Derive the relation between the resistors in series and parallel connection
Answers
Let the current through resistor R1 be I1 and the current through R2 be I2
The voltage drop across both R1 and R2 is equal to the supply voltage V
Therefore from Ohm's Law
I1 = V / R1
and
I2 = V / R2
But from Kirchoff's Current Law, we know the current entering a node (connection point) is equal to the current leaving the node
Therefore
I = I1 + I2
Substituting the values derived for I1 and I2 gives us
I = V / R1 + V / R2
= V(1 / R1 + 1 / R2)
The lowest common denominator (LCD) of 1 / R1 and 1 / R2 is R1R2 so we can replace the expression (1 / R1 + 1 / R2) by
R2 / R1R2 + R1 / R1R2
Switching around the two fractions
= R1 / R1R2 + R2 / R1R2
and since the denominator of both fractions is the same
= (R1 + R2) / R1R2
Therefore
I = V(1 / R1 + 1 / R2) = V(R1 + R2) / R1R2
Rearranging gives us
V / I = R1R2 / (R1 + R2)
But from Ohm's Law, we know V / I = total resistance of the circuit. Let's call it Rtotal
Therefore
Rtotal = R1R2 / (R1 + R2)
So for two resistors in parallel, the combined resistance is the product of the individual resistances divided by the sum of the resistances.
Explanation: