derive an expression for equivalents resistance for series and parallel combination
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consider three resistors R1, R2, R3connected in series. Suppose a current I flows through the circuit when a cell of V volt is connected across the combination.
By Ohm’s law, the potential differences across the three resistors will be,
V1 = IR1, V2 = IR2, V3 = IR3
If Rs be the equivalent resistance of the series combination, then on applying a potential difference V across it, the same current I must flow through it.
there fore R1+R2+R3=Rq
FOR PARALLAL COMBINATION
consider three resistors R1, R2, R3connected in parallel.
Suppose a current I flows through the circuit when a cell of voltage V is connected across the combination. The current I at point A is divided into three parts I1, I2, I3 through the resistors R1, R2, R3respectively. These three parts recombine at point B to give same current I.
∴ I = I1 + I2 + I3
As all the three resistors have been connected between the same two points A and B, voltage V across each of them is same. By Ohm’s law,

If RP be equivalent resistance of parallel combination, then,
1/req=1/r1+1/r2
By Ohm’s law, the potential differences across the three resistors will be,
V1 = IR1, V2 = IR2, V3 = IR3
If Rs be the equivalent resistance of the series combination, then on applying a potential difference V across it, the same current I must flow through it.
there fore R1+R2+R3=Rq
FOR PARALLAL COMBINATION
consider three resistors R1, R2, R3connected in parallel.
Suppose a current I flows through the circuit when a cell of voltage V is connected across the combination. The current I at point A is divided into three parts I1, I2, I3 through the resistors R1, R2, R3respectively. These three parts recombine at point B to give same current I.
∴ I = I1 + I2 + I3
As all the three resistors have been connected between the same two points A and B, voltage V across each of them is same. By Ohm’s law,

If RP be equivalent resistance of parallel combination, then,
1/req=1/r1+1/r2
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sorry I didn't understand
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