Direct current (DC)

An electric current flowing continuously in one direction through a conductor is called the direct current (DC). The dry cell is a DC voltage source because it has only one polarity of the output voltage which produces direct current in the external circuit

Amperage and Voltage

When there is an electric field in a conductor the free charges within it are set in motion.

Positive charges moving in the same direction as the field and negative charges move in the opposite direction. FIG. 2.1 shows a portion of a conductor within which there is an electric field of intensity E. In a metallic conductor negative charges crossing a section from right to left are equivalent to positive charges crossing from left to right. The entire current is due to the motion of the free charges. The electrons move in the opposite direction to the conventional current in a metallic conductor. The value of current, I, is given by the relation

t Q

I = / (2.1)

where

I = current Q = charge t = time

FIG. 2.1. Positive charges move in the direction of the field in a conductor

The unit of current in MKS system of units is an Ampere, which represents a quantity of one Coulomb of charge flowing per second. Quantitatively it is equal to the flow of 6.25 × 10¹⁸ electrons per second through a given point in a circuit. The basic unit of potential in an electrostatic field is a Volt which is equal to the potential difference between two points for

which one Coulomb of charge will do one Joule of work in going from one point to the other, i.e.

1 Volt = 1 Joule/Coulomb (2.2)

The above relation helps us to define potential difference. The potential difference between two (arbitrary) points is said to be one Volt if the work done in bringing one unit of charge from one point to another is one joule. If the points are lettered A and B, the potential difference between them is simply VA -VB or VB - VA. The potential difference between the terminals of a lead storage battery is about 12 volts with the terminal marked + at higher potential. If we call this terminal A and other terminal (negative) as B, then V_AB = 12 Volts and V_BA = - 12 Volts electrons to move, is called the Electromotive Force (EMF). The potential difference between the terminals of the battery when it is not supplying current to an external circuit is called its EMF. When an EMF is applied across a conductive material, flow of electrical current results.

The unit of EMF is also a Volt.

Ohm’s law and resistance

If a voltage V is applied across a conductor, a current I flows through it. George Ohm discovered that the magnitude of the current in metals is proportional to the applied voltage provided there is no change in the physical state of the conductor. This is known as the Ohm’s law. The relationship is exact within the accuracy of the measurements. Mathematically, it

Resistance in any conducting material is the measure of the opposition to the motion of free electrons due to their continuous collisions against the atoms of the lattice. Resistance depends on the nature, dimension and physical state of the conductor. The unit of electrical resistance is Ohm (Ω). The Ohm is defined as the resistance of a conductor through which a current of one Ampere is flowing when the potential difference across it is one Volt i.e.

1 Ohm = 1 Volt /1Ampere

The amount of resistance in a material is a factor that limits the amount of current that flows through the material for a given applied electromotive force (EMF). Since the resistance in a circuit results in the expenditure of energy, the result is the dissipation of that energy in the form of heat. For a pure conductor resistance is directly proportional to its length L and inversely proportional to its cross section area A i.e.

A

Conductivity is defined as the ability of a material to conduct electric current. It is denoted by σ. The unit of conductivity is Siemens per meter or mho per meter. The conductivity of a conductor decreases with the increase in the temperature. Each element has a unique value of conductivity. Copper, silver and gold have high conductivities where as, carbon has a very low conductivity.

An eddy current is a flow of electrons. The amount of electron flow through an electrically conductive material is directly related to the conductivity of the material. If the conductivity increases, the flow of eddy current increases.

Resistivity is reciprocal of the conductivity. Therefore the materials that have high resistivity have poor conductivity and vice versa. The resistivity is denoted by ρ and is defined as the

‘ratio of electrical intensity (emf) to the current per unit cross-section area.’

Mathematically it can be written as

The value of resistivity can also be derived from relation (2.4), i.e. ρ = RA/L.

The unit of resistivity is Ohm-meter (Ω-m). The Ohm-meter is a bigger unit, the smaller unit is the micro Ohm-centimeter (µΩ-cm).

Resistivity of a material changes with the change in temperature. As the temperature of the conductor rises, the amplitude of the vibration of the atoms in the lattice increases and hence the probability of their collision with the free electrons also increases. It can also be said, that at high temperatures, the atoms offer bigger target area, i.e. the collision cross-section of the atoms increases. This makes the collision between free electrons and the atoms in the lattice more frequent and hence the resistance of the conductor increases.

Experimentally the change in resistivity of a metallic conductor with temperature is found to be nearly linear over a wide range of temperatures below and above 0°C. Over such a range, the fractional change in resistivity per Kelvin is known as the temperature coefficient of resistivity.

In eddy current testing, conductivity is frequently given as a percentage of the international annealed copper standard (%IACS). In this system conductivity of pure annealed copper at 20

°C is set to 100% and conductivity of other materials is given as a percentage of copper.

Conductivity of a material can be calculated from its resistivity.

ρ

Where IACS = international annealed copper standard ρ = resistivity

Conductivity is a one of main variables in eddy current inspection. It makes possible the screening of certain materials based upon their conductivity, the detection of changes in chemistry, lattice distortion, heat treat, hardness, discontinuities, etc. The resistivity and conductivity of various materials are given in Table 2.1.

TABLE 2.1. THE RESISTIVITY AND CONDUCTIVITY VALUES OF VARIOUS