Derivation of mathematical expression of parallel combination : Let, V be the potential difference across the two common points A and B. Then, from Ohm’s law
Current passing through R
1, I
1 = V/R
1 ...(i)
Current passing through R
2, I
2 = V/R
2 ...(ii)
Current passing through R
3, I
3 = V/R
3 ...(iii)
If R is the equivalent resistance, then from Ohm’s law, the total current flowing through the circuit is given by,
I = V/R ...(iv)
and I = I
1 + I
2 + I
3 ...(v)
Substituting the values of I, I
1, I
2 and I
3 in Equation (v),
= ...(vi)
Cancelling common V term, one gets
The equivalent resistance of a parallel combination of resistance is less than each of all the individual resistances.
Important results about parallel combination : (i) Total current through the circuit is equal to the sum of the currents flowing through it.
(ii) In a parallel combination of resistors the voltage (or potential difference) across each resistor is the same and is equal to the applied voltage i.e.
i.e. V
1 = V
2 = V
3 = V :
(iii) (iii) Current flowing through each resistor is inversely proportional to its resistances, thus higher the resistance of a resistors, lower will be the current flowing through it.