Fundamental Electrical and Electronic Principles, Third Edition

Fundamentals 21and distributed by the power companies. This is an alternating or a.c.supply in which the current flows alternately in opposite directionsaround a circuit. Again, the term strictly means ‘ alternating current ’ ,but the emfs and p.d.s associated with this system are referred to as a.c.voltages. Thus, an a.c. generator (or alternator) produces an alternatingvoltage. Most a.c. supplies provide a sinusoidal waveform (a sinewaveshape). Both d.c. and a.c. waveforms are illustrated in Fig. 1.12 . Thetreatment of a.c. quantities and circuits is dealt with in Chapters 6, andneed not concern you any further at this stage.I(A)steady d.c.a.c.0t(s)varying d.c.Fig. 1.12Factors affecting Resistance The resistance of a sample of materialdepends upon four factors(i)(ii)(iii)(iv)its lengthits cross-sectional area (csa)the actual material usedits temperatureSimple experiments can show that the resistance is directlyproportional to the length and inversely proportional to the csa.Combining these two statements we can write:R α where length (in metres) and A csa (in square metres)AThe constant of proportionality in this case concerns the third factorlisted above, and is known as the resistivity of the material. This isdefined as the resistance that exists between the opposite faces of a 1 mcube of that material, measured at a defined temperature. The symbolfor resistivity is ρ . The unit of measurement is the ohm-metre ( Ω m).Thus the equation for resistance using the above factor isR ρ ohm (1.13)A