electric current

An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma.^[1]The SI unit for measuring an electric current is the ampere, which is the flow of electric charge across a surface at the rate of onecoulomb per second. Electric current is measured using a device called an ammeter.^[2]Electric currents can have many effects, notably heating, but they also create magnetic fields, which are used in motors, inductors and generators.

SymbolThe conventional symbol for current is I, which originates from the French phrase intensité de courant, or in English current intensity.^[3][4] This phrase is frequently used when discussing the value of an electric current, but modern practice often shortens this to simply current. The I symbol was used by André-Marie Ampère, after whom the unit of electric current is named, in formulating the eponymous Ampère's force law which he discovered in 1820.^[5] The notation travelled from France to Britain, where it became standard, although at least one journal did not change from using C to I until 1896.

Main article:

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,^[7] one arrives at the usual mathematical equation that describes this relationship:^[8]
$I = \frac{V}{R}$
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.^[9]
The abbreviations AC and DC are often used to mean simply alternating and direct, as when they modify current or voltage.^[10][11]

Direct current
Main article: Direct current
Direct current (DC) is the unidirectional flow of electric charge. Direct current is produced by sources such as batteries, thermocouples, solar cells, and commutator-type electric machines of the dynamo type. Direct current may flow in a conductor such as a wire, but can also flow through semiconductors, insulators, or even through a vacuum as in electron or ion beams. The electric charge flows in a constant direction, distinguishing it from alternating current (AC). A term formerly used for direct current was galvanic current.^[12]

Alternating current
Main article: Alternating current
In alternating current (AC, also ac), the movement of electric charge periodically reverses direction. In direct current (DC, also dc), the flow of electric charge is only in one direction.
AC is the form in which electric power is delivered to businesses and residences. The usual waveform of an AC power circuit is a sine wave. In certain applications, different waveforms are used, such as triangular or square waves. Audio and radio signals carried on electrical wires are also examples of alternating current. In these applications, an important goal is often the recovery of information encoded (or modulated) onto the AC signal
Voltage, electrical potential difference, electric tension or electric pressure (denoted $∆ V$ and measured in units of electric potential:volts, or joules per coulomb) is the electric potential difference between two points, or the difference in electric potential energy of a unitcharge transported between two points.^[1] Voltage is equal to the work done per unit charge against a static electric field to move the charge between two points. A voltage may represent either a source of energy (electromotive force), or lost, used, or stored energy (potential drop). A voltmeter can be used to measure the voltage (or potential difference) between two points in a system; often a common reference potential such as the ground of the system is used as one of the points. Voltage can be caused by static electric fields, byelectric current through a magnetic field, by time-varying magnetic fields, or some combination of these three.
An electric current is a flow of electric charge. In electric circuits this charge is often carried by moving electrons in a wire. It can also be carried by ions in an electrolyte, or by both ions and electrons such as in a plasma

Ohm's law

AC and DC

Current Electricity - Lesson 3 - Electrical Resistance

Resistance

An electron traveling through the wires and loads of the external circuit encounters resistance. Resistance is the hindrance to the flow of charge. For an electron,

the journey from terminal to terminal is not a direct route. Rather, it is a zigzag path that results from countless collisions with fixed atoms within the conducting material. The electrons encounter resistance - a hindrance to their movement. While the electric potential difference established between the two terminals encourages the movement of charge, it is resistance that discourages it. The rate at which charge flows from terminal to terminal is the result of the combined effect of these two quantities.

Variables Affecting Electrical Resistance

The flow of charge through wires is often compared to the flow of water through pipes. The resistance to the flow of charge in an electric circuit is analogous to the frictional effects between water and the pipe surfaces as well as the resistance offered by obstacles that are present in its path. It is this resistance that hinders the water flow and reduces both its flow rate and its drift speed. Like the resistance to water flow, the total amount of resistance to charge flow within a wire of an electric circuit is affected by some clearly identifiable variables.

First, the total length of the wires will affect the amount of resistance. The longer the wire, the more resistance that there will be. There is a direct relationship between the amount of resistance encountered by charge and the length of wire it must traverse. After all, if resistance occurs as the result of collisions between charge carriers and the atoms of the wire, then there is likely to be more collisions in a longer wire. More collisions mean more resistance.

Second, the cross-sectional area of the wires will affect the amount of resistance. Wider wires have a greater cross-sectional area. Water will flow through a wider pipe at a higher rate than it will flow through a narrow pipe. This can be attributed to the lower amount of resistance that is present in the wider pipe. In the same manner, the wider the wire, the less resistance that there will be to the flow of electric charge. When all other variables are the same, charge will flow at higher rates through wider wires with greater cross-sectional areas than through thinner wires.

A third variable that is known to affect the resistance to charge flow is the material that a wire is made of. Not all materials are created equal in terms of their conductive ability. Some materials are better conductors than others and offer less resistance to the flow of charge. Silver is one of the best conductors but is never used in wires of household circuits due to its cost. Copper and aluminum are among the least expensive materials with suitable conducting ability to permit their use in wires of household circuits. The conducting ability of a material is often indicated by its resistivity. The resistivity of a material is dependent upon the material's electronic structure and its temperature. For most (but not all) materials, resistivity increases with increasing temperature. The table below lists resistivity values for various materials at temperatures of 20 degrees Celsius.

Material	Resistivity (ohm•meter)
Silver	1.59 x 10-8
Copper	1.7 x 10-8
Gold	2.4 x 10-8
Aluminum	2.8 x 10-8
Tungsten	5.6 x 10-8
Iron	10 x 10-8
Platinum	11 x 10-8
Lead	22 x 10-8
Nichrome	150 x 10-8
Carbon	3.5 x 10-5
Polystyrene	107 - 1011
Polyethylene	108 - 109
Glass	1010 - 1014
Hard Rubber	1013

As seen in the table, there is a broad range of resistivity values for various materials. Those materials with lower resistivities offer less resistance to the flow of charge; they are better conductors. The materials shown in the last four rows of the above table have such high resistivity that they would not even be considered to be conductors.

Look it Up!

Use the Resistivity of a Material widget to look up the resistivity of a given material. Type the name of the material and click the Submit button to find its resistivity.

Resistivity of a Material

Mathematical Nature of Resistance

Resistance is a numerical quantity that can be measured and expressed mathematically. The standard metric unit for resistance is the ohm, represented by the Greek letter omega -

. An electrical device having a resistance of 5 ohms would be represented as R = 5

. The equation representing the dependency of the resistance (R) of a cylindrically shaped conductor (e.g., a wire) upon the variables that affect it is

where L represents the length of the wire (in meters), A represents the cross-sectional area of the wire (in meters2), and

represents the resistivity of the material (in ohm•meter). Consistent with the discussion above, this equation shows that the resistance of a wire is directly proportional to the length of the wire and inversely proportional to the cross-sectional area of the wire. As shown by the equation, knowing the length, cross-sectional area and the material that a wire is made of (and thus, its resistivity) allows one to determine the resistance of the wire.

Investigate!

Resistors are one of the more common components in electrical circuits. Most resistors have stripes or bands of colors painted on them. The colors reveal information about the resistance value. Perhaps you're doing a lab and need to know the resistance of a resistor used in the lab. Use the widget below to determine the resistance value from the colored stripes.

Resistor Color Code

MAKE IT EASY

Sabado, Agosto 30, 2014

PHYSICS