# Resistors in series and parallel; power

Components are in series if they are connected end to end in a line. Resistors in series having values R1, R2, R3, etc., may be replaced by a single equivalent resistor having a value R, such that

R = R 1 + R2 + R3 . . . . . .

When connected across a supply, the same current will flow through each resistor.

Components are in parallel when, laid side by side, like ends are connected together. Resistors in parallel having values R1, R2, R3 etc., may be replaced by a single equivalent resistor having a value R, such that

(very easy on a calculator with reciprocals).

When connected across a supply the same p.d. will be across each resistor.

Energy is the capacity for doing work. Its unit is the joule (J). Power is the rate of doing work and is therefore measured in joules per second, simplified to the watt (W). The power (P) dissipated in a resistor is given by the expression

P = VI

Substituting in this expression the relationship V = IR gives:

The total power in any circuit is the sum of the individual powers.

The maximum power that can be taken from a DC supply occurs when the load resistance equals the internal resistance of that supply plus any other line resistance.

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