Series and Parallel Circuits
Series and Parallel Circuits
Understanding the rules for current, potential difference, and resistance in series and parallel circuits is essential for GCSE Physics. These rules explain how components share energy and how circuits behave.
Series Circuits
In a series circuit, components are connected one after another in a single loop. There is only one path for the current to flow.
Rules for series circuits:
Current is the same everywhere in a series circuit. The same charge flows through every component.
I(total) = I(1) = I(2) = I(3)
Potential difference is shared between components. The total p.d. of the supply equals the sum of the p.d.s across each component.
V(total) = V(1) + V(2) + V(3)
Resistance — the total resistance is the sum of all the individual resistances. Adding more resistors in series increases the total resistance.
R(total) = R(1) + R(2) + R(3)
Example: Two resistors of 4 ohms and 6 ohms are connected in series to a 10 V battery.
- Total resistance = 4 + 6 = 10 ohms
- Current = V / R = 10 / 10 = 1 A (the same through both)
- P.d. across 4-ohm resistor = I x R = 1 x 4 = 4 V
- P.d. across 6-ohm resistor = 1 x 6 = 6 V (4 + 6 = 10 V, as expected)
Parallel Circuits
In a parallel circuit, components are connected on separate branches. There are multiple paths for the current.
Rules for parallel circuits:
Potential difference is the same across each branch. Every branch is connected directly across the supply.
V(total) = V(1) = V(2) = V(3)
Current is shared between branches. The total current from the supply is the sum of the currents through each branch.
I(total) = I(1) + I(2) + I(3)
Resistance — the total resistance is less than the smallest individual resistance. Adding more resistors in parallel provides more paths for current, decreasing total resistance.
1/R(total) = 1/R(1) + 1/R(2)
Example: Two resistors of 6 ohms and 3 ohms are connected in parallel across a 6 V supply.
- P.d. across each = 6 V (same in parallel)
- Current through 6-ohm resistor = 6 / 6 = 1 A
- Current through 3-ohm resistor = 6 / 3 = 2 A
- Total current = 1 + 2 = 3 A
- Total resistance = V / I = 6 / 3 = 2 ohms (less than the smallest resistor)
Comparing Series and Parallel
| Feature | Series | Parallel |
|---|---|---|
| Current | Same everywhere | Splits between branches |
| P.d. | Shared between components | Same across each branch |
| Total resistance | Sum of all resistors | Less than the smallest |
| If one component breaks | Whole circuit stops | Other branches still work |
| Brightness of identical bulbs | Dimmer (shared p.d.) | Full brightness (full p.d.) |
Practical Applications
- Christmas tree lights were traditionally in series — if one bulb blew, they all went out. Modern sets use parallel circuits.
- Household circuits are wired in parallel so each appliance gets the full mains voltage and can be switched independently.
- Ammeters are placed in series (to measure current flowing through a component).
- Voltmeters are placed in parallel (to measure p.d. across a component).
Exam Tips
- If asked to calculate values in a mixed (series-parallel) circuit, work out the parallel section first to find its combined resistance, then treat it as a single resistor in series with any others
- Draw a table of V, I, and R for each component to keep track of your working
- Remember: adding resistors in series INCREASES total resistance; adding in parallel DECREASES it