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www.GOALias.blogspot.comCurrentElectricityThe last two equations should be the same and henceεreqeqε r=rrr1 2=r + r+ ε r+ r1 2 2 11 21 2We can put these equations in a simpler way,1 1 1= +r r req1 2ε ε εr r r(3.73)(3.74)(3.75)eq 1 2= + (3.76)eq 1 2In Fig. (3.21), we had joined the positive terminalstogether and similarly the two negative ones, so that thecurrents I 1, I 2flow out of positive terminals. If the negativeterminal of the second is connected to positive terminalof the first, Eqs. (3.75) and (3.76) would still be valid withε2 → –ε 2Equations (3.75) and (3.76) can be extended easily.If there an n cells of emf ε 1, . . . ε nand of internal resistancesr 1, . . . r nrespectively, connected in parallel, thecombination is equivalent to a single cell of emf ε eqandinternal resistance r eq, such that1 1 1= + L +r r req1ε ε εr r rn(3.77)eq 1n= + L +(3.78)eq 1nGustav Robert Kirchhoff(1824 – 1887) Germanphysicist, professor atHeidelberg and atBerlin. Mainly known forhis development ofspectroscopy, he alsomade many importantcontributions to mathematicalphysics, amongthem, his first andsecond rules for circuits.GUSTAV ROBERT KIRCHHOFF (1824 – 1887)3.13 KIRCHHOFF’S RULESElectric circuits generally consist of a number of resistors and cellsinterconnected sometimes in a complicated way. The formulae we havederived earlier for series and parallel combinations of resistors are notalways sufficient to determine all the currents and potential differencesin the circuit. Two rules, called Kirchhoff’s rules, are very useful foranalysis of electric circuits.Given a circuit, we start by labelling currents in each resistor by asymbol, say I, and a directed arrow to indicate that a current I flowsalong the resistor in the direction indicated. If ultimately I is determinedto be positive, the actual current in the resistor is in the direction of thearrow. If I turns out to be negative, the current actually flows in a directionopposite to the arrow. Similarly, for each source (i.e., cell or some othersource of electrical power) the positive and negative electrodes are labelledas well as a directed arrow with a symbol for the current flowing throughthe cell. This will tell us the potential difference, V = V (P) – V (N) = ε – I r115