME
MORE SHOPPING...



Ohm's law, named after its discoverer (''Mathematical treatment of the electrical circuit''), facsimile (PDF)], states that the Potential Difference between two points along a connected path and the Current flowing through it are Proportional at a given (i.e., fixed) Temperature :

:V = IR
where ''V'' is the potential difference, ''I'' is the current, and ''R'' is a constant called the Electrical Resistance of the conductor.

The ''potential difference'' is the Voltage Drop from one point to a second point within the connected path of a Conductor , and is sometimes designated by ''E'' or ''U'' instead of ''V''. The unit of resistance is the Ohm , which is equal to one Volt per Ampere , or one volt- Second per Coulomb .

The inverse of resistance, 1/R, is Conductance , and its SI unit is the Siemens (also unofficially called the Mho ). Conductance, or its alternating current (and frequency-dependent) analog Admittance , is widely used in certain types of electrical and electronic analysis.


AN ELEMENTARY EXPLANATION OF ELECTRICAL CIRCUITS, THE ABOVE DIAGRAM, AND HOW OHM'S LAW IS USED


Electrical circuits consist of electrical devices connected by wires (or other suitable conductors). (See the article Electrical Circuits for some basic combinations.) The above diagram is about as simple an electrical circuit as can be constructed. One electrical device in the electrical circuit is shown as a circle with + and - terminals. This symbol represents a voltage source such as a battery. The other electrical device in the electrical circuit is illustrated by a zig-zag symbol and has an R beside it. This symbol represents a resistor, and the value of its resistance is designated by the R. The + or positive terminal of the voltage source is connected to one of the terminals of the resistor using a wire of negligible resistance, and through this wire a current I is shown to be passing, and in a specified direction illustrated by the arrow. The other terminal of the resistor is connected to the - or negative terminal of the voltage source by a second wire. A complete circuit is formed by this configuration because all the current that leaves one terminal of the voltage source must return to the other terminal of the voltage source. (While not shown, because electrical engineers know that it would be redundant, there is an implied current I, and an arrow pointing to the left, associated with the second wire.)

Voltage is the term used to describe the force that moves charge (electrons) through wires and electrical devices, current is the rate of flow of charge, and resistance is the property of a resistor that limits current to the amount that must flow under the applied voltage. So, for a voltage source producing a voltage V, and a resistor of resistance R, Ohm's law provides the equation (I=V/R) for computing the current I.

The term 'conductor' is used in the statement of Ohm's law to indicate that the circuit element across which a voltage is to be measured conducts electricity. Resistors are conductors that limit the passage of electricity to some degree. A resistor with a high value of resistance, say above 10 megaohms, is a poor conductor, while a resistor with a low value of resistance, say below 0.1 ohm, is a good conductor. (Insulators are electrical devices that, for most practical purposes, do not conduct electricity.) The only circuit element in the above diagram that may be identified as the 'conductor' of Ohm's law is the resistor, and the two points across which a voltage is measured are the resistor's leads or terminal interconnection conductors. These interconnection points are not explicitly shown in circuit diagrams such as the one above because including all interconnection points in a circuit diagram, in order to illustrate more completely the interconnection features of an actual circuit, would introduce unnecessary detail to the diagram.


OVERVIEW


The law as published by Ohm applied specifically to his Experiment s with conduction in metallic wires. Later, when electronic circuits were created that required a wider range of resistances in a compact form, Resistor s were manufactured from nonmetals that obeyed Ohm's Law. Metallic and nonmetallic resistors are called '' Ohmic Device s'', because they obey Ohm's Law, at least within certain limits of voltage and current.

Outside these limits, the resistance of an ohmic device varies with the voltage and current. Finally, at extremely high voltages, the device may suffer from electric breakdown or arcing, causing a short circuit; or, at high currents, the device may overheat and melt, causing an open circuit. The resistance of most devices also varies with their temperature, and more specialised devices have resistances that vary with magnetic field intensity, light intensity or many other stimuli.

The relation V = IR can also be applied to non-ohmic devices, but it then ceases to represent Ohm's Law. In non-ohmic cases, ''R'' depends on ''V'' and is no longer a constant of proportionality but a variable called ''differential resistance''. To check whether a given device is ohmic or not, one plots ''V'' versus ''I'' and checks that the curve is a straight line.


PHYSICS


Physicists often use the continuum form of Ohm's Law:
:
\mathbf{J} = \sigma \cdot \mathbf{E}

where J is the Current Density (current per unit area), σ is the Conductivity (which can be a Tensor in anisotropic materials) and '''E''' is the Electric Field .

The common form V = I \cdot R used in circuit design is the macroscopic, averaged-out version.

The equation above is only valid in the Reference Frame of the conducting material. If the material is moving at velocity v relative to a Magnetic Field '''B''', a term must be added as follows
:
\mathbf{J} = \sigma \cdot \left( \mathbf{E} + \mathbf{v} imes\mathbf{B} ight)

The analogy to the Lorentz Force is obvious, and in fact Ohm's law can be derived from the Lorentz force and the assumption that there is a drag on the charge carriers proportional to their velocity.

