Current and charge definitions
Electric current = the rate of flow of electric charge; ; unit ampere (A). Two elements: (1) rate of flow (not just flow); (2) of charge (not of electrons). Charge is in coulombs (C); ; 1 C is the charge when 1 A flows for 1 s. Both are two-mark definitions — one mark per element, and the unit (A or C) is always a marking point. E.g. 0.6 A for 45 s: C.
P.d. = energy per unit charge (E = QV)
Potential difference (p.d. / voltage) between two points = the energy transferred per unit charge passing: , or ( in volts, in J, in C). The volt is one joule per coulomb (1 V = 1 J/C). Two elements: (1) energy transferred per unit charge; (2) unit V (or J/C). E.g. 50 C through a 12 V component transfers J.
Conventional current vs electron flow
In solid metals current is carried by negatively charged electrons, which flow from − to + in the external circuit. Conventional current is defined the opposite way, from + to − (a convention set before the electron was discovered). "Direction of current" means conventional (+ to −); "direction of electron flow" means − to +; the two are always opposite. In electrolytes and gases positive ions also carry current, so the "electrons only" picture applies just to metals.
Drawn from real examiner reports.
Current defined as "flow of electrons"
"Current is the flow of electrons" fails twice: it omits rate (current is charge per second, measured in A = C/s), and it names the carrier (electrons) rather than the quantity (charge) — current also flows as ions in electrolytes and gases. Mark-scheme form: "current is the rate of flow of electric charge", with as the second marking point.
November 2024 Paper 2P Q2(b): many candidates correctly mentioned that movement of charge is equivalent to a current but could not score the second mark because they did not state that charge movement stopping caused the current to return to zero — showing the definition must include the "rate" element, not just the word "current".
Milliseconds not converted in Q = It
needs time in seconds. Convert milliseconds first (): 250 ms = 0.250 s, so C, not 125 C. Worse is treating "ms" as minutes — a factor-of-60 000 error (1 min = 60 s, 1 ms = 0.001 s). Check the time unit and convert to seconds before substituting; ms never stands for minutes.
November 2024 Paper 2P Q2(c): the examiner noted that despite the formula Q = It being provided on the formula sheet, common errors included incorrect rearrangement and — in particular — converting milliseconds to seconds incorrectly, with a note that "ms never stands for minutes".
Current and electron flow same direction
Conventional current runs + to − in the external circuit; electrons, being negative, are repelled by − and attracted to +, so they run − to + — the opposite way. If the current arrow points clockwise, electrons move anticlockwise. A question asking "which way do electrons flow?" wants the opposite of the current arrows on the diagram.
November 2024 Paper 2P Q2(a): a number of candidates incorrectly referred to the movement of positive charges rather than electrons in a static-charging context, suggesting the conventional/electron direction distinction was not secure.
Current and charge treated as the same
Current (, in A) is the rate at which charge flows; charge (, in C) is the total that has passed. They are linked by : doubling the time doubles the charge but does not change a steady current. Do not quote a charge in amperes or a current in coulombs — the unit tells you which quantity you have.
Voltage confused with energy
Voltage (p.d., in V) is energy per unit charge and does not depend on how much charge flows; the energy transferred (, in J) does, via . A 9 V battery always supplies 9 V p.d., but the energy it delivers depends on the charge drawn. Do not answer a "voltage" question with an energy in joules, or vice versa.
Ammeter/voltmeter placed wrongly
An ammeter goes in series with the component (it reads the current through it); wired in parallel it short-circuits the component. A voltmeter goes in parallel across the component (it reads the p.d. across it); wired in series it blocks the current. Swapping the two is a fatal circuit-diagram error.
Write the unit conversion on its own line
For with non-SI units (ms, mA, minutes), write the conversion on its own line first, e.g. " ms s", then substitute. The substitution mark is awarded even if the final arithmetic slips — hiding the conversion inside the working loses it.
Rearrange before substituting
Write the formula, rearrange it, then substitute — e.g. → → put the numbers in. Rearranging after substituting, or inverting the ratio, is a common way to lose marks even when the physics is understood.
Give both elements of the definition
Definition and "explain" answers here need BOTH elements: for current, "rate of flow" AND "of charge"; for the current/electron directions, name both and state they are opposite. A one-element answer scores only half the marks.
| Formula | Meaning | Symbols |
|---|---|---|
| Charge = current × time | = charge (C), = current (A), = time (s) | |
| p.d. = energy per unit charge | = potential difference (V), = energy transferred (J), = charge (C) | |
| Energy transferred = charge × p.d. | All symbols as above |
Unit definitions:
Mark-scheme definitions (two elements each — both required):
Electric current: the rate of flow of electric charge; ; unit ampere (A).
Potential difference (p.d.): the energy transferred per unit charge between two points; ; unit volt (V) = joule per coulomb (J/C).
Charge: measured in coulombs (C); ; 1 C is the charge flowing when 1 A flows for 1 s.
Define electric current.
A torch bulb carries a current of 0.6 A. Calculate the charge that flows through the bulb in 45 seconds.
State the formula you use, substitute the values, and give the unit in your answer.
