IB Physics electricity & circuits questions become much more manageable once you treat them as a sequence of decisions rather than a search for the right formula. Identify the circuit structure, label known quantities, apply the correct series or parallel rule, and check that your answer is physically reasonable.
In the current course, electricity and circuits appears primarily in B.5 Current and circuits. Questions commonly test current, potential difference, resistance, resistivity, power, internal resistance, variable resistors, circuit diagrams, and data interpretation. The formats are predictable, but recurring traps involving series and parallel connections, graph gradients, and terminal potential difference cost many marks.
How electricity and circuits is examined in IB Physics
The current IB Physics course, first assessed in 2025, places current and circuits within theme B, The particulate nature of matter. The topic is taught at both SL and HL, although it may be connected to other parts of the course in extended questions.
According to the official IB Physics subject brief, external assessment consists of Paper 1 and Paper 2:
| Assessment | Format | How circuits may appear |
|---|---|---|
| Paper 1A | Multiple-choice questions | Equivalent resistance, current ratios, conceptual circuit changes, power, graphs |
| Paper 1B | Data-based questions | Current-voltage data, gradients, uncertainties, experimental circuits |
| Paper 2 | Short-answer and extended-response questions | Multi-stage calculations, explanations, circuit analysis, links to energy or practical work |
Calculators are permitted, and students use a clean Physics data booklet. The IB emphasizes interpretation and application rather than simple formula recall, as explained in the official Physics curriculum update. You should therefore know what each equation means and when it applies.
The RevisionDojo IB Physics data booklet is useful for becoming familiar with the location and form of the B.5 equations before attempting timed questions.
The essential circuit relationships
Most calculations begin with a small set of definitions and conservation rules.
| Quantity | Relationship | Meaning |
|---|---|---|
| Current | I = Δq/Δt | Rate at which charge passes a point |
| Potential difference | V = W/q | Energy transferred per unit charge |
| Resistance | R = V/I | Ratio of potential difference to current |
| Resistivity | ρ = RA/L | Material property related to wire resistance |
| Power | P = IV = I²R = V²/R | Rate of electrical energy transfer |
| Internal resistance | ε = I(R + r) | Emf supplies energy to both external and internal resistance |
Do not use every equation containing the quantities given. Select the equation whose physical conditions match the circuit. For example, P = V²/R is convenient only when the potential difference across that particular component is known.
Series and parallel rules
For components in series:
- Current is the same through every component.
- Potential differences add.
- Resistances add: Rₛ = R₁ + R₂ + ...
For components in parallel:
- Potential difference is the same across every branch.
- Branch currents add to the total current.
- Reciprocals of resistance add: 1/Rₚ = 1/R₁ + 1/R₂ + ...
A useful check is that a parallel equivalent resistance must be smaller than the smallest branch resistance. If it is not, the calculation is wrong.
A reliable method for answering circuit questions
1. Redraw and label the circuit
Mark the emf, resistances, currents, and potential differences directly on a simplified diagram. Identify junctions and decide which components genuinely share one unbranched path.
Components that look adjacent are not automatically in series. They are in series only if the same current must pass through both without encountering a junction.
2. Simplify from the inside outward
Replace an obvious series or parallel group with its equivalent resistance. Repeat until the source sees one total external resistance, then calculate total current before reconstructing branch quantities.
3. Apply conservation consistently
At a junction, current entering equals current leaving. Around a complete loop, energy supplied per unit charge equals energy transferred per unit charge. These principles explain the series and parallel rules, so they also help when a circuit cannot be reduced immediately.
4. Show substitutions and units
In written-response questions, show the equation, substituted values, and final unit. A bare calculator answer hides the method and makes it difficult to recover credit if an arithmetic error occurs.
5. Test the result physically
Ask whether adding a parallel branch decreased total resistance, whether a real cell's terminal voltage is below its emf while delivering current, and whether branch currents add to the total. These checks take seconds and catch many mistakes.
Worked example: a mixed resistor circuit
A 4.0 Ω resistor is connected in series with a parallel combination of 6.0 Ω and 3.0 Ω. The combination is connected to an ideal 12 V supply.
First calculate the parallel resistance:
1/Rₚ = 1/6.0 + 1/3.0 = 1/2.0, so Rₚ = 2.0 Ω.
The total resistance is therefore:
Rtotal = 4.0 + 2.0 = 6.0 Ω.
