Question

what are series and parallel combinations? what is the practical utility of these combinations?

Answer 1

these are the way of connecting the resistors
in parllel or series.
the practical utility of these combinations
is to control the power of current passed.

Answer 2

Hey Friend,

1. Series circuit - 
   (i) Resistance is increased
   (ii) Same current flows through all parts or through each resistor in a circuit.
   (iii) Potential difference across each resistor is different.
   (iv) Total potential difference across the circuit is equal to sum of all       potential differences across each resistor.
   (v) R(s) = R1 + R2 + R3 + ...
   (vi) Total resistance in series is always greater than the value of each individual resistor.

2. Parallel circuit - 
    (i) Resistance is decreased.
    (ii) Same potential difference flows through all parts or through each resistor in a circuit.
    (iii) Current across each resistor is different.
    (iv) Total current across circuit is equal to the sum of reciprocal of individual resistor.
    (v) 1/R(p) = 1/R1 + 1/R2+ 1/R3 + ...
    (vi) Total resistance in parallel is always smaller than the value of each individual resistor.

The circuit diagrams are in the attachement

Hope it helps!