P2.74SD0 · Section P2: Electricity

Current, voltage and resistance in circuits

Key learnings

V = I x R; resistance = V / I
The central relation is $V = I \times R$ ( $V$ in volts, $I$ in amps, $R$ in ohms). Resistance is the RATIO of the p.d. across a component to the current through it ( $R = V/I$ ; unit ohm, Ω) — two elements: the ratio and the unit; "opposition to current" alone is not enough. Potential difference is the energy transferred per unit charge (volt); current is the rate of flow of charge (amp). For a fixed resistor at constant temperature $R$ is constant, so $I \propto V$ .
Series vs parallel rules
SERIES (single loop): the current is the SAME everywhere; the voltage is SHARED — the component p.d.s add up to the supply, so two equal resistors on a 6 V cell get 3 V each. PARALLEL (separate branches): the voltage across each branch is the SAME and equals the supply voltage; the current is SHARED — the branch currents add up to the total. A parallel advantage: each branch works INDEPENDENTLY with the full supply voltage, so one failing does not stop the others.
I-V graphs; LDR and thermistor
An I–V characteristic plots current against p.d. A metal wire or fixed resistor at constant temperature gives a STRAIGHT line through the origin ( $I \propto V$ , constant resistance — ohmic). A filament lamp gives an S-shaped curve that gets shallower, because the hot filament's resistance INCREASES. A diode conducts ONE way only. An LDR's resistance FALLS as light brightens; an NTC thermistor's FALLS as temperature rises; a lamp or LED lights only when current flows.

Common mistakes

Drawn from real examiner reports.

Series voltage is shared, not full

The biggest series error is giving each series resistor the WHOLE supply voltage. In series the p.d. is shared: the component p.d.s ADD to the supply. So a 6.0 V cell with two identical resistors puts 3.0 V across each, and the current follows from that shared p.d. ( $I = 3.0/R$ ), not the full 6.0 V. Only in PARALLEL does each branch get the full supply voltage.

Nov 2024 1P Q9b(iv)

Ammeter in series, voltmeter in parallel

A voltmeter must be connected in PARALLEL (across) the component whose p.d. is being measured; an ammeter in SERIES so the same current passes through it. Putting the voltmeter in series is a fatal circuit error. Also draw the VARIABLE RESISTOR correctly — a rectangle with an arrow through it — included so the current, and the p.d. across the test component, can be varied.

Nov 2024 1P Q9a

Parallel LOWERS total resistance

Adding a resistor in PARALLEL DECREASES the total resistance, so the total current INCREASES (it can even double). The dominant misconception — that connecting resistors in parallel ADDS resistance like a series chain — scored most candidates zero. Extra parallel paths give the current more routes, which lowers the overall resistance.

Jun 2023 1P Q11b

Resistance is a ratio, not just "opposition"

Writing only "opposition to current", or "how hard current flows", does not earn full credit. The mark-scheme definition is the RATIO of the p.d. across a component to the current through it ( $R = V/I$ ), measured in OHMS (Ω) — the ratio and the unit. Read from a V–I graph, the resistance is the gradient; give its unit as ohms, never "V/mA".

Jun 2024 1PR Q7b(i)-(ii)

LDR and thermistor both go DOWN

An LDR's resistance FALLS as the light gets brighter; an NTC thermistor's resistance FALLS as the temperature rises. Both DECREASE when their controlling condition increases. Students often state the opposite — that more light or more heat means more resistance — which reverses the direction of the current change and loses the mark.

Filament lamp I-V is not a straight line

A filament lamp's I–V graph is NON-LINEAR: as the p.d. rises the current still increases, but by a smaller amount each time (the curve levels off), because the hot filament's resistance increases. Saying "current DECREASES at higher voltages" is wrong. Do not draw a lamp's graph as a straight line like a fixed resistor's.

Jun 2023 1P Q2b(i)

"Voltage" defined vaguely loses the mark

Fewer than a quarter of candidates could correctly define potential difference. The mark-scheme form is the ENERGY TRANSFERRED PER UNIT CHARGE (per coulomb) between two points, with the unit the volt (V). Vague answers — "the push", or "the power of the cell" — score nothing. Learn the two-element form: energy per unit charge, and the unit.

Jun 2024 1P Q7c(i)

Exam tips

Series-or-parallel first, then the rule
Decide the arrangement first — series (one loop) or parallel. Then: series → same current, shared voltage; parallel → same voltage, shared current. Find the p.d. across the ACTUAL component before using $V = I \times R$ — in series only its share, never the full supply.
Link the parallel advantage to "parallel"
When asked the advantage of a parallel arrangement, explicitly LINK "each lamp works independently" to the components being in PARALLEL — only the abler candidates made that link. Stating the advantage without naming the parallel connection did not gain full credit.
Read resistance as a graph gradient
On a V–I (or I–V) graph, a straight line through the origin means $V \propto I$ and a constant resistance; the resistance is the gradient. Give its unit as ohms (Ω), never "V/mA", and convert kΩ to Ω to avoid a power-of-ten error.

Revision notes