Parallel Circuits: Current, PD & Resistance Revision Notes | GCSE AQA Higher Physics

1. What Makes Parallel Circuits Different

Think about the lights in your house. When you turn off the kitchen light, the living room light stays on. This happens because your house uses parallel circuits - each light has its own separate path to the mains supply.

In a parallel circuit, components sit on separate branches rather than in a single loop. Each component gets its own direct connection to both terminals of the power supply. This creates multiple pathways for current to flow through.

This arrangement gives parallel circuits three key properties that make them behave very differently from series circuits. Understanding these properties will help you tackle any parallel circuit question in your exam.

2. The Three Rules of Parallel Circuits

Parallel circuits follow three fundamental rules that govern how current, voltage and resistance behave. These rules are the opposite of what happens in series circuits, so many students get confused between them.

Rule 1: Voltage is the same everywhere. Every component in a parallel circuit experiences the same potential difference as the power supply. If you have a 12V battery, every bulb gets 12V across it, no matter how many bulbs you add.

Rule 2: Current splits and adds up. The total current from the power supply divides between the branches. Each branch carries some current, and when you add up all the branch currents, you get the total current from the supply.

Rule 3: Adding resistors decreases total resistance. This sounds backwards, but it makes sense when you think about it. More branches means more pathways for current, so it's easier for current to flow overall.

3. How Current Behaves in Parallel Circuits

Current behaviour in parallel circuits follows a simple rule: it splits at junctions and rejoins later. Think of current like water flowing through pipes that branch and then come back together.

At the first junction, the total current from the power supply divides between the available branches. How much current each branch gets depends on the resistance of that branch - branches with lower resistance get more current.

The mathematical relationship is: $I_{total} = I_1 + I_2 + I_3 + ...$. This means if you measure the current in each branch and add them up, you'll get the total current leaving the power supply.

Worked example: Calculating Total Current in Parallel Circuits

A parallel circuit has three branches with currents of 2A, 3A and 1.5A. Calculate the total current from the power supply.

1. $I_{total} = I_1 + I_2 + I_3$ — In parallel, total current equals sum of branch currents
2. $I_{total} = 2 + 3 + 1.5$ — Add all the branch currents together
3. $I_{total} = 6.5$ A — Always include units in your final answer

Answer: 6.5 A

4. Why Voltage Stays Constant

The voltage rule in parallel circuits often surprises students because it's the opposite of series circuits. In parallel, every component receives the full supply voltage, regardless of how many components you add.

This happens because each component has its own direct connection to both terminals of the power supply. It's like having multiple separate circuits, each getting the full 'push' from the battery.

You can prove this with a voltmeter. Measure across any component in a parallel circuit and you'll get the same reading as measuring across the power supply terminals. The potential difference is identical everywhere.

This constant voltage is why parallel circuits are used in homes. Every appliance needs the full mains voltage (230V in the UK) to work properly. If we used series circuits, adding more appliances would reduce the voltage to each one.

5. Understanding Resistance in Parallel

Here's where parallel circuits get really interesting. Adding more resistors actually decreases the total resistance of the circuit. This seems backwards until you understand what's really happening.

Think of resistance like narrow doorways that restrict the flow of people. In series, people must go through every doorway one after another - each doorway adds to the total restriction. In parallel, people can choose between multiple doorways - more doorways means easier flow overall.

When you add a parallel branch, you're giving current another pathway to flow through. Even if the new branch has high resistance, it still provides some additional route for current. More routes means less overall resistance.

The total resistance in parallel is always less than the smallest individual resistor. For example, if you have resistors of 10Ω and 20Ω in parallel, the overall resistance is lower than 10Ω. For GCSE, you are not required to perform any calculations for total resistance; instead, focus on understanding qualitatively that adding more branches decreases the overall resistance.

6. Comparing Series and Parallel Circuits

The key differences between series and parallel circuits often appear in exam questions. Understanding these contrasts will help you identify which type of circuit you're dealing with and apply the correct rules.

In series circuits, there's only one path for current, so current is the same everywhere but voltage divides between components. In parallel circuits, there are multiple paths, so voltage is the same everywhere but current divides between branches.

Series circuits have a major disadvantage: if one component breaks, the whole circuit stops working because there's only one path. Parallel circuits are more reliable because each component has its own path - if one breaks, the others keep working.

For resistance, series circuits add up all the resistances, making the total higher. Parallel circuits do the opposite - the total resistance decreases as you add more branches. This is why your house lights don't get dimmer when you turn on more of them.

7. Exam Technique and Common Mistakes

Parallel circuit questions test your understanding of the three key rules. Examiners often try to catch you by mixing up series and parallel rules, so always identify the circuit type first.

When calculating currents, remember that current splits at junctions and rejoins later. Draw arrows on your circuit diagram to show current flow - this helps you visualise which currents to add together.

For voltage questions, remember that every component in parallel gets the full supply voltage. Don't divide the voltage between components - that's what happens in series circuits.

Watch out for questions about adding components. In parallel, adding more components increases total current (more branches to carry current) but decreases total resistance (more pathways available). This is the opposite of series behaviour.