Series vs Parallel Circuits: Key Differences

Electric charge flows only around a complete loop, pushed by a cell, battery or power supply. When a circuit has more than one component, there are two basic ways to connect them. In a series circuit the components sit one after another in a single loop. In a parallel circuit each component has its own branch connected across the supply. The same lamps, resistors and battery behave very differently in each arrangement.

Three quantities describe what is happening in a circuit:

• Current (I) is the rate of flow of charge, measured in amperes (A) with an ammeter connected in series.
• Potential difference (V), also called voltage, is the energy transferred per unit of charge, measured in volts (V) with a voltmeter connected in parallel across a component.
• Resistance (R) is how much a component opposes the current, measured in ohms (Ω).

They are linked by V = IR. A component with 6 V across it and a resistance of 6 Ω carries a current of 1 A.

With only one path, the current is the same everywhere in a series circuit: an ammeter reads the same before and after each lamp, because current is not used up. The supply voltage is shared between the components, and the total resistance is the sum of the individual resistances. Adding another lamp raises the total resistance, lowers the current and makes every lamp dimmer. If one lamp breaks, the loop is broken and everything goes out.

In a parallel circuit, each branch gets the full supply voltage, and the current from the supply splits between the branches. The branch currents add up to the total current. Each extra branch gives the charge another path, so the total resistance falls below that of the smallest resistor. Lamps added in parallel stay as bright as a single lamp, but the battery has to supply more current and runs down faster. For older students, the total resistance of two resistors in parallel comes from 1/R = 1/R₁ + 1/R₂.

Take two 6 Ω resistors and a 6 V battery. In series, the total resistance is 12 Ω, so the current is 6 ÷ 12 = 0.5 A and each resistor has 3 V across it. In parallel, each resistor has the full 6 V across it and carries 6 ÷ 6 = 1 A, so the battery supplies 2 A.

The lights and sockets in homes are wired in parallel so that every appliance gets the full mains voltage and can be switched on and off by itself. A switch is placed in series with whatever it controls, because opening it must break that loop and stop the current.