Two-element definitions: I, V, R
Three definitions, each worth two marks (one per element). Current = rate of flow of electric charge; ; unit ampere (A) — elements: (1) rate of flow, (2) of charge (not "electrons"). Potential difference (p.d.) = energy transferred per unit charge; ; unit volt (V) — elements: (1) energy transferred, (2) per unit charge. Resistance = ratio of p.d. to current; ; unit ohm (Ω) — elements: (1) ratio of p.d. to current, (2) unit Ω.
Series vs parallel: the six rules
Series: current is the same everywhere (); component voltages add to the supply (); resistances add (). Parallel: each branch has the full supply voltage (); branch currents add (); combined resistance is less than the smallest branch, from . These are exact laws, not approximations. Mini-example: 3 Ω + 6 Ω in series → Ω.
Three I-V characteristic shapes
Ohmic conductor (resistor at constant temperature): a straight line through the origin; gradient = , so R is constant. Filament lamp: a curve that gets less steep as current rises — the tungsten filament heats up, so resistance increases. Diode: almost no current in reverse; a steep rise in the forward direction once past the threshold (~0.6 V for silicon). Explain each shape with the physics, do not just describe it.
Drawn from real examiner reports.
Current is not "flow of electrons"
Two faults with "current is the flow of electrons": (1) it drops "rate", so it is not a per-second quantity; (2) it names the carrier (electrons) not the quantity (charge) — in an electrolyte the carriers are ions. The mark-scheme form is "rate of flow of electric charge", with as the second marking point. Both elements are needed for two marks.
November 2024 Paper 1P Q3: candidates who gave vague answers about safety (e.g. "burns" or "harm") scored zero because they were not specific enough — mirroring the precision trap seen on definition questions throughout the paper.
Full supply V on each series resistor
In series the supply is shared, not duplicated. A 12 V supply on two resistors does NOT put 12 V across each; the component voltages add to 12 V ( V). To find one voltage: total resistance , then current , then . Applying the full supply to each resistor breaks Kirchhoff's voltage law.
November 2024 Paper 1P Q9(b)(iv): the examiner reported that most candidates attempted to calculate the current in a series circuit by applying the full EMF to each resistor independently, showing a lack of understanding of voltages in series circuits.
Ammeter in parallel / voltmeter in series
An ammeter goes in series — it has near-zero resistance and is damaged if placed across the supply. A voltmeter goes in parallel across the component — it has very high resistance and would block the current in series. A meter placed the wrong way loses that part's marks. Check: ammeter in the wire, voltmeter bridging the component.
November 2024 Paper 1P Q9(a): some candidates made mistakes when drawing the voltmeter in a suitable position in the circuit, losing the mark for correct meter placement.
Current vs charge (rate vs total)
Current (I) is the RATE of flow of charge, in amperes (A); charge (Q) is the total amount that has flowed, in coulombs (C). They are linked by . A 2 A current flowing for 5 s transfers C, not 2 C. Do not quote a current value when the question asks for charge, or leave the answer in A when it should be C.
Parallel resistance is smaller, not bigger
Adding resistors in parallel gives a combined resistance SMALLER than any single branch — like opening extra motorway lanes, easing the flow. Use , then invert; never add them as in series. Self-check: if the answer is larger than the smallest branch, you added instead of using reciprocals. Example: 6 Ω ∥ 12 Ω → Ω.
Filament lamp: describe vs explain
For a filament lamp I-V graph, marks split between describing (a curve that gets less steep as current rises) and explaining (the filament heats up, so its resistance increases). Many answers describe the shape but never mention temperature or resistance, so they miss the explanation mark. Always give the physical reason, not just the shape.
Substitute first, then rearrange
Write the formula and put numbers in before rearranging. If your algebra then slips, a correct substitution can still earn the substitution mark. E.g. → → A. Rearranging first and getting it wrong can score zero even with the right data.
Definitions: give both elements
Definition questions are worth two marks — one per element. For current, p.d. and resistance, quote the relationship AND the unit (or the two physical parts). "Rate of flow of charge" plus "" scores two; "flow of electrons" scores zero. Learn the two-part form.
Explain I-V shapes, do not just describe
When a question says "explain" an I-V characteristic, link the shape to the physics: filament lamp — heating raises resistance; diode — conducts one way past a threshold; ohmic — constant resistance at constant temperature. A pure description misses the explanation marks.
Convert units before substituting
Before using , or the power formulae, convert to base units: mA → A (÷1000), minutes → s (×60), kΩ → Ω (×1000). A current left in mA or a time in minutes gives an answer wrong by a large factor. Convert first, then substitute, then add the unit.
| Quantity | Symbol | Formula | Unit |
|---|---|---|---|
| Charge | coulomb, C | ||
| Current | ampere, A | ||
| Potential difference | volt, V | ||
| Resistance | ohm, Ω | ||
| Ohm's law | — | — |
Where = current (A), = time (s), = charge (C), = energy/work done (J), = p.d. (potential difference, V), = resistance (Ω).
Factors affecting resistance of a wire:
Define electric current.
A resistor has a resistance of 30 Ω. The potential difference across it is 9.0 V.
Calculate the current through the resistor. Give the unit of your answer.
