3.2.8Movement and Interactions
Resistors In Series and Parallel — AQA GCSE Combined Science Synergy 8465 - Physical Sciences Revision Notes & Topical Questions
5 min read 8 parts Lesson 25 of 59
Section 1 of 8Series circuits
Current
- Current There is only one path, so the same current passes through every component.
Potential difference
- Potential difference The supply voltage is shared between the components, so the individual potential differences add up to the supply voltage.
- If two identical resistors share a supply in series, each takes half of the supply voltage.
Total resistance
- Total resistance Rtotal = R1 + R2 + R3.
- Adding another resistor in series increases the total resistance.
Section 2 of 8Parallel circuits
Potential difference
- Potential difference Each branch is connected directly across the supply, so every branch has the same potential difference.
Current
- Current The supply current splits between branches; the branch currents add up to the total current.
Total resistance
- Total resistance Adding a resistor in parallel gives the current another path, so the total resistance decreases.
- The total is always smaller than the smallest individual resistor.
- For two identical resistors in parallel, the total resistance is half the value of one resistor.
Section 3 of 8Comparing the two
| Property | Series | Parallel |
|---|---|---|
| Current | Same everywhere | Splits between branches |
| Potential difference | Shared out between components | Same across each branch |
| Total resistance | Sum of all resistances | Less than the smallest resistance |
| If one component fails | All components stop | Other branches keep working |
Section 4 of 8Series calculation
1
A 4 Ω and a 6 Ω resistor are connected in series to a 12 V battery.
2
Total resistance R = 4 + 6 = 10 Ω.
3
Current I = V ÷ R = 12 ÷ 10 = 1.2 A, the same through both resistors.
4
The potential difference across the 4 Ω resistor is V = I × R = 1.2 × 4 = 4.8 V.
5
The 6 Ω resistor has 7.2 V, and 4.8 + 7.2 = 12 V.
Section 5 of 8Parallel calculation
1
A 6 Ω and a 3 Ω resistor are connected in parallel across a 12 V battery.
2
Each branch has the full 12 V across it, so the potential difference does not need to be shared.
3
Current in the 6 Ω branch: I = V ÷ R = 12 ÷ 6 = 2 A.
4
Current in the 3 Ω branch: I = 12 ÷ 3 = 4 A.
5
The battery supplies 2 + 4 = 6 A in total, so the total resistance is 12 ÷ 6 = 2 Ω, which is smaller than both resistors.
Section 6 of 8Everyday uses
Why the type of circuit matters
Household lighting and sockets
- Household lighting and sockets Mains circuits use parallel wiring so each appliance gets the full 230 V and can be switched on and off independently.
Old-style fairy lights
- Old-style fairy lights Bulbs wired in series all go out when one fails, because the single loop is broken.
Ammeters and voltmeters
- Ammeters and voltmeters An ammeter is placed in series to measure current through a component,
- and a voltmeter is placed in parallel to measure its potential difference.
Section 7 of 8Key terms
| Term | Meaning |
|---|---|
| Series circuit | A circuit in which components are connected end to end in a single loop. |
| Parallel circuit | A circuit in which components are connected on separate branches across the same supply. |
| Total resistance | The single resistance that could replace all components in a circuit. |
| Branch | One of the separate paths for current in a parallel circuit. |
| Potential divider | Resistors in series used to share a supply voltage in a fixed ratio. |
| Ammeter | A meter connected in series that measures current in amperes. |
| Voltmeter | A meter connected in parallel that measures potential difference in volts. |
Section 8 of 8Exam technique
1
Adding resistances in parallel in the same way as in series.
2
Thinking that adding resistors in parallel raises the total resistance; it lowers it.
3
Saying current is shared between series components; the current is the same.
4
Not checking that the potential differences in a series circuit add up to the supply voltage.
5
Connecting a voltmeter in series with a component instead of across it.
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