Resistance

RESISTANCE

Electric resistance is the property of a material which offers opposition to the current and dissipates energy. In a conductor the resistance is due to the collision between moving electrons and fixed conductor ions. Heat is generated because of these collisions. In other words, conductors are heated when they carry current. The resistance of a conductor depends upon its length, cross-sectional area, material and temperature.

Ohm’s law states that the current through any conductor is directly proportional to the potential difference between its ends. It is assumed that the physical conditions of the conductor do not change. That is, temperature, length and cross-sectional area of the conductor are unchanged.

If V= voltage between the two ends of the conductor in volts (V)

I= current through the conductor in amperes (A)

then

I ∞ V ; I = 1/R x V

V = RI

Where R is called the resistance of the conductor. It is measured in ohms. The Greek letter Omega(Ω) is used for the symbol of ohm.

Resistance is the electrical property of a material which opposes the current. A physical device which has the principal characteristic of resistance is called a resistor. A resistor is an electrical circuit element.

OPEN CIRCUIT AND SHORT CIRCUIT :

An open is a gap, break, or interruption in a circuit path. A circuit element is said to be open circulated or simply open if the current through it is zero regardless of the voltage across it. Any open circuit may be considered as a resistor of infinite resistance. Therefore by Ohm’s law

I = V/R = V/∞ = 0

whatever be the value of V.

A circuit element is said to be short- circuited or simply shorted if the voltage across it is zero regardless of the current through it. Ashort circuit may be considered as a resistor of zero resistance. Therefore by Ohm’s law

V = RI = 0 x I = 0

whatever be the value of I.

CONDUCTANCE :

The parameter G is called the conductance of the conductor. It is reciprocal of resistance, that is,

G = 1/R

Also

G = I/V

RESISTANCE OF METALLIC CONDUCTOR :

Four factors affect the resistance of a metallic conductor : length, cross-sectional area, the type of material from which the conductor is made, and temperature.

The resistance of a metallic conductor is directly proportional to its length l in the direction of current. That is, as the length of the conductor increases, its resistance increases.

The resistance of a metallic conductor is inversely proportional to the cross-sectional area a through which current passes. That is, as the thickness of the conductor increases its resistance decreases, and as its thickness decreases, its resistance increases.

The resistance of a metallic conductor depends on the type of material of the conductor.

The resistance of a metallic conductor depends on the working temperature.

Let the temperature be assumed constant.

Mathematically,

R ∞ l

R ∞ 1/a

so R ∞ l/a

and R = ƿ x l/a

where the Greek letter ƿ (rho) is the proportionality constant at a given temperature. It is called the resistivity or specific resistance of the conductor material.

TYPES OF RESISTORS :