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- 1 decade agoFavorite Answer
Ohm's law applies to electrical circuits; it states that the current passing through a conductor between two points is directly proportional to the potential difference (i.e. voltage drop or voltage) across the two points, and inversely proportional to the resistance between them.

The mathematical equation that describes this relationship is:

where I is the current in amperes, V is the potential difference between two points of interest in volts, and R is a circuit parameter, measured in ohms (which is equivalent to volts per ampere), and is called the resistance. The potential difference is also known as the voltage drop, and is sometimes denoted by U, E or emf (electromotive force) instead of V.[1]

The law was named after the physicist Georg Ohm, who, in a treatise published in 1827, described measurements of applied voltage and current passing through simple electrical circuits containing various lengths of wire. He presented a slightly more complex equation than the one above to explain his experimental results. The above equation is the modern form of Ohm's law. Well before Georg Ohm's work, Henry Cavendish found experimentally (January 1781) that current varies in direct proportion to applied voltage, but he did not communicate his results to other scientists at the time.[2]

The resistance of most resistive devices (resistors) is constant over a large range of values of current and voltage. When a resistor is used under these conditions, the resistor is referred to as an ohmic device because a single value for the resistance suffices to describe the resistive behavior of the device over the range. When sufficiently high voltages are applied to a resistor, forcing a high current to flow through it, the device is no longer ohmic because its resistance, when measured under such electrically stressed conditions, is different (typically greater) from the value measured under standard conditions (see temperature effects, below).

Ohm's law, in the form above, is an extremely useful equation in the field of electrical/electronic engineering because it describes how voltage, current and resisitance are interrelated on a macroscopic level, that is, commonly, as circuit elements in an electrical circuit. Physicists who study the electrical properties of matter at the microsopic level use a closely related and more general vector equation, sometimes also referred to as Ohm's law, having variables that are closely related to the I, V and R scalar variables of Ohm's law, but are each functions of position within the conductor. See the Physics section and the Relation to heat conduction section below.

Contents [hide]

1 Elementary description and use

2 Physics

3 How electrical and electronic engineers use Ohm's law

3.1 Hydraulic analogs

3.2 Sheet resistance

4 Temperature effects

5 Strain (mechanical) effects

6 Transients and ac circuits

7 Relation to heat conduction

8 History

9 See also

10 References

11 External links

[edit] Elementary description and use

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 shows one of the simplest electrical circuits that can be constructed. One electrical device is shown as a circle with + and - terminals, which represents a voltage source such as a battery. The other device is illustrated by a zig-zag symbol and has an R beside it. This symbol represents a resistor, and the R designates its resistance. 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, 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. This configuration forms a complete circuit 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 assume that it exists, there is an implied current I, and an arrow pointing to the left, associated with the second wire.)

Voltage is the electrical force that moves (negatively charged) electrons through wires and electrical devices, current is the rate of electron flow, and resistance is the property of a resistor (or other device that obeys Ohm's law) that limits current to an amount proportional to the applied voltage. So, for a given resistance R (ohms), and a given voltage V (volts) established across the resistance, Ohm's law provides the equation (I=V/R) for calculating the current through the resistor (or device).

The 'conductor' mentioned by Ohm's law is a circuit element across which the voltage is measured. Resistors are conductors that slow down the passage of electric charge. A resistor with a high value of resistance, say greater than 10 megohms, is a poor conductor, while a resistor with a low value, say less than 0.1 ohm, is a good conductor. (Insulators are materials that, for most practical purposes, do not allow a current when a voltage is applied.)

In a circuit diagram like the one above, the various components may be joined by connectors, contacts, welds or solder joints of various kinds, but for simplicity these connections are usually not shown.

[edit] Physics

Physicists often use the continuum form of Ohm's Law:[3]

where J is the current density (current per unit area, unlike the simpler I, units of amperes, of Ohm's law), σ is the conductivity (which can be a tensor in anisotropic materials) and E is the electric field (units of volts per meter, unlike the simpler V, units of volts, of Ohms's law). While the notation above does not explicitly depict the variables, each are vectors and each are functions of three position variables. (Normally, and in some places below, the dot means the vector dot product. Here the dot just means multiplication.) That is, in the case of J, using cartesian coordinates, there are actually three separate equations, one for each component of the vector, each equation having three independent position variables. For example, the components of J in the x, y and z directions would be Jx(x,y,z), Jy(x,y,z) and Jz(x,y,z).

