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  Electric Current, Physics tutorial

Introduction:

We are familiar that apart from the gravity, the mere force among the two electric charges is the Coulomb force. In terms of applications, the significance of electrostatics is not well acknowledged to the ordinary person. Historically, things in reality got exciting if the charges began moving to form an electric current. It was by noticing electrical currents which the connection between electricity and magnetism was irreversibly established.

The whole electrical appliances we make use of, like the electric heater, radio, electric fan refrigerator and so forth based on the flow of charge which is electric current.

The motion of charge generally takes place in conductors that have free electrons; in the ionized gases of fluorescent lamps that include charge carriers of both signs, and as well in an evacuated area, for illustration: electrons in a TV picture tube.

Electric Current and Current Density:

If there is a total flow of charge across any area, we state that there is a current across that area. For illustration: when the ends of a conductor, state copper wire are joined or connected to a battery, an electric field 'E' will be set up at each and every point in the conductor. As an outcome of the field, the electrons in the wire will move in the direction opposite to that of the field and give mount to an electric current in the wire.

Define: The electric current is stated as the amount of charge passing via a given cross-section of the wire per unit time.

In equation form, the electric current 'I' is stated to be:

I = ΔQ/Δt

Here, ΔQ is the amount or quantity of charge passing via a given area in time Δt

The Current is a scalar quantity. However we will shortly observe that a current in a wire is symbolized by an arrow. Such an arrow just represents direction of flow of charges all along a conductor.

By convention, the direction of current is stated as that direction in which a positive charge moves. When the moving charge is negative, as by means of electrons in a metal, then the current is opposite to the flow of the real charges.

Definition of current density:

The current density, 'J' is stated as the charge flowing per unit time per unit area normal to the surface and consists of the similar direction as a positive charge.

J = I/A

Here,

J = current density in amperes/m2

I = current via a conductor, in amperes

A = cross-sectional area of the conductor, m2

Resistance, Resistivity and Conductivity:

Resistance:

In physics, resistance is a measurement of the tendency of a material to resist the flow of an electrical current. This is basically dependent on the nature of the material, its length and thickness and on temperature. Resistance is low in substances, like metals, which are good conductors, and high in materials, like plastic and rubber, which are insulators. If an electrical current encounters resistance, portion of its energy is transformed into heat, and at times light, decreasing the current. This phenomenon can be a dilemma, however as well have numerous uses.

Define: Resistance is stated as inversely proportional to current.

I ∝ 1/R

By joining the relationships of current to voltage and current to resistance gives:

I = V/R

V = IR

The above relationship is as well termed as Ohm's law. Ohm's law in this form in reality states resistance for quite a few materials.

Resistivity:

Electrical resistivity (as well termed as resistivity, specific electrical resistance or volume resistivity) is an intrinsic property which measures how strongly a given material opposes the flow of the electric current. A low resistivity points out a material which readily lets the movement of electric charge. Resistivity is generally symbolized by the Greek letter ρ (rho). The SI unit of electrical resistivity is the ohm⋅metre (Ω ⋅ m)

R = (l/A) ρ

ρ = (A/l) R

Conductivity:

Electrical conductivity or simply EC is a property which is employed to explain how well materials let electrons to flow. This is determined by using experiments and math equations. Conductivity is basically the reciprocal of resistivity, signifying the higher the conductivity, the lower the resistivity. A conductor is a material which consists of a high electrical conductivity, and an insulator is a material which consists of a high electrical resistivity. Both the properties based on the temperature and purity of materials.

Define: The Electrical conductivity is the proportionality factor associating to the current which flows in a medium to the electric force field which is applied. This is a measure of the capability of the material to conduct an electrical current to move via the material.  Resistivity is the reciprocal of the conductivity. The units of conductivity are Siemens per meter (S/m).  The practical unit is MilliSiemens per meter (mS/m).  As Siemen, the unit of conductance, is the reciprocal of the Ohm, the unit of resistance, the units of conductivity are at times given as mhos/meter. Resistivity is the inverse of conductivity.

σ = 1/ρ

σ = l/RA

The Ohm's law can be written in terms of resistivity, electric field 'E', and current density 'J':

E = J ρ

Electromotive Force:

Electromotive Force or EMF is the difference in voltage among the terminals of a battery, generator, thermocouple or other electrical device. This is generally stated as the electrical potential energy that lets current to pass from one end of a circuit to the other. Charge differences are generally created if particles termed electrons collect at one terminal and there are some of them at the other end. Amperes, voltage and internal resistance are computed mathematically to find out the electromotive force that is usually less than the net voltage of the system.

Define: The electromotive force of a source (that is, a battery, generator and so on) is the energy (that is, chemical, mechanical and so on) transformed into electrical energy, if unit charge passes via it.

Electromotive force, as well termed as emf (symbolized ξ and measured in volts), is the voltage developed through any source of electrical energy like a battery or dynamo. It is usually stated as the potential for a source in a circuit.

Internal Resistance:

A practical electrical power source that is a linear electric circuit might, according to Thevenin's theorem, be symbolized as an ideal voltage source in series by impedance. This impedance is known as the internal resistance of the source. If the power source delivers current, the measured voltage output is lower than the no-load voltage; the difference is the voltage drop (that is, the product of current and resistance) caused through the internal resistance. The theory of internal resistance applies to all types of electrical sources and is helpful for analyzing numerous kinds of electrical circuits.

The internal resistance of a battery might be computed from its open circuit voltage VNL, load voltage VFL and the load resistance RL:

Rint = [(VNL/VFL) - 1] RL

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