Series and parallel circuitsIB MYP Sciences: Revision notes

A circuit diagram uses standard symbols so anyone can read it. The main ones are: a cell (one long line and one short line), a battery (several cells), a switch, a lamp (a circle with a cross), a fixed resistor (a rectangle), a variable resistor (a rectangle with an arrow), an ammeter (a circle with A) and a voltmeter (a circle with V). Draw straight wires with a ruler, leave no gaps, and put the ammeter in series and the voltmeter in parallel with the component.

In a series circuit the components are connected one after another in a single loop. There is only one path for the current.

• The current is the same at every point: I = I₁ = I₂
• The potential differences add up to the supply p.d.: V = V₁ + V₂
• If one component breaks, the whole circuit stops.

Worked example: a 9.0 V battery is connected to two lamps in series. The p.d. across lamp 1 is 4.0 V, so the p.d. across lamp 2 is 9.0 − 4.0 = 5.0 V.

In a parallel circuit the components are on separate branches, so the current has more than one path.

• The p.d. across each branch is the same as the supply p.d.
• The currents in the branches add up to the current from the supply: I = I₁ + I₂
• If one branch breaks, the other branches keep working.

Household lamps and sockets are wired in parallel so that each works on its own at the full mains p.d.

In series, the total resistance is the sum of the resistances: R = R₁ + R₂. Adding more resistors makes the total resistance larger.

In parallel, the current has more paths to take, so the total resistance is smaller than the smallest resistor in the circuit. Adding another branch decreases the total resistance.

To find missing values, write down what is known and use V = I × R together with the series or parallel rules.

Worked example (series): 6.0 Ω and 3.0 Ω resistors are in series with an 18 V supply. Total R = 9.0 Ω. I = 18 ÷ 9.0 = 2.0 A. The p.d. across the 3.0 Ω resistor is 2.0 × 3.0 = 6.0 V.

Worked example (parallel): a 12 V supply is connected to a 4.0 Ω and a 6.0 Ω resistor in parallel. The currents are 12 ÷ 4.0 = 3.0 A and 12 ÷ 6.0 = 2.0 A, so the supply current is 5.0 A.

Choose the circuit to suit the job. A parallel circuit is used for lights and sockets in a house because every device gets the full p.d. and works independently. A series circuit is used when one switch or one fault should affect everything, such as some safety circuits. When evaluating a choice (for example for a string of lights), compare reliability, cost, safety and waste.