The potential difference between two points is defined as

or, in the case where the electric field is independent of the choice of path (as it is in a circuit),

where L is the distance between points of interest. Since the current per unit area, J, is equal to I / A, Ohm's Law becomes:

The electrical resistance of a conductor is defined in terms of conductivity, length, and cross sectional area:

From this, it can be seen that Ohm's law takes on the more familiar, yet macroscopic and averaged version:

The continuum form of the equation 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:

See Lorentz force for more on this and Hall effect for some other implications of a magnetic field. This equation is not a modification to Ohm's law. Rather, it is analogous in circuit analysis terms to taking into account inductance as well as resistance.

A perfect crystal lattice, with no thermal motions or other deviations from periodic structure, would have no resistivity,[4] but a real metal has crystallographic defects, impurities, multiple isotopes, and thermal motion of the atoms. Electrons scatter from all of these, resulting in resistance to their flow.

[edit] How electrical and electronic engineers use Ohm's law

This section does not cite any references or sources. (March 2008)

Please improve this section by adding citations to reliable sources. Unverifiable material may be challenged and removed.

Ohm's law is one of the equations used in the analysis of electrical circuits, whether the analysis is done by engineers or by computers. Though computers running electronic computer-aided design and analysis programs do the bulk of the work predicting and optimizing the performance of electrical circuits, most electrical engineers still use Ohm's law every working day. Whether designing or debugging an electrical circuit, electrical engineers must have a working knowledge of the practical aspects of Ohm's law.

Virtually all electronic circuits have resistive elements, which are usually treated as ideal ohmic devices, that is, they obey Ohm's law. From the engineer's point of view, resistors (devices that "resist" the electric current) develop a voltage across their terminals (the two wires emerging from the device) proportional to the amount of current through the device.

More specifically, the voltage measured across a resistor at a given instant is strictly proportional to the current 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.

The DC resistance of a resistor is always a positive quantity, and the current through a resistor generates heat in it. Voltages can be either positive or negative, depending on the ordering of the terminals and the direction of current flow. Currents can be either positive or negative, the sign of the current indicating the direction of current.

Ohm's law applies to conductors whose resistance is (substantially) independent of the applied voltage (or equivalently the injected current). That is, Ohm's law only applies to the linear portion of the I vs. V curve centered around the origin. The equation is too simple to e

Source(s): http://en.wikipedia.org/wiki/Ohm%27s_law- Login to reply the answers

- Anonymous6 years ago
Ohm's Law was developed by Gorge Simon Ohm who created the law to help with Electronics in the early 19 hundreds. Ohms Law is used to calculate how Amps, Volts and Resistance in an electric appliance. The Volts is at the top of the triangle and it's represented as a V and the Volts is the push of the Electricity. The Amps is the first down of the Triangle and the Amps is the current or flow of the Electricity and can be represented as an I or an A. The Resistance is the stuff that slows the Electricity down and can be called Ohms and can be represented as an A or an Omega symbol.

Now Ohm's Law has a formula and that is for example you know what the Amps and the Ohms are but you don't know what the volts are. You times the Amps by the Resistance. If you know what the Amps are and the Volts but you don't know what the Ohms are. You divide the volts by the amps. Now that's Ohm's Law.

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- andranoLv 43 years ago
Ohm got here across that as quickly as he positioned extra voltage during an straight forward cloth like metallic twine, he have been given extra modern. Double the voltage, double the present. So he called this property (voltage divided by modern) resistance. Ohm's regulation is barely valid for undemanding components, no longer e.g. semiconductors. most of the sequence/parallel issues incredibly contain Kirchoff's rules - certainly, what is going in ought to come out, and circuits ought to style loops in any different case the can charge won't bypass everywhere and not something will paintings.

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- 1 decade ago
OHMS LAW: Voltage = Current * Resistance

UNITS: Voltage - Volts

Current - Amps

Resistance - Ohms

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- 6 years ago
Ohm's law states that at constant temperature the steady current flowing through a conductor is directly proportional to the potential difference between its ends.

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- 1 decade ago
When the potential difference (measured in volts) between two ends of a conductor, and the current (measured in amps) flowing along that conductor is constant then the ratio is proportional to the resistance of the conductor (measured in Ohms) ie; V/R = I

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- Anonymous1 decade ago
E = IR

The voltage E in a circuit is equal to the current I multiplied by the resistance R.

Volts = amps * ohms

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- RobertLv 44 years ago
I would like to explain Ohm's law by having an analogy with water behaviour: The flow of water from overhead tank to the tap is directly proportional to the height difference between tank and tap; similarly the amount of current flowing through a conductor in a ideal condition is proportional to the voltage difference between two point.(considering, power flow is DC and closed path).

Try to understand the electrical parameters with water analogy.

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- Anonymous1 decade ago
v = i x r where v = voltage i = current r = resistance

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