All About Current







Electromotive Force


The force that causes the electrons to move in an electrical circuit is called the electromotive force, or EMF. Sometimes it is convenient to think of EMF as electrical pressure. In other words, it is the force that makes electrons move in a certain direction within a conductor. There are many sources of EMF, the most common being batteries and electrical generators.


Resistance


Resistance is the opposition of a body or substance to the flow of electrical current through it, resulting in a change of electrical energy into heat, light, or other forms of energy. The amount of resistance depends on the type of material. Materials with low resistance are good conductorsof electricity.  Materials with high resistance are good insulators.


Lenz's Law


Soon after Faraday proposed his law of induction, Heinrich Lenz developed a rule for determining the direction of the induced current in a loop. Basically, Lenz's law states that an induced current has a direction such that its magnetic field opposes the change in magnetic field that induced the current. This means that the current induced in a conductor will oppose the change in current that is causing the flux to change.


Skin effect

The skin effect arises when the eddy currents flowing in the test object at any depth produce magnetic fields which oppose the primary field, thus reducing the net magnetic flux and causing a decrease in current flow as the depth increases. The depth that eddy currents penetrate into a material is affected by the frequency of the excitation current and the electrical conductivity and magnetic permeability of the specimen. The depth of penetration decreases with increasing frequency and increasing conductivity and magnetic permeability.

Ferranti effect

The Ferranti effect is a rise in voltage occurring at the receiving end of a long transmission line, relative to the voltage at the sending end, which occurs when the line is energized but there is a very light load or the load is disconnected. This effect is due to the voltage drop across the line inductance (due to charging current) being in phase with the sending end voltages. Therefore both capacitance and inductance are responsible for producing this phenomenon.  The Ferranti Effect will be more pronounced the longer the line and the higher the voltage applied. The relative voltage rise is proportional to the square of the line length. Due to high capacitance, the Ferranti effect is much more pronounced in underground cables, even in short lengths.

Proximity Effect

A changing magnetic field will influence the distribution of an electric current flowing within an electrical conductor. When an alternating current (AC) flows through an isolated conductor, it creates an associated alternating magnetic field. The alternating magnetic field induces eddy currents in adjacent conductors, altering the overall distribution of current flowing through them. The proximity effect can significantly increase the AC resistance of adjacent conductors when compared to its resistance to a DC current. The effect increases with frequency. At higher frequencies, the AC resistance of a conductor can easily exceed ten times its DC resistance.

Surge impedance

Surge impedance of a uniform transmission line  is the ratio of the amplitudes of a single pair of voltage and current waves propagating along the line in the absence of reflections. The SI unit of characteristic impedance is the ohm. The characteristic impedance of a lossless transmission line is purely real, that is, there is no imaginary component . Characteristic impedance appears like a resistance , such that power generated by a source on one end of an infinitely long lossless transmission line is transmitted through the line but is not dissipated in the line itself.

Heat  loss

Copper is a good electrical conductor, but it is also its drawbacks. Copper is responsible  to loss in the form of heat.  When electrical currents flowing through a copper wire pass through a conductor, some of the electrical energy is given off in the form of heat. This decreases the amount of electricity that is supplied to the end destination. Another type of electrical loss that occurs with copper wire is surface loss. DC power flows through the wire in a constant, uniform fashion. When AC power is flowing through copper, the electricity moves along the outer sections of the wire. This pushes the current to the outside layers of the wire and creates line loss.

Dielectric loss

Loss can also occur because of the dielectric material in between conductors. The dielectric material shifts the electrons flowing through the conductor and pushes them off of a path. This disperses the electrons and distorts their electrical field, resulting in heat dissipation into the dielectric material. The electrons use more energy dispersing than they do on a set course, so the result is line loss.


Why Power is taken as CONSTANT:


It must be understood that neither voltage nor current by themselves constitute power. Rather, power is the combination of both voltage and current in a circuit.
We must remember that:
        ·      Power is the measure of how much work can be done in a given amount of time.
·                    Current is the rate at which electric charges move through a conductor.
·                    A voltage may represent either a source of energy (electromotive force), or it may  represent lost or stored energy.


Voltage (specific work) is analogous to the work done in lifting a weight against the pull of gravity. Current (rate) is analogous to the speed at which that weight is lifted. Together as a product (multiplication), voltage (work) and current (rate) constitute power.
Now assume that if a boy wants to lift a stone from ground floor to top floor, higher the number of floor he wants to go, less will be his average speed because his energy is constant. We can also say this in terms of electricity that greater the rate of charge transfer , less will be potential difference (assuming total charge is constant) or , a circuit with high voltage and low current may be dissipating the same amount of power as a circuit with low voltage and high current. In this way power is conserved.