Construct and compare series and parallel circuits, explaining and calculating current, potential difference and resistance, including equivalent resistance in series.
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GCSE Physics Revision
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GCSE Physics revision
Series and parallel circuits
Series and parallel circuits
Your specification
AQA student objectives
Learning pathway
All · Most · Some
Apply the scientific explanation of series and parallel circuits to a relevant example.
Analyse a new situation involving series and parallel circuits and explain the scientific reasoning.
Revision summary
Key knowledge
Read on screen, then print for Cornell-style active revision.
What is a Parallel Circuit?
- A parallel circuit contains more than one loop, with each loop typically containing a single component.
- Unlike series circuits, if one component in a parallel circuit breaks, the rest of the circuit continues to work.
- Real-world circuits are often a mix of series and parallel sections, meaning some loops may contain more than one component.
Potential Difference in Parallel Circuits
- In a purely parallel circuit, every component receives the full source potential difference.
- For example, if a battery supplies 12 V, then each component across every loop will have 12 V across it.
- This is a key advantage over series circuits, where the potential difference is shared between components.
Current in Parallel Circuits
- The total current in a parallel circuit is shared between all the loops.
- The individual branch currents always add up to equal the total current supplied by the source.
- For example, if the total current is 4 A across two loops, the split could be 3 A and 1 A, or 2 A and 2 A.
How Resistance Affects Current Distribution
- The way current splits between loops depends on the resistance of each individual component.
- Loops with greater resistance carry a smaller share of the current, whilst loops with lower resistance carry a larger share.
- This can be compared to water flowing downhill — most water takes the path of least resistance.
Resistance in Parallel Circuits
- Adding more components in parallel decreases the overall resistance of the circuit.
- This is because each additional loop provides another path for current to flow, making it easier for charge to move through the circuit.
- Even if a new loop contains a high-resistance component, the total resistance of the circuit still decreases.
Key Equations and Rules Summary
- The potential difference across each branch in a parallel circuit is equal:
- Vtotal = V1 = V2 = V3
- The total current is the sum of the individual branch currents: Itotal = I1 + I2 + I3
- The total resistance in a parallel circuit is always less than the smallest individual resistance in any branch.
Types of Circuit Connections
- Electrical components can be connected in two ways: in series (a single loop) or in parallel (multiple branches).
- In a series circuit, all components are joined in one single pathway, so removing any one component breaks the entire circuit.
- In a parallel circuit, components are connected across separate branches, providing multiple pathways for current to flow.
Current in a Series Circuit
- The current through every component in a series circuit is the same throughout the entire loop.
- For example, if an ammeter reads 0.3 A, then 0.3 A flows through every component in that series circuit.
- Because the current is equal through identical lamps in series, they will glow with equal brightness.
Potential Difference in a Series Circuit
- The total potential difference (voltage) of the power supply is shared between all the components in a series circuit.
- For two identical lamps connected to a 9 V supply, each lamp has a potential difference of 4.5 V across it.
- The individual potential differences across each component add up to equal the total supply voltage.
Resistance in a Series Circuit
- The total resistance in a series circuit is the sum of the resistances of all individual components: Rtotal = R_1 + R_2.
- For example, two resistors each of 10 Ω connected in series give a total resistance of 20 Ω.
- The unit of resistance is the ohm (Ω), represented by the Greek letter omega.
Potential Difference in a Parallel Circuit
- In a parallel circuit, the potential difference across each component (in each branch) is the same.
- For example, if the supply voltage is 9 V, then 9 V appears across every branch of the parallel circuit.
Current in a Parallel Circuit
- The total current from the power supply is the sum of the currents through each separate branch: Itotal = I_1 + I_2.
- For example, if the total current is 0.4 A and there are two identical branches, each branch carries 0.2 A.
- The branch currents recombine after the components so that the total current returns to 0.4 A back at the power supply.
Resistance in a Parallel Circuit
- The total resistance of a parallel circuit is always less than the resistance of the smallest individual resistor.
- For example, two resistors of 45 Ω each in parallel give a total resistance of less than 45 Ω.
- The formula for calculating total resistance in parallel is frac{1}{Rtotal} = 1 / R_1 + 1 / R_2, though for GCSE you only need to know the rule, not apply this equation.
- Using V = IR in Circuit Calculations The equation V = IR links potential difference (V), current (I), and resistance (R), and can be rearranged to I = V / R to find current.
- In a parallel circuit, the same supply voltage applies to each branch, so you use the branch voltage and branch resistance to find each branch current.
- For a branch containing components in series, you must first add their resistances together before applying I = V / R.
Mixed Series and Parallel Circuits
- Some circuits contain both series and parallel sections, requiring you to apply the rules for both types.
- For a branch that itself contains two resistors in series, the total resistance of that branch is Rbranch = R_1 + R_2.
- The total current from the supply equals the sum of all individual branch currents, combining both series and parallel rules.
Key Rules to Remember for Your Exam
- Series circuit rules: current is the same everywhere; voltage is shared; total resistance is the sum of individual resistances.
- Parallel circuit rules: voltage is the same across each branch; total current is the sum of branch currents; total resistance is less than the smallest individual resistance.
- Always identify whether components are in series or parallel before applying the correct rule in an exam question.