Recall and apply: charge flow (C) = current (A) × time (s)

The idea

Charge flow is the total charge that passes a point in a circuit, measured in coulombs (C). Current is the rate of flow of charge, measured in amperes (A), where one ampere is one coulomb per second. Time is measured in seconds (s). The relationship is Q = I × t, so a current of 2 A flowing for 30 s transfers 2 × 30 = 60 C. Rearranged, I = Q ÷ t and t = Q ÷ I. Always convert time to seconds first: 3 minutes is 180 s, and 0.5 minutes is 30 s. If a question gives charge and time and asks for current, divide charge by time. Check units: coulombs divided by seconds gives amperes. This equation links the definition of current to measurable quantities and is recalled and applied in both foundation and higher tiers.

What earns the marks

• Recalls the relationship charge flow = current × time and writes it as Q = I × t.
• Substitutes current in amperes and time in seconds to obtain charge in coulombs.
• Rearranges the relationship to I = Q ÷ t or t = Q ÷ I when current or time is the unknown.
• Converts a time given in minutes to seconds by multiplying by 60 before calculating.
• Checks that the answer carries the unit coulomb and is consistent with the values used.

Common mistakes and how to put them right

• Uses time in minutes without converting to seconds.

  Put it right: Multiply the minutes by 60 to obtain seconds.

• Multiplies charge by time instead of current by time.

  Put it right: Apply Q = I × t with current and time as the factors.

• Gives the answer in amperes when charge was asked for; state the unit as coulombs for charge flow.

• Divides time by current when finding current.

  Put it right: Use I = Q ÷ t so charge is divided by time.

Exam tips

• Write Q = I × t, then rearrange before substituting numbers.
• Convert every time to seconds and every current to amperes before calculating.
• Carry the unit through the calculation so the final answer is clearly in coulombs, amperes or seconds.