Total resistance of combined resistors - series and parallel circuits


Series and parallel circuits GCSE A Level IGCSE
Resistors in series and parallel

Resistors can be combined in two different methods; they are in series and in parallel.
If two resistors, R1 and R22, are in series, the total resistance, Rt, is given by the following formula.
Rt = R1 + R2
If the same resistors are in parallel, the total resistance is as follows:
Rt = 1/R1 + 1/R2

E.g. 1
Two resistors, 6 and 3 are connected in series. Find the total resistance.
Rt = R1 + R2
Rt = 6 + 3
Rt = 9

Two resistors, 10 and 15 are in parallel. Find the total resistance. 
1/Rt = 1/10 + 1/15
1/Rt = 5/30
Rt = 6

If resistors are in series
  • The total resistance is bigger than the highest individual resistance of the circuit.
  • The current through each resistor is the same.
  • The total voltage splits up across each resistor.
  • If one resistors is removed, the currents does not flow through the entire circuit
E.g. Christmas tree decorating lights

If resistors are in parallel

  • The total resistance is smaller than the highest individual resistance of the circuit.

  • The voltage across each resistor is the same.
  • The total current splits up across each resistor
  • The loss of a resistor does not affect the functioning of the circuit
E.g. The domestic electric circuits

You can practise the resistors in series and parallel with the following applet:

For a more comprehensive tutorial with lots of worked examples on electricity, please use the following link on the main site:
Electricity tutorial


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