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AS & A-Level Physics 09 — Charge, Current and Resistance

AS & A-Level Physics 09 — Charge, Current and Resistance

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Independent Deckloop revision aligned with the Cambridge International AS & A Level Physics (9702) syllabus, 2025–2027. Not affiliated with or endorsed by Cambridge International Education. Chapter 9 of 26: Charge, current and resistance. Concepts, worked applications and misconception checks.

Physics EN AS & A-Level
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Relating Charge, Current, and Time

Electric current (II) is defined as the rate of flow of charge (QQ) past a point in a circuit. This relationship is expressed by the fundamental equation Q=ItQ = It. Here, QQ represents the total charge that flows, II is the current, and tt is the time duration over which the charge flows. The SI unit for charge is the coulomb (C), for current is the ampere (A), and for time is the second (s). One coulomb is defined as the amount of charge that passes a point in a circuit when a current of one ampere flows for one second. Therefore, 1 C=1 A s1 \text{ C} = 1 \text{ A s}. It is crucial to ensure all quantities are in their base SI units for calculations.

Key points

  • Current is the rate of flow of charge.
  • The relationship is given by Q=ItQ = It.
  • Units: Charge (Coulomb, C), Current (Ampere, A), Time (second, s).
  • One Coulomb is equivalent to one Ampere-second (1 C=1 A s1 \text{ C} = 1 \text{ A s}).

Worked example

Question

A small electric motor draws a constant current of 150 mA150 \text{ mA} for 2.52.5 minutes. Calculate the total charge that flows through the motor during this time.

Solution

1. First, convert the current to Amperes: I=150 mA=150×10−3 A=0.150 AI = 150 \text{ mA} = 150 \times 10^{-3} \text{ A} = 0.150 \text{ A}.
2. Next, convert the time to seconds: t=2.5 minutes=2.5×60 s=150 st = 2.5 \text{ minutes} = 2.5 \times 60 \text{ s} = 150 \text{ s}.
3. Use the formula Q=ItQ = It.
4. Q=(0.150 A)×(150 s)Q = (0.150 \text{ A}) \times (150 \text{ s}).
5. Q=22.5 CQ = 22.5 \text{ C}.

The total charge that flows through the motor is 22.5 C22.5 \text{ C}.

Common pitfalls

  • Forgetting to convert current from milliamperes (mA) or time from minutes/hours into base SI units (Amperes and seconds) before calculation.
  • Mixing up the variables in the formula, e.g., calculating I/tI/t instead of ItIt for charge.

Prerequisites

  • Understanding of electric current as charge flow.
  • Knowledge of SI base units for current and time.
  • Ability to use SI prefixes like milli.
Further resources