Ohm's law can be used to calculate the amount of current that will flow from a power supply, when it is connected to a single resistor, I = V/R.
However, in most circuits there will be more than one resistor and these resistors may be connected in different ways. In these circuits, before we can calculate the current that will flow from the power supply, we first need to calculate the combined resistance of the individual resistors in the circuit. In this section we will look at the two possible ways of connecting resistors together and how to calculate the total resistance in each case.
The diagram below shows three resistors connected in series. To show how to calculate the total resistance, we will use Kirchoff's voltage law and ohms law.
According to Kirchoff's voltage law, the voltage of the e.m.f. is equal to the sum of the voltage drops across each resistor.
According to Ohm's law, V = IR. Therefore substituting for V gives;
dividing both sides of the equation by I, gives;
Therefore to find the total resistance of series resistors, we simply add the individual resistor values together.
RT = R1 + R2 + R3.