N1 Engineering Science|The Easy Way! 107


Solution: R = 20 ohms I = 3 A (a)


P = I2R = 32 × 20 = 180 W


(b) \ V = P


P = VI I


= 180 3 = 60 V


6.9 Grouping of resistors 6.9.1 Resistors in series


or V = IR


= 3 × 20 = 60 V


Fig. 6.5 Resistors in series


If resistors R1, R2 and R3 are connected in series across a supply as shown in fig. 6.5, then applying Kirchhoff’s second law:


VT = V1 + V2 + V3 ........................................................................................................ (6.5) Applying Ohm’s law:


IRT = IR1 + IR2 + IR3 IT = I1 = I2 = I3 ........................................................................................................... (6.6)


Dividing by I as it is a constant: RT = R1 + R2 + R3 ......................................................................................................... (6.7)


Example 6.13


Tree resistors with values of 4 ohms, 6 ohms and 2,75 ohms respectively are connected in series across a 24 v supply. Calculate; (a) (b) (c)


the total resistance of the circuit, the current through each resistor, and the pd (or volt drop) across each resistor.


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