A perfect metal lattice would have no Resistivity , but a real metal has Crystallographic Defect s, impurities, multiple Isotope s, and thermal motion of the atoms. Electrons Scatter from all of these, resulting in resistance to their flow.

HOW ELECTRICAL AND ELECTRONIC ENGINEERS USE OHM'S LAW


Ohm's Law is one of the equations used in the analysis of electrical circuits, whether the analysis is done by engineers or computers. Even though, today, computers running electronic computer aided design and analysis programs do the bulk of the work predicting and optimizing the performance of electrical circuits (in particular, those circuits to be fabricated on silicon chips), most electrical engineers still use Ohm's Law every working day. Whether designing or debugging an electrical circuit, electrical enginners must have a working knowledge of the practical aspects of Ohm's law.

Virtually all electronic circuits have resistive elements which are almost always considered ideal ohmic devices, i.e. they obey Ohm's Law. From the engineer's point of view, resistors (devices that "resist" the flow of electrical current) develop a voltage across their terminal conductors (e.g. the two wires emerging from the device) proportional to the amount of current flowing through the device.

More specifically, the voltage measured across a resistor at a given instant is strictly proportional to the current passing through the resistor at that instant. When a functioning electrical circuit drives a current I, measured in amperes, through a resistor of resistance '''R''', the voltage that develops across the resistor is I '''R''', the value of '''R''' serving as the proportionality factor. Thus resistors act like current to voltage converters (just as springs act like displacement to force converters).

Similarly, resistors act like voltage to current converters when a desired voltage is established across the resistor because a current I equal to 1/'''R''' times '''V''' must be flowing through the resistor. That current must have been supplied by a circuit element functioning as a current source and it must be passed on to a circuit element that serves as a current sink.

The DC resistance of a resistor is always a positive quantity, and the current flowing through a resistor generates (waste) heat in the resistor as it does in one of Ohm's wires. Voltages can be either positive or negative, and are always measured with respect to a reference point. When we say that a point in a circuit has a certain voltage, it is understood that this voltage is really a voltage difference (a two terminal measurement) and that there is an understood, or explicitly stated, reference point, often called ground or common. Currents can be either positive or negative, the sign of the current indicating the direction of current flow. Current flow in a wire consists of the slow drift of electrons due to the influence of a voltage established between two points on the wire.



where X was the reading from the galvanometer, '''l''' was the length of the test conductor, '''a''' depended only on the thermocouple junction temperature, and '''b''' was a constant of the entire setup. From this, Ohm determined his eponymous law and published his results in {Link without Title} .

Ohm's law was probably the most important of the early quantitative descriptions of the physics of electricity. We consider it almost obvious today. When Ohm first published his work, this was not the case; critics reacted to his treatment of the subject with hostility. They called his work a "web of naked fancies" and proclaimed that Ohm was "a professor who preached such heresies was unworthy to teach science." The prevailing scientific philosophy in Germany at the time, led by Hegel , asserted that experiments need not be performed to develop an understanding of nature because nature is so well ordered, and that scientific truths may be deduced through reasoning alone. Also, Ohm's brother Martin, a mathematician, was battling the German educational system. These factors hindered the acceptance of Ohm's work, and his work did not become widely accepted until the 1840s. Fortunately, Ohm received recognition for his contributions to science well before he died.

While the old term for electrical conductance, the Mho , is still used, a new name, the Siemens , was adopted in 1971, honoring Ernst Werner von Siemens. The siemens is preferred in formal papers.

Benjamin Franklin chose the sign convention used today for assigning the sign of a measured voltage, such as when measuring the voltage across a battery. He did not know what was actually "flowing."

Unstated in the above definition of Ohm's Law is the necessity of making measurements of voltage and current that are averaged over a sufficiently long period of time in order to compute accurately a resistor's actual resistance. This constraint would not have been understood by Ohm in the 1820s because instruments for making current and voltage measurements over short periods of time were not available. A hundred years later, the effect now known as Johnson Noise , or thermal noise, was discovered. This thermal effect implies that measurements of current and voltage that are taken over sufficiently short periods of time will yield, usually, ratios of V/I that are different from the time averaged value of R, although the Ensemble Average of the ratios computed from a large number of statistically independent current and voltage measurements will yield the time-averaged value of R.

Ohm's work long preceded Maxwell's Equations and any understanding of frequency dependent effects in AC circuits. Modern developments in electromagnetic theory and circuit theory do not contradict Ohm's law when they are evaluated within the appropriate limits.

In a sense, Ohm's law is not a law at all. Rather, Ohm's law may be more properly seen as the definition of resistance. (In the hierarchy of 'physical' or 'logical' terms, definitions are more fundamental than laws or theories.) Joules, coulombs and time are basic units of measure. A Volt can be defined based on those units of measure. Ohm's "law" defines the ohm. An ohm, the unit of resistance, is derived from those more basic units. In essence, Ohm discovered that V/I is a constant for a given conductor at a given temperature, and over a large range of V and I. Sanford P. Bordeau. ''Volts to Hertz...the Rise of Electricity.'' Burgess Publishing Company, Minneapolis, MN. pp.86-107.


SEE ALSO




REFERENCES



EXTERNAL LINKS