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How to i calculate total resistance in a circuit?

Total resistance in a circuit depends on whether the resistors are in series or parallel since this determines which equation is to be used.

In series, resistors are essentially lined up next to each other so the total is simple since it is just the sum of the resistors resistance in ohms.

e.g. if a circuit has one 10ohm, one 45ohm and one 235ohm resistor in it, all three of which are in series, the total resistance is (10+45+235)ohm = 290ohms

In parallel each resistor is on its own path for the electricity to follow and as a result the equation is not quite the same, we use the equation:

 1/Rtotal = 1/R1 + 1/R+ 1/R..etc

e.g. a circuit has 3 resistors all in parallel. Their resistances are 25ohm, 5ohm and 100ohm respectivley, what is the total resistance of the circuit?

1/Rt = 1/25 + 1/5 + 1/100

1/R= 4/100 + 20/100 + 1/100

1/R= 25/100

100 = 25Rt

R= 4ohms

often the answer will be a fraction when calculating a parallel due to the fractions involved in the total calculation however i used a simpler calculation for the example to demonstrate the equation's use

Matthew K. GCSE Maths tutor, GCSE Physics tutor

2 years ago

Answered by Matthew, a GCSE Physics tutor with MyTutor


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