5.3 How to investigate the effect on the total resistance of a circuit by adding identical resistors together in series

(a) Adding resistors in series

(a) The circuit of identical resistors wired in series

You set up a relatively simple circuit of a power supply, ammeter and one or more identical resistors, all wired in series.

You start by putting one resistor in the circuit and measuring the current flowing through it (I) and the p.d. across it (V).

Then, add a second resistor in series and remeasure the p.d. and current again.

Just repeat by adding more resistors and for each pair of readings calculate the total resistance (R = V/I).

From your recorded results plot a graph of total resistance versus number of resistors in series.

You find the graph is linear proving the rule that:

R_total = R₁ + R₂ + R₃ ohms etc.

for resistors wired in series.

This addition applies to any resistance values, the resistors do not have to be identical.

e.g. if R₁ is 4Ω, R₂ 6Ω and R₃ 2Ω, R_tot = 4 + 6 + 2 = 12Ω

In other words the total resistance steadily increases the more resistors you connect in series,

and you should notice that the current flowing steadily decreases as the total resistance increases.

Water flow analogy: The longer the thin pipe the greater the friction is to oppose the flow.

 

(b)

For comparison, the graph for wiring identical resistors, shows a negative gradient, the complete opposite effect to wiring the resistors in series.

See Part 5.5 for the comparison with resistors wired in parallel