P2.144SD0 · Section P2: Electricity

Charge, energy and circuit rules

Key learnings

Current and charge — Q = I x t
Current is the RATE OF FLOW of electric charge ( $I = Q/t$ ; unit ampere, A, where 1 A = 1 coulomb per second) — two elements: rate of flow of CHARGE (not just "electrons"), and the unit. Rearranged, the charge transferred is $Q = I \times t$ ( $Q$ in coulombs, $t$ in SECONDS). A current of 0.30 A for 40 s moves $0.30 \times 40 = 12$ C. In a metal, free electrons drift from negative to positive — OPPOSITE to conventional current (positive to negative).
Voltage and energy — E = Q x V
Potential difference (voltage) is the ENERGY TRANSFERRED PER UNIT CHARGE ( $V = E/Q$ ; unit volt, V, where 1 V = 1 joule per coulomb) — two elements: energy per coulomb, and the unit. Rearranged, the energy transferred is $E = Q \times V$ ( $E$ in joules). When 8.0 C passes through a lamp with 6.0 V across it, $E = 8.0 \times 6.0 = 48$ J. A 12 V supply gives each coulomb twice the energy of a 6 V supply.
Series and parallel circuit rules
From conservation of charge and energy: SERIES (one loop) — the current is the same everywhere (charge cannot pile up), and the source voltage is SHARED, so the p.d.s add up ( $V_\text{supply} = V_1 + V_2$ ). PARALLEL (branches) — the voltage across each branch is the same (= supply), and the current SPLITS, with current into a junction equal to current out ( $I_\text{total} = I_1 + I_2$ ). Parallel lamps thus work independently, each with the full supply voltage.

Common mistakes

Drawn from real examiner reports.

Q = I x t needs seconds and amps

$Q = I \times t$ needs current in amps and time in SECONDS to give coulombs. Over 1 hour, forgetting to convert (using $t = 1$ instead of 3600 s) gave the frequent wrong answer "1". Convert minutes by multiplying by 60, hours by 3600, and milliamps by dividing by 1000, BEFORE substituting.

Jun 2024 1P Q11a

Compare the charge, do not just calculate it

In "is the power bank big enough?" style questions, candidates calculate the total charge but omit the COMPARISON. You must compare the total charge NEEDED with the charge AVAILABLE and state a justified conclusion — e.g. "220 C needed exceeds 180 C stored, so it is insufficient". A bare number without the comparison loses that mark.

Jun 2024 1P Q11c

Longer cable adds resistance, not delay

A common misconception is that a longer connecting cable makes electrons "take more time to travel", reasoning from speed = distance/time. In fact a longer cable has MORE RESISTANCE, which reduces the charging current. It is the resistance, not the electrons' travel time, that matters.

Jun 2024 1P Q11b(ii)

Series voltage is shared, not full

In series the source voltage is SHARED — not applied in full to each component. A common error is treating two series resistors as if the whole cell voltage acts across each. Correct: the component p.d.s ADD to the supply, so if a 6.0 V cell drives A and B in series with 4.0 V across A, then B has $6.0 - 4.0 = 2.0$ V. Only in PARALLEL does each branch get the full supply.

Nov 2024 1P Q9b(iv)

Current is charge/second, not "electrons"

Defining current as only "the flow of electrons" does not gain full credit. The mark-scheme form is the RATE OF FLOW OF ELECTRIC CHARGE ( $I = Q/t$ ), unit the ampere (A). In a metal the carriers happen to be electrons, but the definition is about charge per second, not the particles. Always give the meaning AND the unit.

Voltage is not "battery strength"

Saying voltage is "the power of the battery" or "how strong the cell is" gains nothing, and confuses it with power. The mark-scheme form is the ENERGY TRANSFERRED PER UNIT CHARGE ( $V = E/Q$ ), unit the volt (V). Voltage is joules per COULOMB (J/C); power is joules per SECOND (J/s) — same numerator, different denominator.

Jun 2024 1P Q7c(i)

Exam tips

Name the arrangement, then apply the rule
Decide the arrangement first and STATE it (examiners credit naming it): series → same current, shared voltage (p.d.s add); parallel → same voltage, shared current. Then apply $Q = I \times t$ (seconds, amps) for charge and $E = Q \times V$ for energy, always with the unit.
Substitute first, then state the unit
Write the equation, substitute the numbers, THEN rearrange if needed — substituting first secures a method mark even if the algebra slips. Always finish with the correct unit (C for charge, J for energy, A for current); a missing or wrong unit is an easy dropped mark.
Match your working to the marks
Use the mark tariff as a guide to how many steps or comparisons are needed — a 2-mark charge-comparison wants BOTH the calculation and an explicit compare-and-conclude. Show every substitution so method marks are safe even if the final value is wrong.

Revision notes