Series and Parallel CircuitsAQA GCSE Physics: Revision notes
Section 1
What are the key differences between series and parallel circuits?
Series circuits have components connected in a single continuous loop, so there is only one path for current to flow. Parallel circuits have components connected in separate branches, creating multiple paths for current to flow.
| Feature | Series | Parallel |
|---|---|---|
| Current path | Single loop | Multiple branches |
| Components | Connected end-to-end | Connected side-by-side |
| Failure | If one component fails, circuit breaks | If one branch fails, others still work |
Understanding the physical layout is essential for answering questions about how current and potential difference behave in each type.
Think of a series circuit like a single lane road where all traffic flows through the same route. A parallel circuit is like a motorway with multiple lanes — traffic can split and take different routes, but all lanes connect the same two places.
Section 2
How does current behave in series circuits?
In a series circuit, current is the same at all points throughout the circuit. This is because there is only one path for charge to flow, so the same amount of charge passes through every component each second.
If you measure current at any point in a series circuit using an ammeter, you will always get the same reading. This follows the principle of conservation of charge — charge cannot accumulate or disappear, so the same amount must flow everywhere.
Example: In a series circuit with three resistors:
- Current through resistor 1 = 2 A
- Current through resistor 2 = 2 A
- Current through resistor 3 = 2 A
- Total current from the power supply = 2 A
Examiners expect you to state that current is 'the same' or 'identical' at all points in a series circuit. Never say current 'decreases' or 'is shared' in series — these are parallel circuit behaviours.
Students often confuse this and think current decreases as it passes through components in series. It doesn't — the current remains constant because charge is conserved.
Section 4
How do current and potential difference behave in parallel circuits?
Potential difference is the same across all components in a parallel circuit. Each branch experiences the full voltage from the power supply, regardless of resistance.
Current is shared between branches. The total current from the power supply splits at junctions and flows through each branch separately. The amount of current in each branch depends on the resistance of that branch:
- Branches with lower resistance carry more current
- Branches with higher resistance carry less current
Key rules:
- V_total = V₁ = V₂ = V₃ = ... (all equal)
- I_total = I₁ + I₂ + I₃ + ... (currents add up)
Example: A parallel circuit with a 12 V power supply and two resistors:
- Potential difference across branch 1 = 12 V
- Potential difference across branch 2 = 12 V
- Current in branch 1 = 3 A
- Current in branch 2 = 2 A
- Total current = 3 + 2 = 5 A
Remember: same voltage (potential difference), different currents in each branch. Examiners expect you to clearly state these two facts separately when describing parallel circuits.
Students often think potential difference is 'shared' in parallel circuits like it is in series. It isn't — voltage is the same across each parallel branch, not divided up.
Section 5
How do you calculate total resistance in series circuits?
In a series circuit, resistances add directly together. The total resistance is the sum of all individual resistances:
R_total = R₁ + R₂ + R₃ + ...
This is because components resist the flow of current one after another, so their resistances accumulate.
Step-by-step calculation:
- Identify all resistances in the circuit
- Add them together using the formula
- State the final answer with units (ohms, Ω)
Example: Three resistors in series with values 5 Ω, 3 Ω, and 2 Ω:
- R_total = 5 + 3 + 2
- R_total = 10 Ω
Important property: The total resistance in a series circuit is always greater than the largest individual resistance.
A series circuit has resistors of 10 Ω, 5 Ω, and 15 Ω. Calculate total resistance: R_total = 10 + 5 + 15 = 30 Ω. This is straightforward addition — examiners expect you to show all values and the final answer clearly.
Section 6
How do you calculate total resistance in parallel circuits?
In a parallel circuit, resistances do not add directly. Instead, use the reciprocal formula:
1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...
Then rearrange to find R_total by taking the reciprocal of the sum.
Step-by-step calculation:
- Write down all individual resistances
- Calculate the reciprocal of each (1/R)
- Add all reciprocals together
- Take the reciprocal of the total to find R_total
Example: Two resistors in parallel with values 6 Ω and 3 Ω:
- 1/R_total = 1/6 + 1/3
- 1/R_total = 1/6 + 2/6 = 3/6
- 1/R_total = 0.5
- R_total = 1 ÷ 0.5 = 2 Ω
Important property: The total resistance in a parallel circuit is always less than the smallest individual resistance. This is because parallel branches provide alternative paths for current.
Examiners mark carefully on parallel resistance calculations. Always show: the reciprocal formula, individual reciprocals added together, and the final answer after taking the reciprocal. Calculator errors are common — show your working clearly.
Three resistors in parallel: 4 Ω, 4 Ω, and 4 Ω. Calculate: 1/R_total = 1/4 + 1/4 + 1/4 = 3/4. Therefore R_total = 4/3 ≈ 1.33 Ω. Notice it's much smaller than any individual resistor.
Must Know
- Series circuits have one path for current; parallel circuits have multiple branches. Current is the same everywhere in series but splits between branches in parallel. Potential difference is shared in series (adds to total) but the same across all branches in parallel.
- In series: R_total = R₁ + R₂ + R₃ + ...
- In parallel: 1/R_total = 1/R₁ + 1/R₂ + 1/R₃ + ...
- Total resistance in series is always greater than any individual resistance; in parallel it is always less.
- Use the principle of conservation of charge to explain why current doesn't change in series circuits.
- Total potential difference always equals the sum of individual potential differences in series: V_total = V₁ + V₂ + ...
- Total current always equals the sum of branch currents in parallel: I_total = I₁ + I₂ + ...
That's the notes covered.
Carry on to the next subtopic.