Electrical Engineering ⇒ Topic : Electrical resistance

ELECTRICAL RESISTANCE

    Every material offers some resistance (or opposition) to the flow of electric current through it. Good conductors like copper and aluminium offer very low resistance to current flow. On the other hand, insulators such as wood, plastics and mica which are poor conductors of electricity offers very high resistance to current flow through them.

     The size and type of material of wires (or conductors) used for carrying current are chosen such that their electrical resistance is very low. In this way current can flow easily through them, just as water flows easily through the pipes between tanks, as shown in Fig. (a). More water will flow if the diameter

                                              FIGURE (a) Water analogy of potential difference and current flow

of the pipe is large. Similarly, if the diameter of the conductor is large, current flow through it will be high and its resistance will be small. Thus, larger the diameter of the electric wires (or conductors), the lower will be their electrical resistance. In the case of water flow, the friction of the pipe's surface opposes the flow of water. Similarly, the resistance of the wire opposes the flow of current.The pipe friction is similar to the electrical resistance of the wire. The electrical resistance of the wires depends upon (1) the length of wire, (2) the diameter of the wire and (3) the material of the wire (copper, aluminium, etc.).The resistance of a conductor R varies (a) directly with its length and (b) inversely with the area of cross-section of the conductor, that is

                                                 ........... (1)

where, R is the resistance in Ω, lis the length in m, a is the area of cross-section of the conductor in m² and ρ (greek symbol, Rho) is a constant which depends on the nature of the material of the conductor and is called the specific resistance or resistivity of the material.

From Eq. (1), we have

                                                            ........ (2)

Thus, the unit of ρ is ohm-metre.