DC Circuits, Kirchhoff's Laws and Power
Analysing series, parallel and multi-loop direct-current circuits using conservation of charge and energy, equivalent resistance and electrical power.
How to study A-level Physics
Define the system, represent the physics, select a justified model, calculate transparently, then test the result against units, limits and evidence.
Core concepts
Concept 1
Junction current conservation follows conservation of charge; loop potential changes follow conservation of energy.
Exam cue: Mark assumed current directions and potential rises or drops before writing equations.
Concept 2
Series and parallel combinations impose different current and potential-difference constraints.
Exam cue: Reduce only combinations that are genuinely in series or parallel.
Concept 3
Electrical power may be written in equivalent forms selected from the known circuit quantities.
Exam cue: Check that total power supplied equals total power transferred within rounding.
Risk pitfalls and guardrails
Assuming components are in parallel merely because they look visually adjacent.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Adding resistances directly for a parallel combination.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Applying a loop equation without a consistent sign convention.
Guardrail: Do not substitute a memorised formula until you have identified the system, variables, direction, assumptions and valid range of the model.
Memory anchors
Junction Rule
Total current entering a junction equals total current leaving it.
Loop Rule
The algebraic sum of potential changes around a closed loop is zero.
Series Current
Components in one unbranched path carry the same current.
Parallel Voltage
Branches connected across the same two nodes have the same potential difference.
Electrical Power
Power equals potential difference times current.
Checkpoint rule
Do the check-up only after you can summarize each concept in one sentence and identify one dangerous pitfall from memory.
Knowledge Check (after reading)
Short check-up to confirm understanding of this module.
Check-up Questions
What does Kirchhoff's first law express at a junction?
Currents of 1.2 A and 0.80 A enter a junction. A 0.50 A current leaves by one branch. What current leaves by the other?
Answer all questions to submit.
Next step personalized recommendations
Continue learning
Move forward only after this module is stable.
What is Pass Harbor?
Completely free exam prep for 247 UK exams.
- Practice questions
- Flashcards
- Study guides
- Mock exams
- No registration
- No paywall
- Start instantly
“No more expensive exam prep. Quality study tools should be accessible to everyone.”