SITEMAP   School Physics Notes: Electricity 5.6 Investigating identical resistors in parallel

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Electricity section 5: Part 5.6 How to investigate the effect on the total resistance of a circuit by adding identical resistors together in parallel

Doc Brown's Physics exam study revision notes: There are various sections to work through, after 1 they can be read and studied in any order.

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

(b) Adding resistors wired in parallel

You set up a relatively simple circuit of a power supply, ammeter and one or more identical resistors, all wired in parallel. You can use filament bulbs as the resistors.

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 parallel 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 parallel.

You should find the graph differs from the series graph in two ways.

(i) Its a curve, not a linear graph,

(ii) and most significantly, the total resistance dramatically decreases the more resistors you have wired in parallel - the complete opposite effect to adding resistors in series.

Therefore the more resistors wired in parallel, the lower the total resistance.

This is the complete opposite effect, compared to resistances wired in series.

Water flow analogy: There are more 'shorter' pipes for the water to flow through offers less resistance than one 'long' so the total water flow can increase.

Note 1. For comparison -  the effect of resistors in series - linear graph with a positive constant gradient

See Part 5.3 For comparison with resistors wired in series

Note 2 - some courses may require you to know the following

To calculate the total resistance of resistors wired in parallel, the following equation applies:

1/Rtotal = 1/R1 + 1/R2 + 1/R3 etc.

Keywords, phrases and learning objectives for wiring resistors in parallel

Know how to investigate the effect on the total resistance of a circuit by adding identical resistors together in parallel circuit.

Be able to draw the circuit diagram, describe the method and how to analyse and interpret the results with a graph.

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