The total current, which also passes through the 4.0 Ω resistor, is:
I = 12/6.0 = 2.0 A.
The potential difference across the 4.0 Ω resistor is V = IR = 2.0 × 4.0 = 8.0 V. This leaves 4.0 V across the parallel combination. The branch currents are consequently 4.0/6.0 = 0.67 A and 4.0/3.0 = 1.33 A, which add to 2.0 A as required.
Internal resistance and terminal potential difference
A real cell is modeled as an emf ε in series with an internal resistance r. When the cell supplies current, some energy per unit charge is transferred inside the cell, giving:
ε = I(R + r) and Vterminal = ε - Ir.
Suppose a cell has ε = 12 V, r = 0.50 Ω, and external resistance R = 5.5 Ω. The current is I = 12/(5.5 + 0.50) = 2.0 A, while the terminal potential difference is V = IR = 11 V. The missing 1.0 V is the internal voltage drop Ir.
A frequent mistake is to set the terminal potential difference equal to the emf while current is flowing. They are equal only in the idealized case of zero internal resistance, or when no current is drawn.
Graph and experimental questions
Electricity questions often present current-voltage data rather than a completed circuit calculation. Read the axes before interpreting the gradient:
- On a graph of V against I, gradient = R.
- On a graph of I against V, gradient = 1/R.
- A straight line through the origin indicates constant resistance under the measured conditions.
- A filament lamp is non-ohmic because heating changes its resistance.
For a wire, R = ρL/A. Increasing length increases resistance, while increasing cross-sectional area decreases it. If diameter is given, remember that A = πd²/4, so doubling diameter makes the area four times larger.
When asked to design or interpret an experiment, place an ammeter in series and a voltmeter in parallel with the tested component. Unless a question states otherwise, the course treats these meters as ideal. The official IB sample examination papers page provides the authoritative route for checking the current assessment style.
Common traps and how to avoid them
- Using total voltage in a branch calculation: Parallel branches share the voltage across the parallel section, which may not equal the source voltage if another component is in series.
- Adding parallel resistances directly: Use reciprocal addition, then invert the result.
- Confusing emf with terminal voltage: Include internal resistance whenever the cell is modeled as non-ideal.
- Assuming V = IR proves Ohm's law: Resistance is defined by V/I, but an ohmic conductor requires proportionality between V and I at constant temperature.
- Using the wrong graph gradient: State the axes before interpreting the slope.
- Rounding too early: Keep unrounded calculator values until the final answer.
- Ignoring prefixes: Convert mA to A, kΩ to Ω, and mm² to m² before substitution.
- Confusing electron flow and conventional current: Conventional current is defined in the direction positive charge would move.
The fastest way to improve your circuit method
Re-reading notes can clarify definitions, but it does not reveal whether you can recognize a circuit structure under exam pressure. For most students, the fastest route is to attempt an electricity question independently and then watch a complete worked video solution.
Use this cycle:
- Attempt the question without notes.
- Mark where your reasoning first diverged from the solution.
- Watch the circuit being labeled and simplified in a worked IB Physics video solution.
- Write one correction rule, such as “calculate the voltage across the parallel section before branch currents.”
- Redo the question from a blank page the next day.
The B.5 Current and circuits Questionbank allows targeted practice instead of waiting for circuits to appear in a full paper. Pair it with the B.5 circuit notes when you identify a knowledge gap, or use the structured B.5 lessons when the method itself is unclear. Jojo AI can help diagnose whether an error came from physics, algebra, units, or interpretation.
Conclusion
IB Physics electricity & circuits questions repeatedly test the same foundations: correct circuit classification, conservation of current and energy, appropriate equation choice, and careful interpretation of graphs. Label the circuit, simplify it systematically, reconstruct branch values, and finish with a physical check.
Effective revision should place question practice before passive review. RevisionDojo's B.5 Questionbank, worked video solutions, Study Notes, and Jojo AI provide a practical sequence for attempting, diagnosing, and correcting circuit problems.
Sources and referenced URLs
- Official IB Physics subject brief
- Official IB Physics curriculum update
- Official IB sample examination papers
- RevisionDojo IB Physics data booklet
- RevisionDojo IB Physics video solutions
- RevisionDojo B.5 Current and circuits Questionbank
- RevisionDojo B.5 Current and circuits notes
- RevisionDojo B.5 Current and circuits lessons
