RESISTANCE

RESISTANCE:

The resistance of a material is  defined as

                      “The ratio of voltage to current passing through the conductor”

                           OR

“the resistance between two points in a conductor when a constant electric potential of one volt applied at the two points produces a current flow of one ampere in the conductor”

REPRESENTATION:

Resistance is denoted by the symbol “R”

UNITS:

The unit of resistance is Ohm(),named after a scientist named sir George ohm.

FORMULA:

R=V/I

where  “V” is the potential difference across the two points in volts and “ I” is the current

flowing between the two points in amperes.

FACTORS AFFECTING RESISTANCE:

The resistance of an electrical conductor depends on four factors, these being:

 (a) the length of the conductor

 (b) the cross-sectional area of the conductor

(c) the type of material

(d) the temperature of the material.

•  Resistance R is directly proportional to length, l, of a conductor. For example, if the length of a piece of wire is doubled, then the resistance is doubled. Resistance (R)is inversely proportional to cross-sectional area (A)of a conductor, i.e. R is proportional to 1/A.Thus, for example, if the cross-sectional area of a piece of wire is doubled, then the resistance is halved.  
•   The resistance of some materials increases by increase in temperature(called positive temperature cofficient),while the resistance of some materials increases with decrease in temperature(called negative temperature coffeicient)

Since  R is proportional to land Ris proportional to 1/a,then Ris proportional to l/a. By inserting a constant of proportionality into this relationship, the type of material used may  be taken into account. The constant of proportionality is known as the resistivity of the  material and is given the symbol ρ(Greek rho).

The value of the resistivity is the resistance of a unit cube of the material measured between opposite faces of the cube.

RESISTIVITY OF SOME SOME MATERIALS:

CONDUCTANCE:

The reciprocal of resistance is called conductance and is measured in siemens (S). Thus,

conductance,  in  siemens  G

G=1/R

where  R is the resistance in ohms.

EXAMPLE:

The resistance of a 5  m  length  of  wire  is  600 Ω. Determine (a) the resistance of an 8   m

length of the same wire, and (b) the length of the same wire when the resistance is 420 Ω.

SOLUTION: