D.C. circuits

Circuit symbols, series and parallel rules, effective resistance and whole-circuit calculations.

SEC G3 Combined Science Physics component 2027
3 lessons

Before you begin

D.C. circuits is about working out the current and p.d. anywhere in a circuit. The lessons start with circuit diagrams and how to connect meters, then use the rules for series and parallel connections, and finish by combining resistors and solving whole circuits with R = V/I.

Be comfortable with: electric charge and current, especially p.d. and R = V/I.

Learning goals

Recognise and interpret circuit symbols for cells, batteries, switches, lamps, LEDs, resistors, fuses, ammeters and voltmeters
Draw circuit diagrams with cells, batteries, switches, lamps, LEDs, fixed and variable resistors, fuses, ammeters and voltmeters
Apply the same-current rule in series circuits
Apply the potential-difference sum in series circuits
Apply current conservation at parallel junctions
Apply equal potential difference across parallel branches
Calculate effective resistance in series
Calculate effective resistance in parallel
Solve whole-circuit problems using consistent quantities

Syllabus statements covered

draw circuit diagrams with power sources (cell or battery), switches, lamps, light-emitting diodes (LEDs), resistors (fixed and variable), fuses, ammeters and voltmeters
state that the current at every point in a series circuit is the same and apply the principle to new situations or to solve related problems
state that the sum of the potential differences in a series circuit is equal to the potential difference across the whole circuit and apply the principle to new situations or to solve related problems
state that the sum of the currents in the separate branches of a parallel circuit is equal to the current from the source and apply the principle to new situations or to solve related problems
state that the potential difference across the separate branches of a parallel circuit is the same and apply the principle to new situations or to solve related problems
recall and apply the formulae for the effective resistance of a number of resistors in series and in parallel to new situations or to solve related problems
recall and apply the relevant relationships, including R = V / I and those for current, potential differences and resistors in series and in parallel circuits, in calculations involving a whole circuit.

Lessons

Work through them in order.

Practise and check

Or choose

Topic reference

Quick reference

Quantity	Series	Parallel
current	same at every point	branch currents add: I = I₁ + I₂ + …
p.d.	p.d.s add: V = V₁ + V₂ + …	same across each branch
effective resistance	R = R₁ + R₂ + …	1/R = 1/R₁ + 1/R₂ + …
size of the result	larger than each resistance	smaller than the smallest branch resistance

Meter	How it is connected
ammeter	in series with the component
voltmeter	in parallel, across the component

Core knowledge to remember

The current at every point in a series circuit is the same.
The sum of the potential differences in a series circuit is equal to the potential difference across the whole circuit.
The sum of the currents in the separate branches of a parallel circuit is equal to the current from the source.
The potential difference across the separate branches of a parallel circuit is the same.
In a whole-circuit calculation, replace one series or parallel group at a time by its effective resistance, then use R = V/I with values that belong to the same part of the circuit.

Common mistakes

Connecting an ammeter in parallel or a voltmeter in series.
Saying the current is the same in parallel branches, or the p.d. is the same across series components.
Adding parallel resistances, or forgetting to take the reciprocal at the end.
Using R = V/I with a p.d. and a current from different parts of the circuit.