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Lesson: Chapter 19 — Current Electricity

19.7. Resistors in series 3 of 3

A series example

The order of work in every network problem is the same: find the equivalent resistance first, then treat the whole thing as one resistor.

The problem

The figure shows a 10 Ω resistor and a 2 Ω resistor connected to a 6 V electricity supply.

  1. Find the equivalent resistance of the system.
  2. What is the current flowing through the circuit?

Figure to be added

Figure for example 1 — a 10 ohm resistor and a 2 ohm resistor drawn one after the other in a single loop, with a 6 V cell closing the circuit.

Part 1 — the equivalent resistance

equivalent resistance = R1 + R2

= 10 Ω + 2 Ω

= 12 Ω

Part 2 — the current

Applying V = IR to the whole circuit:

I = V ÷ R

= 6 ÷ 12

= 0.5 A

Use the equivalent resistance, not one of the two

The 6 V is across the pair, so it has to be divided by the 12 Ω of the pair. Dividing 6 by 10 or by 2 answers a question nobody asked.